// SPDX-License-Identifier: GPL-2.0
#include <linux/kernel.h>
#include <linux/irqflags.h>
#include <linux/string.h>
#include <linux/errno.h>
#include <linux/bug.h>
#include "printk_ringbuffer.h"
#include "internal.h"
/**
* DOC: printk_ringbuffer overview
*
* Data Structure
* --------------
* The printk_ringbuffer is made up of 3 internal ringbuffers:
*
* desc_ring
* A ring of descriptors and their meta data (such as sequence number,
* timestamp, loglevel, etc.) as well as internal state information about
* the record and logical positions specifying where in the other
* ringbuffer the text strings are located.
*
* text_data_ring
* A ring of data blocks. A data block consists of an unsigned long
* integer (ID) that maps to a desc_ring index followed by the text
* string of the record.
*
* The internal state information of a descriptor is the key element to allow
* readers and writers to locklessly synchronize access to the data.
*
* Implementation
* --------------
*
* Descriptor Ring
* ~~~~~~~~~~~~~~~
* The descriptor ring is an array of descriptors. A descriptor contains
* essential meta data to track the data of a printk record using
* blk_lpos structs pointing to associated text data blocks (see
* "Data Rings" below). Each descriptor is assigned an ID that maps
* directly to index values of the descriptor array and has a state. The ID
* and the state are bitwise combined into a single descriptor field named
* @state_var, allowing ID and state to be synchronously and atomically
* updated.
*
* Descriptors have four states:
*
* reserved
* A writer is modifying the record.
*
* committed
* The record and all its data are written. A writer can reopen the
* descriptor (transitioning it back to reserved), but in the committed
* state the data is consistent.
*
* finalized
* The record and all its data are complete and available for reading. A
* writer cannot reopen the descriptor.
*
* reusable
* The record exists, but its text and/or meta data may no longer be
* available.
*
* Querying the @state_var of a record requires providing the ID of the
* descriptor to query. This can yield a possible fifth (pseudo) state:
*
* miss
* The descriptor being queried has an unexpected ID.
*
* The descriptor ring has a @tail_id that contains the ID of the oldest
* descriptor and @head_id that contains the ID of the newest descriptor.
*
* When a new descriptor should be created (and the ring is full), the tail
* descriptor is invalidated by first transitioning to the reusable state and
* then invalidating all tail data blocks up to and including the data blocks
* associated with the tail descriptor (for the text ring). Then
* @tail_id is advanced, followed by advancing @head_id. And finally the
* @state_var of the new descriptor is initialized to the new ID and reserved
* state.
*
* The @tail_id can only be advanced if the new @tail_id would be in the
* committed or reusable queried state. This makes it possible that a valid
* sequence number of the tail is always available.
*
* Descriptor Finalization
* ~~~~~~~~~~~~~~~~~~~~~~~
* When a writer calls the commit function prb_commit(), record data is
* fully stored and is consistent within the ringbuffer. However, a writer can
* reopen that record, claiming exclusive access (as with prb_reserve()), and
* modify that record. When finished, the writer must again commit the record.
*
* In order for a record to be made available to readers (and also become
* recyclable for writers), it must be finalized. A finalized record cannot be
* reopened and can never become "unfinalized". Record finalization can occur
* in three different scenarios:
*
* 1) A writer can simultaneously commit and finalize its record by calling
* prb_final_commit() instead of prb_commit().
*
* 2) When a new record is reserved and the previous record has been
* committed via prb_commit(), that previous record is automatically
* finalized.
*
* 3) When a record is committed via prb_commit() and a newer record
* already exists, the record being committed is automatically finalized.
*
* Data Ring
* ~~~~~~~~~
* The text data ring is a byte array composed of data blocks. Data blocks are
* referenced by blk_lpos structs that point to the logical position of the
* beginning of a data block and the beginning of the next adjacent data
* block. Logical positions are mapped directly to index values of the byte
* array ringbuffer.
*
* Each data block consists of an ID followed by the writer data. The ID is
* the identifier of a descriptor that is associated with the data block. A
* given data block is considered valid if all of the following conditions
* are met:
*
* 1) The descriptor associated with the data block is in the committed
* or finalized queried state.
*
* 2) The blk_lpos struct within the descriptor associated with the data
* block references back to the same data block.
*
* 3) The data block is within the head/tail logical position range.
*
* If the writer data of a data block would extend beyond the end of the
* byte array, only the ID of the data block is stored at the logical
* position and the full data block (ID and writer data) is stored at the
* beginning of the byte array. The referencing blk_lpos will point to the
* ID before the wrap and the next data block will be at the logical
* position adjacent the full data block after the wrap.
*
* Data rings have a @tail_lpos that points to the beginning of the oldest
* data block and a @head_lpos that points to the logical position of the
* next (not yet existing) data block.
*
* When a new data block should be created (and the ring is full), tail data
* blocks will first be invalidated by putting their associated descriptors
* into the reusable state and then pushing the @tail_lpos forward beyond
* them. Then the @head_lpos is pushed forward and is associated with a new
* descriptor. If a data block is not valid, the @tail_lpos cannot be
* advanced beyond it.
*
* Info Array
* ~~~~~~~~~~
* The general meta data of printk records are stored in printk_info structs,
* stored in an array with the same number of elements as the descriptor ring.
* Each info corresponds to the descriptor of the same index in the
* descriptor ring. Info validity is confirmed by evaluating the corresponding
* descriptor before and after loading the info.
*
* Usage
* -----
* Here are some simple examples demonstrating writers and readers. For the
* examples a global ringbuffer (test_rb) is available (which is not the
* actual ringbuffer used by printk)::
*
* DEFINE_PRINTKRB(test_rb, 15, 5);
*
* This ringbuffer allows up to 32768 records (2 ^ 15) and has a size of
* 1 MiB (2 ^ (15 + 5)) for text data.
*
* Sample writer code::
*
* const char *textstr = "message text";
* struct prb_reserved_entry e;
* struct printk_record r;
*
* // specify how much to allocate
* prb_rec_init_wr(&r, strlen(textstr) + 1);
*
* if (prb_reserve(&e, &test_rb, &r)) {
* snprintf(r.text_buf, r.text_buf_size, "%s", textstr);
*
* r.info->text_len = strlen(textstr);
* r.info->ts_nsec = local_clock();
* r.info->caller_id = printk_caller_id();
*
* // commit and finalize the record
* prb_final_commit(&e);
* }
*
* Note that additional writer functions are available to extend a record
* after it has been committed but not yet finalized. This can be done as
* long as no new records have been reserved and the caller is the same.
*
* Sample writer code (record extending)::
*
* // alternate rest of previous example
*
* r.info->text_len = strlen(textstr);
* r.info->ts_nsec = local_clock();
* r.info->caller_id = printk_caller_id();
*
* // commit the record (but do not finalize yet)
* prb_commit(&e);
* }
*
* ...
*
* // specify additional 5 bytes text space to extend
* prb_rec_init_wr(&r, 5);
*
* // try to extend, but only if it does not exceed 32 bytes
* if (prb_reserve_in_last(&e, &test_rb, &r, printk_caller_id(), 32)) {
* snprintf(&r.text_buf[r.info->text_len],
* r.text_buf_size - r.info->text_len, "hello");
*
* r.info->text_len += 5;
*
* // commit and finalize the record
* prb_final_commit(&e);
* }
*
* Sample reader code::
*
* struct printk_info info;
* struct printk_record r;
* char text_buf[32];
* u64 seq;
*
* prb_rec_init_rd(&r, &info, &text_buf[0], sizeof(text_buf));
*
* prb_for_each_record(0, &test_rb, &seq, &r) {
* if (info.seq != seq)
* pr_warn("lost %llu records\n", info.seq - seq);
*
* if (info.text_len > r.text_buf_size) {
* pr_warn("record %llu text truncated\n", info.seq);
* text_buf[r.text_buf_size - 1] = 0;
* }
*
* pr_info("%llu: %llu: %s\n", info.seq, info.ts_nsec,
* &text_buf[0]);
* }
*
* Note that additional less convenient reader functions are available to
* allow complex record access.
*
* ABA Issues
* ~~~~~~~~~~
* To help avoid ABA issues, descriptors are referenced by IDs (array index
* values combined with tagged bits counting array wraps) and data blocks are
* referenced by logical positions (array index values combined with tagged
* bits counting array wraps). However, on 32-bit systems the number of
* tagged bits is relatively small such that an ABA incident is (at least
* theoretically) possible. For example, if 4 million maximally sized (1KiB)
* printk messages were to occur in NMI context on a 32-bit system, the
* interrupted context would not be able to recognize that the 32-bit integer
* completely wrapped and thus represents a different data block than the one
* the interrupted context expects.
*
* To help combat this possibility, additional state checking is performed
* (such as using cmpxchg() even though set() would suffice). These extra
* checks are commented as such and will hopefully catch any ABA issue that
* a 32-bit system might experience.
*
* Memory Barriers
* ~~~~~~~~~~~~~~~
* Multiple memory barriers are used. To simplify proving correctness and
* generating litmus tests, lines of code related to memory barriers
* (loads, stores, and the associated memory barriers) are labeled::
*
* LMM(function:letter)
*
* Comments reference the labels using only the "function:letter" part.
*
* The memory barrier pairs and their ordering are:
*
* desc_reserve:D / desc_reserve:B
* push descriptor tail (id), then push descriptor head (id)
*
* desc_reserve:D / data_push_tail:B
* push data tail (lpos), then set new descriptor reserved (state)
*
* desc_reserve:D / desc_push_tail:C
* push descriptor tail (id), then set new descriptor reserved (state)
*
* desc_reserve:D / prb_first_seq:C
* push descriptor tail (id), then set new descriptor reserved (state)
*
* desc_reserve:F / desc_read:D
* set new descriptor id and reserved (state), then allow writer changes
*
* data_alloc:A (or data_realloc:A) / desc_read:D
* set old descriptor reusable (state), then modify new data block area
*
* data_alloc:A (or data_realloc:A) / data_push_tail:B
* push data tail (lpos), then modify new data block area
*
* _prb_commit:B / desc_read:B
* store writer changes, then set new descriptor committed (state)
*
* desc_reopen_last:A / _prb_commit:B
* set descriptor reserved (state), then read descriptor data
*
* _prb_commit:B / desc_reserve:D
* set new descriptor committed (state), then check descriptor head (id)
*
* data_push_tail:D / data_push_tail:A
* set descriptor reusable (state), then push data tail (lpos)
*
* desc_push_tail:B / desc_reserve:D
* set descriptor reusable (state), then push descriptor tail (id)
*
* desc_update_last_finalized:A / desc_last_finalized_seq:A
* store finalized record, then set new highest finalized sequence number
*/
#define DATA_SIZE(data_ring) _DATA_SIZE((data_ring)->size_bits)
#define DATA_SIZE_MASK(data_ring) (DATA_SIZE(data_ring) - 1)
#define DESCS_COUNT(desc_ring) _DESCS_COUNT((desc_ring)->count_bits)
#define DESCS_COUNT_MASK(desc_ring) (DESCS_COUNT(desc_ring) - 1)
/* Determine the data array index from a logical position. */
#define DATA_INDEX(data_ring, lpos) ((lpos) & DATA_SIZE_MASK(data_ring))
/* Determine the desc array index from an ID or sequence number. */
#define DESC_INDEX(desc_ring, n) ((n) & DESCS_COUNT_MASK(desc_ring))
/* Determine how many times the data array has wrapped. */
#define DATA_WRAPS(data_ring, lpos) ((lpos) >> (data_ring)->size_bits)
/* Determine if a logical position refers to a data-less block. */
#define LPOS_DATALESS(lpos) ((lpos) & 1UL)
#define BLK_DATALESS(blk) (LPOS_DATALESS((blk)->begin) && \
LPOS_DATALESS((blk)->next))
/* Get the logical position at index 0 of the current wrap. */
#define DATA_THIS_WRAP_START_LPOS(data_ring, lpos) \
((lpos) & ~DATA_SIZE_MASK(data_ring))
/* Get the ID for the same index of the previous wrap as the given ID. */
#define DESC_ID_PREV_WRAP(desc_ring, id) \
DESC_ID((id) - DESCS_COUNT(desc_ring))
/*
* A data block: mapped directly to the beginning of the data block area
* specified as a logical position within the data ring.
*
* @id: the ID of the associated descriptor
* @data: the writer data
*
* Note that the size of a data block is only known by its associated
* descriptor.
*/
struct prb_data_block {
unsigned long id;
char data[];
};
/*
* Return the descriptor associated with @n. @n can be either a
* descriptor ID or a sequence number.
*/
static struct prb_desc *to_desc(struct prb_desc_ring *desc_ring, u64 n)
{
return &desc_ring->descs[DESC_INDEX(desc_ring, n)];
}
/*
* Return the printk_info associated with @n. @n can be either a
* descriptor ID or a sequence number.
*/
static struct printk_info *to_info(struct prb_desc_ring *desc_ring, u64 n)
{
return &desc_ring->infos[DESC_INDEX(desc_ring, n)];
}
static struct prb_data_block *to_block(struct prb_data_ring *data_ring,
unsigned long begin_lpos)
{
return (void *)&data_ring->data[DATA_INDEX(data_ring, begin_lpos)];
}
/*
* Increase the data size to account for data block meta data plus any
* padding so that the adjacent data block is aligned on the ID size.
*/
static unsigned int to_blk_size(unsigned int size)
{
struct prb_data_block *db = NULL;
size += sizeof(*db);
size = ALIGN(size, sizeof(db->id));
return size;
}
/*
* Sanity checker for reserve size. The ringbuffer code assumes that a data
* block does not exceed the maximum possible size that could fit within the
* ringbuffer. This function provides that basic size check so that the
* assumption is safe.
*/
static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
{
struct prb_data_block *db = NULL;
if (size == 0)
return true;
/*
* Ensure the alignment padded size could possibly fit in the data
* array. The largest possible data block must still leave room for
* at least the ID of the next block.
*/
size = to_blk_size(size);
if (size > DATA_SIZE(data_ring) - sizeof(db->id))
return false;
return true;
}
/* Query the state of a descriptor. */
static enum desc_state get_desc_state(unsigned long id,
unsigned long state_val)
{
if (id != DESC_ID(state_val))
return desc_miss;
return DESC_STATE(state_val);
}
/*
* Get a copy of a specified descriptor and return its queried state. If the
* descriptor is in an inconsistent state (miss or reserved), the caller can
* only expect the descriptor's @state_var field to be valid.
*
* The sequence number and caller_id can be optionally retrieved. Like all
* non-state_var data, they are only valid if the descriptor is in a
* consistent state.
*/
static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
unsigned long id, struct prb_desc *desc_out,
u64 *seq_out, u32 *caller_id_out)
{
struct printk_info *info = to_info(desc_ring, id);
struct prb_desc *desc = to_desc(desc_ring, id);
atomic_long_t *state_var = &desc->state_var;
enum desc_state d_state;
unsigned long state_val;
/* Check the descriptor state. */
state_val = atomic_long_read(state_var); /* LMM(desc_read:A) */
d_state = get_desc_state(id, state_val);
if (d_state == desc_miss || d_state == desc_reserved) {
/*
* The descriptor is in an inconsistent state. Set at least
* @state_var so that the caller can see the details of
* the inconsistent state.
*/
goto out;
}
/*
* Guarantee the state is loaded before copying the descriptor
* content. This avoids copying obsolete descriptor content that might
* not apply to the descriptor state. This pairs with _prb_commit:B.
*
* Memory barrier involvement:
*
* If desc_read:A reads from _prb_commit:B, then desc_read:C reads
* from _prb_commit:A.
*
* Relies on:
*
* WMB from _prb_commit:A to _prb_commit:B
* matching
* RMB from desc_read:A to desc_read:C
*/
smp_rmb(); /* LMM(desc_read:B) */
/*
* Copy the descriptor data. The data is not valid until the
* state has been re-checked. A memcpy() for all of @desc
* cannot be used because of the atomic_t @state_var field.
*/
if (desc_out) {
memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
}
if (seq_out)
*seq_out = info->seq; /* also part of desc_read:C */
if (caller_id_out)
*caller_id_out = info->caller_id; /* also part of desc_read:C */
/*
* 1. Guarantee the descriptor content is loaded before re-checking
* the state. This avoids reading an obsolete descriptor state
* that may not apply to the copied content. This pairs with
* desc_reserve:F.
*
* Memory barrier involvement:
*
* If desc_read:C reads from desc_reserve:G, then desc_read:E
* reads from desc_reserve:F.
*
* Relies on:
*
* WMB from desc_reserve:F to desc_reserve:G
* matching
* RMB from desc_read:C to desc_read:E
*
* 2. Guarantee the record data is loaded before re-checking the
* state. This avoids reading an obsolete descriptor state that may
* not apply to the copied data. This pairs with data_alloc:A and
* data_realloc:A.
*
* Memory barrier involvement:
*
* If copy_data:A reads from data_alloc:B, then desc_read:E
* reads from desc_make_reusable:A.
*
* Relies on:
*
* MB from desc_make_reusable:A to data_alloc:B
* matching
* RMB from desc_read:C to desc_read:E
*
* Note: desc_make_reusable:A and data_alloc:B can be different
* CPUs. However, the data_alloc:B CPU (which performs the
* full memory barrier) must have previously seen
* desc_make_reusable:A.
*/
smp_rmb(); /* LMM(desc_read:D) */
/*
* The data has been copied. Return the current descriptor state,
* which may have changed since the load above.
*/
state_val = atomic_long_read(state_var); /* LMM(desc_read:E) */
d_state = get_desc_state(id, state_val);
out:
if (desc_out)
atomic_long_set(&desc_out->state_var, state_val);
return d_state;
}
/*
* Take a specified descriptor out of the finalized state by attempting
* the transition from finalized to reusable. Either this context or some
* other context will have been successful.
*/
static void desc_make_reusable(struct prb_desc_ring *desc_ring,
unsigned long id)
{
unsigned long val_finalized = DESC_SV(id, desc_finalized);
unsigned long val_reusable = DESC_SV(id, desc_reusable);
struct prb_desc *desc = to_desc(desc_ring, id);
atomic_long_t *state_var = &desc->state_var;
atomic_long_cmpxchg_relaxed(state_var, val_finalized,
val_reusable); /* LMM(desc_make_reusable:A) */
}
/*
* Given the text data ring, put the associated descriptor of each
* data block from @lpos_begin until @lpos_end into the reusable state.
*
* If there is any problem making the associated descriptor reusable, either
* the descriptor has not yet been finalized or another writer context has
* already pushed the tail lpos past the problematic data block. Regardless,
* on error the caller can re-load the tail lpos to determine the situation.
*/
static bool data_make_reusable(struct printk_ringbuffer *rb,
unsigned long lpos_begin,
unsigned long lpos_end,
unsigned long *lpos_out)
{
struct prb_data_ring *data_ring = &rb->text_data_ring;
struct prb_desc_ring *desc_ring = &rb->desc_ring;
struct prb_data_block *blk;
enum desc_state d_state;
struct prb_desc desc;
struct prb_data_blk_lpos *blk_lpos = &desc.text_blk_lpos;
unsigned long id;
/* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
blk = to_block(data_ring, lpos_begin);
/*
* Load the block ID from the data block. This is a data race
* against a writer that may have newly reserved this data
* area. If the loaded value matches a valid descriptor ID,
* the blk_lpos of that descriptor will be checked to make
* sure it points back to this data block. If the check fails,
* the data area has been recycled by another writer.
*/
id = blk->id; /* LMM(data_make_reusable:A) */
d_state = desc_read(desc_ring, id, &desc,
NULL, NULL); /* LMM(data_make_reusable:B) */
switch (d_state) {
case desc_miss:
case desc_reserved:
case desc_committed:
return false;
case desc_finalized:
/*
* This data block is invalid if the descriptor
* does not point back to it.
*/
if (blk_lpos->begin != lpos_begin)
return false;
desc_make_reusable(desc_ring, id);
break;
case desc_reusable:
/*
* This data block is invalid if the descriptor
* does not point back to it.
*/
if (blk_lpos->begin != lpos_begin)
return false;
break;
}
/* Advance @lpos_begin to the next data block. */
lpos_begin = blk_lpos->next;
}
*lpos_out = lpos_begin;
return true;
}
/*
* Advance the data ring tail to at least @lpos. This function puts
* descriptors into the reusable state if the tail is pushed beyond
* their associated data block.
*/
static bool data_push_tail(struct printk_ringbuffer *rb, unsigned long lpos)
{
struct prb_data_ring *data_ring = &rb->text_data_ring;
unsigned long tail_lpos_new;
unsigned long tail_lpos;
unsigned long next_lpos;
/* If @lpos is from a data-less block, there is nothing to do. */
if (LPOS_DATALESS(lpos))
return true;
/*
* Any descriptor states that have transitioned to reusable due to the
* data tail being pushed to this loaded value will be visible to this
* CPU. This pairs with data_push_tail:D.
*
* Memory barrier involvement:
*
* If data_push_tail:A reads from data_push_tail:D, then this CPU can
* see desc_make_reusable:A.
*
* Relies on:
*
* MB from desc_make_reusable:A to data_push_tail:D
* matches
* READFROM from data_push_tail:D to data_push_tail:A
* thus
* READFROM from desc_make_reusable:A to this CPU
*/
tail_lpos = atomic_long_read(&data_ring->tail_lpos); /* LMM(data_push_tail:A) */
/*
* Loop until the tail lpos is at or beyond @lpos. This condition
* may already be satisfied, resulting in no full memory barrier
* from data_push_tail:D being performed. However, since this CPU
* sees the new tail lpos, any descriptor states that transitioned to
* the reusable state must already be visible.
*/
while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
/*
* Make all descriptors reusable that are associated with
* data blocks before @lpos.
*/
if (!data_make_reusable(rb, tail_lpos, lpos, &next_lpos)) {
/*
* 1. Guarantee the block ID loaded in
* data_make_reusable() is performed before
* reloading the tail lpos. The failed
* data_make_reusable() may be due to a newly
* recycled data area causing the tail lpos to
* have been previously pushed. This pairs with
* data_alloc:A and data_realloc:A.
*
* Memory barrier involvement:
*
* If data_make_reusable:A reads from data_alloc:B,
* then data_push_tail:C reads from
* data_push_tail:D.
*
* Relies on:
*
* MB from data_push_tail:D to data_alloc:B
* matching
* RMB from data_make_reusable:A to
* data_push_tail:C
*
* Note: data_push_tail:D and data_alloc:B can be
* different CPUs. However, the data_alloc:B
* CPU (which performs the full memory
* barrier) must have previously seen
* data_push_tail:D.
*
* 2. Guarantee the descriptor state loaded in
* data_make_reusable() is performed before
* reloading the tail lpos. The failed
* data_make_reusable() may be due to a newly
* recycled descriptor causing the tail lpos to
* have been previously pushed. This pairs with
* desc_reserve:D.
*
* Memory barrier involvement:
*
* If data_make_reusable:B reads from
* desc_reserve:F, then data_push_tail:C reads
* from data_push_tail:D.
*
* Relies on:
*
* MB from data_push_tail:D to desc_reserve:F
* matching
* RMB from data_make_reusable:B to
* data_push_tail:C
*
* Note: data_push_tail:D and desc_reserve:F can
* be different CPUs. However, the
* desc_reserve:F CPU (which performs the
* full memory barrier) must have previously
* seen data_push_tail:D.
*/
smp_rmb(); /* LMM(data_push_tail:B) */
tail_lpos_new = atomic_long_read(&data_ring->tail_lpos
); /* LMM(data_push_tail:C) */
if (tail_lpos_new == tail_lpos)
return false;
/* Another CPU pushed the tail. Try again. */
tail_lpos = tail_lpos_new;
continue;
}
/*
* Guarantee any descriptor states that have transitioned to
* reusable are stored before pushing the tail lpos. A full
* memory barrier is needed since other CPUs may have made
* the descriptor states reusable. This pairs with
* data_push_tail:A.
*/
if (atomic_long_try_cmpxchg(&data_ring->tail_lpos, &tail_lpos,
next_lpos)) { /* LMM(data_push_tail:D) */
break;
}
}
return true;
}
/*
* Advance the desc ring tail. This function advances the tail by one
* descriptor, thus invalidating the oldest descriptor. Before advancing
* the tail, the tail descriptor is made reusable and all data blocks up to
* and including the descriptor's data block are invalidated (i.e. the data
* ring tail is pushed past the data block of the descriptor being made
* reusable).
*/
static bool desc_push_tail(struct printk_ringbuffer *rb,
unsigned long tail_id)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
enum desc_state d_state;
struct prb_desc desc;
d_state = desc_read(desc_ring, tail_id, &desc, NULL, NULL);
switch (d_state) {
case desc_miss:
/*
* If the ID is exactly 1 wrap behind the expected, it is
* in the process of being reserved by another writer and
* must be considered reserved.
*/
if (DESC_ID(atomic_long_read(&desc.state_var)) ==
DESC_ID_PREV_WRAP(desc_ring, tail_id)) {
return false;
}
/*
* The ID has changed. Another writer must have pushed the
* tail and recycled the descriptor already. Success is
* returned because the caller is only interested in the
* specified tail being pushed, which it was.
*/
return true;
case desc_reserved:
case desc_committed:
return false;
case desc_finalized:
desc_make_reusable(desc_ring, tail_id);
break;
case desc_reusable:
break;
}
/*
* Data blocks must be invalidated before their associated
* descriptor can be made available for recycling. Invalidating
* them later is not possible because there is no way to trust
* data blocks once their associated descriptor is gone.
*/
if (!data_push_tail(rb, desc.text_blk_lpos.next))
return false;
/*
* Check the next descriptor after @tail_id before pushing the tail
* to it because the tail must always be in a finalized or reusable
* state. The implementation of prb_first_seq() relies on this.
*
* A successful read implies that the next descriptor is less than or
* equal to @head_id so there is no risk of pushing the tail past the
* head.
*/
d_state = desc_read(desc_ring, DESC_ID(tail_id + 1), &desc,
NULL, NULL); /* LMM(desc_push_tail:A) */
if (d_state == desc_finalized || d_state == desc_reusable) {
/*
* Guarantee any descriptor states that have transitioned to
* reusable are stored before pushing the tail ID. This allows
* verifying the recycled descriptor state. A full memory
* barrier is needed since other CPUs may have made the
* descriptor states reusable. This pairs with desc_reserve:D.
*/
atomic_long_cmpxchg(&desc_ring->tail_id, tail_id,
DESC_ID(tail_id + 1)); /* LMM(desc_push_tail:B) */
} else {
/*
* Guarantee the last state load from desc_read() is before
* reloading @tail_id in order to see a new tail ID in the
* case that the descriptor has been recycled. This pairs
* with desc_reserve:D.
*
* Memory barrier involvement:
*
* If desc_push_tail:A reads from desc_reserve:F, then
* desc_push_tail:D reads from desc_push_tail:B.
*
* Relies on:
*
* MB from desc_push_tail:B to desc_reserve:F
* matching
* RMB from desc_push_tail:A to desc_push_tail:D
*
* Note: desc_push_tail:B and desc_reserve:F can be different
* CPUs. However, the desc_reserve:F CPU (which performs
* the full memory barrier) must have previously seen
* desc_push_tail:B.
*/
smp_rmb(); /* LMM(desc_push_tail:C) */
/*
* Re-check the tail ID. The descriptor following @tail_id is
* not in an allowed tail state. But if the tail has since
* been moved by another CPU, then it does not matter.
*/
if (atomic_long_read(&desc_ring->tail_id) == tail_id) /* LMM(desc_push_tail:D) */
return false;
}
return true;
}
/* Reserve a new descriptor, invalidating the oldest if necessary. */
static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
unsigned long prev_state_val;
unsigned long id_prev_wrap;
struct prb_desc *desc;
unsigned long head_id;
unsigned long id;
head_id = atomic_long_read(&desc_ring->head_id); /* LMM(desc_reserve:A) */
do {
id = DESC_ID(head_id + 1);
id_prev_wrap = DESC_ID_PREV_WRAP(desc_ring, id);
/*
* Guarantee the head ID is read before reading the tail ID.
* Since the tail ID is updated before the head ID, this
* guarantees that @id_prev_wrap is never ahead of the tail
* ID. This pairs with desc_reserve:D.
*
* Memory barrier involvement:
*
* If desc_reserve:A reads from desc_reserve:D, then
* desc_reserve:C reads from desc_push_tail:B.
*
* Relies on:
*
* MB from desc_push_tail:B to desc_reserve:D
* matching
* RMB from desc_reserve:A to desc_reserve:C
*
* Note: desc_push_tail:B and desc_reserve:D can be different
* CPUs. However, the desc_reserve:D CPU (which performs
* the full memory barrier) must have previously seen
* desc_push_tail:B.
*/
smp_rmb(); /* LMM(desc_reserve:B) */
if (id_prev_wrap == atomic_long_read(&desc_ring->tail_id
)) { /* LMM(desc_reserve:C) */
/*
* Make space for the new descriptor by
* advancing the tail.
*/
if (!desc_push_tail(rb, id_prev_wrap))
return false;
}
/*
* 1. Guarantee the tail ID is read before validating the
* recycled descriptor state. A read memory barrier is
* sufficient for this. This pairs with desc_push_tail:B.
*
* Memory barrier involvement:
*
* If desc_reserve:C reads from desc_push_tail:B, then
* desc_reserve:E reads from desc_make_reusable:A.
*
* Relies on:
*
* MB from desc_make_reusable:A to desc_push_tail:B
* matching
* RMB from desc_reserve:C to desc_reserve:E
*
* Note: desc_make_reusable:A and desc_push_tail:B can be
* different CPUs. However, the desc_push_tail:B CPU
* (which performs the full memory barrier) must have
* previously seen desc_make_reusable:A.
*
* 2. Guarantee the tail ID is stored before storing the head
* ID. This pairs with desc_reserve:B.
*
* 3. Guarantee any data ring tail changes are stored before
* recycling the descriptor. Data ring tail changes can
* happen via desc_push_tail()->data_push_tail(). A full
* memory barrier is needed since another CPU may have
* pushed the data ring tails. This pairs with
* data_push_tail:B.
*
* 4. Guarantee a new tail ID is stored before recycling the
* descriptor. A full memory barrier is needed since
* another CPU may have pushed the tail ID. This pairs
* with desc_push_tail:C and this also pairs with
* prb_first_seq:C.
*
* 5. Guarantee the head ID is stored before trying to
* finalize the previous descriptor. This pairs with
* _prb_commit:B.
*/
} while (!atomic_long_try_cmpxchg(&desc_ring->head_id, &head_id,
id)); /* LMM(desc_reserve:D) */
desc = to_desc(desc_ring, id);
/*
* If the descriptor has been recycled, verify the old state val.
* See "ABA Issues" about why this verification is performed.
*/
prev_state_val = atomic_long_read(&desc->state_var); /* LMM(desc_reserve:E) */
if (prev_state_val &&
get_desc_state(id_prev_wrap, prev_state_val) != desc_reusable) {
WARN_ON_ONCE(1);
return false;
}
/*
* Assign the descriptor a new ID and set its state to reserved.
* See "ABA Issues" about why cmpxchg() instead of set() is used.
*
* Guarantee the new descriptor ID and state is stored before making
* any other changes. A write memory barrier is sufficient for this.
* This pairs with desc_read:D.
*/
if (!atomic_long_try_cmpxchg(&desc->state_var, &prev_state_val,
DESC_SV(id, desc_reserved))) { /* LMM(desc_reserve:F) */
WARN_ON_ONCE(1);
return false;
}
/* Now data in @desc can be modified: LMM(desc_reserve:G) */
*id_out = id;
return true;
}
/* Determine the end of a data block. */
static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
unsigned long lpos, unsigned int size)
{
unsigned long begin_lpos;
unsigned long next_lpos;
begin_lpos = lpos;
next_lpos = lpos + size;
/* First check if the data block does not wrap. */
if (DATA_WRAPS(data_ring, begin_lpos) == DATA_WRAPS(data_ring, next_lpos))
return next_lpos;
/* Wrapping data blocks store their data at the beginning. */
return (DATA_THIS_WRAP_START_LPOS(data_ring, next_lpos) + size);
}
/*
* Allocate a new data block, invalidating the oldest data block(s)
* if necessary. This function also associates the data block with
* a specified descriptor.
*/
static char *data_alloc(struct printk_ringbuffer *rb, unsigned int size,
struct prb_data_blk_lpos *blk_lpos, unsigned long id)
{
struct prb_data_ring *data_ring = &rb->text_data_ring;
struct prb_data_block *blk;
unsigned long begin_lpos;
unsigned long next_lpos;
if (size == 0) {
/*
* Data blocks are not created for empty lines. Instead, the
* reader will recognize these special lpos values and handle
* it appropriately.
*/
blk_lpos->begin = EMPTY_LINE_LPOS;
blk_lpos->next = EMPTY_LINE_LPOS;
return NULL;
}
size = to_blk_size(size);
begin_lpos = atomic_long_read(&data_ring->head_lpos);
do {
next_lpos = get_next_lpos(data_ring, begin_lpos, size);
if (!data_push_tail(rb, next_lpos - DATA_SIZE(data_ring))) {
/* Failed to allocate, specify a data-less block. */
blk_lpos->begin = FAILED_LPOS;
blk_lpos->next = FAILED_LPOS;
return NULL;
}
/*
* 1. Guarantee any descriptor states that have transitioned
* to reusable are stored before modifying the newly
* allocated data area. A full memory barrier is needed
* since other CPUs may have made the descriptor states
* reusable. See data_push_tail:A about why the reusable
* states are visible. This pairs with desc_read:D.
*
* 2. Guarantee any updated tail lpos is stored before
* modifying the newly allocated data area. Another CPU may
* be in data_make_reusable() and is reading a block ID
* from this area. data_make_reusable() can handle reading
* a garbage block ID value, but then it must be able to
* load a new tail lpos. A full memory barrier is needed
* since other CPUs may have updated the tail lpos. This
* pairs with data_push_tail:B.
*/
} while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos,
next_lpos)); /* LMM(data_alloc:A) */
blk = to_block(data_ring, begin_lpos);
blk->id = id; /* LMM(data_alloc:B) */
if (DATA_WRAPS(data_ring, begin_lpos) != DATA_WRAPS(data_ring, next_lpos)) {
/* Wrapping data blocks store their data at the beginning. */
blk = to_block(data_ring, 0);
/*
* Store the ID on the wrapped block for consistency.
* The printk_ringbuffer does not actually use it.
*/
blk->id = id;
}
blk_lpos->begin = begin_lpos;
blk_lpos->next = next_lpos;
return &blk->data[0];
}
/*
* Try to resize an existing data block associated with the descriptor
* specified by @id. If the resized data block should become wrapped, it
* copies the old data to the new data block. If @size yields a data block
* with the same or less size, the data block is left as is.
*
* Fail if this is not the last allocated data block or if there is not
* enough space or it is not possible make enough space.
*
* Return a pointer to the beginning of the entire data buffer or NULL on
* failure.
*/
static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
struct prb_data_blk_lpos *blk_lpos, unsigned long id)
{
struct prb_data_ring *data_ring = &rb->text_data_ring;
struct prb_data_block *blk;
unsigned long head_lpos;
unsigned long next_lpos;
bool wrapped;
/* Reallocation only works if @blk_lpos is the newest data block. */
head_lpos = atomic_long_read(&data_ring->head_lpos);
if (head_lpos != blk_lpos->next)
return NULL;
/* Keep track if @blk_lpos was a wrapping data block. */
wrapped = (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, blk_lpos->next));
size = to_blk_size(size);
next_lpos = get_next_lpos(data_ring, blk_lpos->begin, size);
/* If the data block does not increase, there is nothing to do. */
if (head_lpos - next_lpos < DATA_SIZE(data_ring)) {
if (wrapped)
blk = to_block(data_ring, 0);
else
blk = to_block(data_ring, blk_lpos->begin);
return &blk->data[0];
}
if (!data_push_tail(rb, next_lpos - DATA_SIZE(data_ring)))
return NULL;
/* The memory barrier involvement is the same as data_alloc:A. */
if (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &head_lpos,
next_lpos)) { /* LMM(data_realloc:A) */
return NULL;
}
blk = to_block(data_ring, blk_lpos->begin);
if (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, next_lpos)) {
struct prb_data_block *old_blk = blk;
/* Wrapping data blocks store their data at the beginning. */
blk = to_block(data_ring, 0);
/*
* Store the ID on the wrapped block for consistency.
* The printk_ringbuffer does not actually use it.
*/
blk->id = id;
if (!wrapped) {
/*
* Since the allocated space is now in the newly
* created wrapping data block, copy the content
* from the old data block.
*/
memcpy(&blk->data[0], &old_blk->data[0],
(blk_lpos->next - blk_lpos->begin) - sizeof(blk->id));
}
}
blk_lpos->next = next_lpos;
return &blk->data[0];
}
/* Return the number of bytes used by a data block. */
static unsigned int space_used(struct prb_data_ring *data_ring,
struct prb_data_blk_lpos *blk_lpos)
{
/* Data-less blocks take no space. */
if (BLK_DATALESS(blk_lpos))
return 0;
if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next)) {
/* Data block does not wrap. */
return (DATA_INDEX(data_ring, blk_lpos->next) -
DATA_INDEX(data_ring, blk_lpos->begin));
}
/*
* For wrapping data blocks, the trailing (wasted) space is
* also counted.
*/
return (DATA_INDEX(data_ring, blk_lpos->next) +
DATA_SIZE(data_ring) - DATA_INDEX(data_ring, blk_lpos->begin));
}
/*
* Given @blk_lpos, return a pointer to the writer data from the data block
* and calculate the size of the data part. A NULL pointer is returned if
* @blk_lpos specifies values that could never be legal.
*
* This function (used by readers) performs strict validation on the lpos
* values to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
* triggered if an internal error is detected.
*/
static const char *get_data(struct prb_data_ring *data_ring,
struct prb_data_blk_lpos *blk_lpos,
unsigned int *data_size)
{
struct prb_data_block *db;
/* Data-less data block description. */
if (BLK_DATALESS(blk_lpos)) {
/*
* Records that are just empty lines are also valid, even
* though they do not have a data block. For such records
* explicitly return empty string data to signify success.
*/
if (blk_lpos->begin == EMPTY_LINE_LPOS &&
blk_lpos->next == EMPTY_LINE_LPOS) {
*data_size = 0;
return "";
}
/* Data lost, invalid, or otherwise unavailable. */
return NULL;
}
/* Regular data block: @begin less than @next and in same wrap. */
if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next) &&
blk_lpos->begin < blk_lpos->next) {
db = to_block(data_ring, blk_lpos->begin);
*data_size = blk_lpos->next - blk_lpos->begin;
/* Wrapping data block: @begin is one wrap behind @next. */
} else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) ==
DATA_WRAPS(data_ring, blk_lpos->next)) {
db = to_block(data_ring, 0);
*data_size = DATA_INDEX(data_ring, blk_lpos->next);
/* Illegal block description. */
} else {
WARN_ON_ONCE(1);
return NULL;
}
/* A valid data block will always be aligned to the ID size. */
if (WARN_ON_ONCE(blk_lpos->begin != ALIGN(blk_lpos->begin, sizeof(db->id))) ||
WARN_ON_ONCE(blk_lpos->next != ALIGN(blk_lpos->next, sizeof(db->id)))) {
return NULL;
}
/* A valid data block will always have at least an ID. */
if (WARN_ON_ONCE(*data_size < sizeof(db->id)))
return NULL;
/* Subtract block ID space from size to reflect data size. */
*data_size -= sizeof(db->id);
return &db->data[0];
}
/*
* Attempt to transition the newest descriptor from committed back to reserved
* so that the record can be modified by a writer again. This is only possible
* if the descriptor is not yet finalized and the provided @caller_id matches.
*/
static struct prb_desc *desc_reopen_last(struct prb_desc_ring *desc_ring,
u32 caller_id, unsigned long *id_out)
{
unsigned long prev_state_val;
enum desc_state d_state;
struct prb_desc desc;
struct prb_desc *d;
unsigned long id;
u32 cid;
id = atomic_long_read(&desc_ring->head_id);
/*
* To reduce unnecessarily reopening, first check if the descriptor
* state and caller ID are correct.
*/
d_state = desc_read(desc_ring, id, &desc, NULL, &cid);
if (d_state != desc_committed || cid != caller_id)
return NULL;
d = to_desc(desc_ring, id);
prev_state_val = DESC_SV(id, desc_committed);
/*
* Guarantee the reserved state is stored before reading any
* record data. A full memory barrier is needed because @state_var
* modification is followed by reading. This pairs with _prb_commit:B.
*
* Memory barrier involvement:
*
* If desc_reopen_last:A reads from _prb_commit:B, then
* prb_reserve_in_last:A reads from _prb_commit:A.
*
* Relies on:
*
* WMB from _prb_commit:A to _prb_commit:B
* matching
* MB If desc_reopen_last:A to prb_reserve_in_last:A
*/
if (!atomic_long_try_cmpxchg(&d->state_var, &prev_state_val,
DESC_SV(id, desc_reserved))) { /* LMM(desc_reopen_last:A) */
return NULL;
}
*id_out = id;
return d;
}
/**
* prb_reserve_in_last() - Re-reserve and extend the space in the ringbuffer
* used by the newest record.
*
* @e: The entry structure to setup.
* @rb: The ringbuffer to re-reserve and extend data in.
* @r: The record structure to allocate buffers for.
* @caller_id: The caller ID of the caller (reserving writer).
* @max_size: Fail if the extended size would be greater than this.
*
* This is the public function available to writers to re-reserve and extend
* data.
*
* The writer specifies the text size to extend (not the new total size) by
* setting the @text_buf_size field of @r. To ensure proper initialization
* of @r, prb_rec_init_wr() should be used.
*
* This function will fail if @caller_id does not match the caller ID of the
* newest record. In that case the caller must reserve new data using
* prb_reserve().
*
* Context: Any context. Disables local interrupts on success.
* Return: true if text data could be extended, otherwise false.
*
* On success:
*
* - @r->text_buf points to the beginning of the entire text buffer.
*
* - @r->text_buf_size is set to the new total size of the buffer.
*
* - @r->info is not touched so that @r->info->text_len could be used
* to append the text.
*
* - prb_record_text_space() can be used on @e to query the new
* actually used space.
*
* Important: All @r->info fields will already be set with the current values
* for the record. I.e. @r->info->text_len will be less than
* @text_buf_size. Writers can use @r->info->text_len to know
* where concatenation begins and writers should update
* @r->info->text_len after concatenating.
*/
bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
struct printk_record *r, u32 caller_id, unsigned int max_size)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
struct printk_info *info;
unsigned int data_size;
struct prb_desc *d;
unsigned long id;
local_irq_save(e->irqflags);
/* Transition the newest descriptor back to the reserved state. */
d = desc_reopen_last(desc_ring, caller_id, &id);
if (!d) {
local_irq_restore(e->irqflags);
goto fail_reopen;
}
/* Now the writer has exclusive access: LMM(prb_reserve_in_last:A) */
info = to_info(desc_ring, id);
/*
* Set the @e fields here so that prb_commit() can be used if
* anything fails from now on.
*/
e->rb = rb;
e->id = id;
/*
* desc_reopen_last() checked the caller_id, but there was no
* exclusive access at that point. The descriptor may have
* changed since then.
*/
if (caller_id != info->caller_id)
goto fail;
if (BLK_DATALESS(&d->text_blk_lpos)) {
if (WARN_ON_ONCE(info->text_len != 0)) {
pr_warn_once("wrong text_len value (%hu, expecting 0)\n",
info->text_len);
info->text_len = 0;
}
if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
goto fail;
if (r->text_buf_size > max_size)
goto fail;
r->text_buf = data_alloc(rb, r->text_buf_size,
&d->text_blk_lpos, id);
} else {
if (!get_data(&rb->text_data_ring, &d->text_blk_lpos, &data_size))
goto fail;
/*
* Increase the buffer size to include the original size. If
* the meta data (@text_len) is not sane, use the full data
* block size.
*/
if (WARN_ON_ONCE(info->text_len > data_size)) {
pr_warn_once("wrong text_len value (%hu, expecting <=%u)\n",
info->text_len, data_size);
info->text_len = data_size;
}
r->text_buf_size += info->text_len;
if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
goto fail;
if (r->text_buf_size > max_size)
goto fail;
r->text_buf = data_realloc(rb, r->text_buf_size,
&d->text_blk_lpos, id);
}
if (r->text_buf_size && !r->text_buf)
goto fail;
r->info = info;
e->text_space = space_used(&rb->text_data_ring, &d->text_blk_lpos);
return true;
fail:
prb_commit(e);
/* prb_commit() re-enabled interrupts. */
fail_reopen:
/* Make it clear to the caller that the re-reserve failed. */
memset(r, 0, sizeof(*r));
return false;
}
/*
* @last_finalized_seq value guarantees that all records up to and including
* this sequence number are finalized and can be read. The only exception are
* too old records which have already been overwritten.
*
* It is also guaranteed that @last_finalized_seq only increases.
*
* Be aware that finalized records following non-finalized records are not
* reported because they are not yet available to the reader. For example,
* a new record stored via printk() will not be available to a printer if
* it follows a record that has not been finalized yet. However, once that
* non-finalized record becomes finalized, @last_finalized_seq will be
* appropriately updated and the full set of finalized records will be
* available to the printer. And since each printk() caller will either
* directly print or trigger deferred printing of all available unprinted
* records, all printk() messages will get printed.
*/
static u64 desc_last_finalized_seq(struct printk_ringbuffer *rb)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
unsigned long ulseq;
/*
* Guarantee the sequence number is loaded before loading the
* associated record in order to guarantee that the record can be
* seen by this CPU. This pairs with desc_update_last_finalized:A.
*/
ulseq = atomic_long_read_acquire(&desc_ring->last_finalized_seq
); /* LMM(desc_last_finalized_seq:A) */
return __ulseq_to_u64seq(rb, ulseq);
}
static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
struct printk_record *r, unsigned int *line_count);
/*
* Check if there are records directly following @last_finalized_seq that are
* finalized. If so, update @last_finalized_seq to the latest of these
* records. It is not allowed to skip over records that are not yet finalized.
*/
static void desc_update_last_finalized(struct printk_ringbuffer *rb)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
u64 old_seq = desc_last_finalized_seq(rb);
unsigned long oldval;
unsigned long newval;
u64 finalized_seq;
u64 try_seq;
try_again:
finalized_seq = old_seq;
try_seq = finalized_seq + 1;
/* Try to find later finalized records. */
while (_prb_read_valid(rb, &try_seq, NULL, NULL)) {
finalized_seq = try_seq;
try_seq++;
}
/* No update needed if no later finalized record was found. */
if (finalized_seq == old_seq)
return;
oldval = __u64seq_to_ulseq(old_seq);
newval = __u64seq_to_ulseq(finalized_seq);
/*
* Set the sequence number of a later finalized record that has been
* seen.
*
* Guarantee the record data is visible to other CPUs before storing
* its sequence number. This pairs with desc_last_finalized_seq:A.
*
* Memory barrier involvement:
*
* If desc_last_finalized_seq:A reads from
* desc_update_last_finalized:A, then desc_read:A reads from
* _prb_commit:B.
*
* Relies on:
*
* RELEASE from _prb_commit:B to desc_update_last_finalized:A
* matching
* ACQUIRE from desc_last_finalized_seq:A to desc_read:A
*
* Note: _prb_commit:B and desc_update_last_finalized:A can be
* different CPUs. However, the desc_update_last_finalized:A
* CPU (which performs the release) must have previously seen
* _prb_commit:B.
*/
if (!atomic_long_try_cmpxchg_release(&desc_ring->last_finalized_seq,
&oldval, newval)) { /* LMM(desc_update_last_finalized:A) */
old_seq = __ulseq_to_u64seq(rb, oldval);
goto try_again;
}
}
/*
* Attempt to finalize a specified descriptor. If this fails, the descriptor
* is either already final or it will finalize itself when the writer commits.
*/
static void desc_make_final(struct printk_ringbuffer *rb, unsigned long id)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
unsigned long prev_state_val = DESC_SV(id, desc_committed);
struct prb_desc *d = to_desc(desc_ring, id);
if (atomic_long_try_cmpxchg_relaxed(&d->state_var, &prev_state_val,
DESC_SV(id, desc_finalized))) { /* LMM(desc_make_final:A) */
desc_update_last_finalized(rb);
}
}
/**
* prb_reserve() - Reserve space in the ringbuffer.
*
* @e: The entry structure to setup.
* @rb: The ringbuffer to reserve data in.
* @r: The record structure to allocate buffers for.
*
* This is the public function available to writers to reserve data.
*
* The writer specifies the text size to reserve by setting the
* @text_buf_size field of @r. To ensure proper initialization of @r,
* prb_rec_init_wr() should be used.
*
* Context: Any context. Disables local interrupts on success.
* Return: true if at least text data could be allocated, otherwise false.
*
* On success, the fields @info and @text_buf of @r will be set by this
* function and should be filled in by the writer before committing. Also
* on success, prb_record_text_space() can be used on @e to query the actual
* space used for the text data block.
*
* Important: @info->text_len needs to be set correctly by the writer in
* order for data to be readable and/or extended. Its value
* is initialized to 0.
*/
bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
struct printk_record *r)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
struct printk_info *info;
struct prb_desc *d;
unsigned long id;
u64 seq;
if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
goto fail;
/*
* Descriptors in the reserved state act as blockers to all further
* reservations once the desc_ring has fully wrapped. Disable
* interrupts during the reserve/commit window in order to minimize
* the likelihood of this happening.
*/
local_irq_save(e->irqflags);
if (!desc_reserve(rb, &id)) {
/* Descriptor reservation failures are tracked. */
atomic_long_inc(&rb->fail);
local_irq_restore(e->irqflags);
goto fail;
}
d = to_desc(desc_ring, id);
info = to_info(desc_ring, id);
/*
* All @info fields (except @seq) are cleared and must be filled in
* by the writer. Save @seq before clearing because it is used to
* determine the new sequence number.
*/
seq = info->seq;
memset(info, 0, sizeof(*info));
/*
* Set the @e fields here so that prb_commit() can be used if
* text data allocation fails.
*/
e->rb = rb;
e->id = id;
/*
* Initialize the sequence number if it has "never been set".
* Otherwise just increment it by a full wrap.
*
* @seq is considered "never been set" if it has a value of 0,
* _except_ for @infos[0], which was specially setup by the ringbuffer
* initializer and therefore is always considered as set.
*
* See the "Bootstrap" comment block in printk_ringbuffer.h for
* details about how the initializer bootstraps the descriptors.
*/
if (seq == 0 && DESC_INDEX(desc_ring, id) != 0)
info->seq = DESC_INDEX(desc_ring, id);
else
info->seq = seq + DESCS_COUNT(desc_ring);
/*
* New data is about to be reserved. Once that happens, previous
* descriptors are no longer able to be extended. Finalize the
* previous descriptor now so that it can be made available to
* readers. (For seq==0 there is no previous descriptor.)
*/
if (info->seq > 0)
desc_make_final(rb, DESC_ID(id - 1));
r->text_buf = data_alloc(rb, r->text_buf_size, &d->text_blk_lpos, id);
/* If text data allocation fails, a data-less record is committed. */
if (r->text_buf_size && !r->text_buf) {
prb_commit(e);
/* prb_commit() re-enabled interrupts. */
goto fail;
}
r->info = info;
/* Record full text space used by record. */
e->text_space = space_used(&rb->text_data_ring, &d->text_blk_lpos);
return true;
fail:
/* Make it clear to the caller that the reserve failed. */
memset(r, 0, sizeof(*r));
return false;
}
/* Commit the data (possibly finalizing it) and restore interrupts. */
static void _prb_commit(struct prb_reserved_entry *e, unsigned long state_val)
{
struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
struct prb_desc *d = to_desc(desc_ring, e->id);
unsigned long prev_state_val = DESC_SV(e->id, desc_reserved);
/* Now the writer has finished all writing: LMM(_prb_commit:A) */
/*
* Set the descriptor as committed. See "ABA Issues" about why
* cmpxchg() instead of set() is used.
*
* 1 Guarantee all record data is stored before the descriptor state
* is stored as committed. A write memory barrier is sufficient
* for this. This pairs with desc_read:B and desc_reopen_last:A.
*
* 2. Guarantee the descriptor state is stored as committed before
* re-checking the head ID in order to possibly finalize this
* descriptor. This pairs with desc_reserve:D.
*
* Memory barrier involvement:
*
* If prb_commit:A reads from desc_reserve:D, then
* desc_make_final:A reads from _prb_commit:B.
*
* Relies on:
*
* MB _prb_commit:B to prb_commit:A
* matching
* MB desc_reserve:D to desc_make_final:A
*/
if (!atomic_long_try_cmpxchg(&d->state_var, &prev_state_val,
DESC_SV(e->id, state_val))) { /* LMM(_prb_commit:B) */
WARN_ON_ONCE(1);
}
/* Restore interrupts, the reserve/commit window is finished. */
local_irq_restore(e->irqflags);
}
/**
* prb_commit() - Commit (previously reserved) data to the ringbuffer.
*
* @e: The entry containing the reserved data information.
*
* This is the public function available to writers to commit data.
*
* Note that the data is not yet available to readers until it is finalized.
* Finalizing happens automatically when space for the next record is
* reserved.
*
* See prb_final_commit() for a version of this function that finalizes
* immediately.
*
* Context: Any context. Enables local interrupts.
*/
void prb_commit(struct prb_reserved_entry *e)
{
struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
unsigned long head_id;
_prb_commit(e, desc_committed);
/*
* If this descriptor is no longer the head (i.e. a new record has
* been allocated), extending the data for this record is no longer
* allowed and therefore it must be finalized.
*/
head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_commit:A) */
if (head_id != e->id)
desc_make_final(e->rb, e->id);
}
/**
* prb_final_commit() - Commit and finalize (previously reserved) data to
* the ringbuffer.
*
* @e: The entry containing the reserved data information.
*
* This is the public function available to writers to commit+finalize data.
*
* By finalizing, the data is made immediately available to readers.
*
* This function should only be used if there are no intentions of extending
* this data using prb_reserve_in_last().
*
* Context: Any context. Enables local interrupts.
*/
void prb_final_commit(struct prb_reserved_entry *e)
{
_prb_commit(e, desc_finalized);
desc_update_last_finalized(e->rb);
}
/*
* Count the number of lines in provided text. All text has at least 1 line
* (even if @text_size is 0). Each '\n' processed is counted as an additional
* line.
*/
static unsigned int count_lines(const char *text, unsigned int text_size)
{
unsigned int next_size = text_size;
unsigned int line_count = 1;
const char *next = text;
while (next_size) {
next = memchr(next, '\n', next_size);
if (!next)
break;
line_count++;
next++;
next_size = text_size - (next - text);
}
return line_count;
}
/*
* Given @blk_lpos, copy an expected @len of data into the provided buffer.
* If @line_count is provided, count the number of lines in the data.
*
* This function (used by readers) performs strict validation on the data
* size to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
* triggered if an internal error is detected.
*/
static bool copy_data(struct prb_data_ring *data_ring,
struct prb_data_blk_lpos *blk_lpos, u16 len, char *buf,
unsigned int buf_size, unsigned int *line_count)
{
unsigned int data_size;
const char *data;
/* Caller might not want any data. */
if ((!buf || !buf_size) && !line_count)
return true;
data = get_data(data_ring, blk_lpos, &data_size);
if (!data)
return false;
/*
* Actual cannot be less than expected. It can be more than expected
* because of the trailing alignment padding.
*
* Note that invalid @len values can occur because the caller loads
* the value during an allowed data race.
*/
if (data_size < (unsigned int)len)
return false;
/* Caller interested in the line count? */
if (line_count)
*line_count = count_lines(data, len);
/* Caller interested in the data content? */
if (!buf || !buf_size)
return true;
data_size = min_t(unsigned int, buf_size, len);
memcpy(&buf[0], data, data_size); /* LMM(copy_data:A) */
return true;
}
/*
* This is an extended version of desc_read(). It gets a copy of a specified
* descriptor. However, it also verifies that the record is finalized and has
* the sequence number @seq. On success, 0 is returned.
*
* Error return values:
* -EINVAL: A finalized record with sequence number @seq does not exist.
* -ENOENT: A finalized record with sequence number @seq exists, but its data
* is not available. This is a valid record, so readers should
* continue with the next record.
*/
static int desc_read_finalized_seq(struct prb_desc_ring *desc_ring,
unsigned long id, u64 seq,
struct prb_desc *desc_out)
{
struct prb_data_blk_lpos *blk_lpos = &desc_out->text_blk_lpos;
enum desc_state d_state;
u64 s;
d_state = desc_read(desc_ring, id, desc_out, &s, NULL);
/*
* An unexpected @id (desc_miss) or @seq mismatch means the record
* does not exist. A descriptor in the reserved or committed state
* means the record does not yet exist for the reader.
*/
if (d_state == desc_miss ||
d_state == desc_reserved ||
d_state == desc_committed ||
s != seq) {
return -EINVAL;
}
/*
* A descriptor in the reusable state may no longer have its data
* available; report it as existing but with lost data. Or the record
* may actually be a record with lost data.
*/
if (d_state == desc_reusable ||
(blk_lpos->begin == FAILED_LPOS && blk_lpos->next == FAILED_LPOS)) {
return -ENOENT;
}
return 0;
}
/*
* Copy the ringbuffer data from the record with @seq to the provided
* @r buffer. On success, 0 is returned.
*
* See desc_read_finalized_seq() for error return values.
*/
static int prb_read(struct printk_ringbuffer *rb, u64 seq,
struct printk_record *r, unsigned int *line_count)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
struct printk_info *info = to_info(desc_ring, seq);
struct prb_desc *rdesc = to_desc(desc_ring, seq);
atomic_long_t *state_var = &rdesc->state_var;
struct prb_desc desc;
unsigned long id;
int err;
/* Extract the ID, used to specify the descriptor to read. */
id = DESC_ID(atomic_long_read(state_var));
/* Get a local copy of the correct descriptor (if available). */
err = desc_read_finalized_seq(desc_ring, id, seq, &desc);
/*
* If @r is NULL, the caller is only interested in the availability
* of the record.
*/
if (err || !r)
return err;
/* If requested, copy meta data. */
if (r->info)
memcpy(r->info, info, sizeof(*(r->info)));
/* Copy text data. If it fails, this is a data-less record. */
if (!copy_data(&rb->text_data_ring, &desc.text_blk_lpos, info->text_len,
r->text_buf, r->text_buf_size, line_count)) {
return -ENOENT;
}
/* Ensure the record is still finalized and has the same @seq. */
return desc_read_finalized_seq(desc_ring, id, seq, &desc);
}
/* Get the sequence number of the tail descriptor. */
u64 prb_first_seq(struct printk_ringbuffer *rb)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
enum desc_state d_state;
struct prb_desc desc;
unsigned long id;
u64 seq;
for (;;) {
id = atomic_long_read(&rb->desc_ring.tail_id); /* LMM(prb_first_seq:A) */
d_state = desc_read(desc_ring, id, &desc, &seq, NULL); /* LMM(prb_first_seq:B) */
/*
* This loop will not be infinite because the tail is
* _always_ in the finalized or reusable state.
*/
if (d_state == desc_finalized || d_state == desc_reusable)
break;
/*
* Guarantee the last state load from desc_read() is before
* reloading @tail_id in order to see a new tail in the case
* that the descriptor has been recycled. This pairs with
* desc_reserve:D.
*
* Memory barrier involvement:
*
* If prb_first_seq:B reads from desc_reserve:F, then
* prb_first_seq:A reads from desc_push_tail:B.
*
* Relies on:
*
* MB from desc_push_tail:B to desc_reserve:F
* matching
* RMB prb_first_seq:B to prb_first_seq:A
*/
smp_rmb(); /* LMM(prb_first_seq:C) */
}
return seq;
}
/**
* prb_next_reserve_seq() - Get the sequence number after the most recently
* reserved record.
*
* @rb: The ringbuffer to get the sequence number from.
*
* This is the public function available to readers to see what sequence
* number will be assigned to the next reserved record.
*
* Note that depending on the situation, this value can be equal to or
* higher than the sequence number returned by prb_next_seq().
*
* Context: Any context.
* Return: The sequence number that will be assigned to the next record
* reserved.
*/
u64 prb_next_reserve_seq(struct printk_ringbuffer *rb)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
unsigned long last_finalized_id;
atomic_long_t *state_var;
u64 last_finalized_seq;
unsigned long head_id;
struct prb_desc desc;
unsigned long diff;
struct prb_desc *d;
int err;
/*
* It may not be possible to read a sequence number for @head_id.
* So the ID of @last_finailzed_seq is used to calculate what the
* sequence number of @head_id will be.
*/
try_again:
last_finalized_seq = desc_last_finalized_seq(rb);
/*
* @head_id is loaded after @last_finalized_seq to ensure that
* it points to the record with @last_finalized_seq or newer.
*
* Memory barrier involvement:
*
* If desc_last_finalized_seq:A reads from
* desc_update_last_finalized:A, then
* prb_next_reserve_seq:A reads from desc_reserve:D.
*
* Relies on:
*
* RELEASE from desc_reserve:D to desc_update_last_finalized:A
* matching
* ACQUIRE from desc_last_finalized_seq:A to prb_next_reserve_seq:A
*
* Note: desc_reserve:D and desc_update_last_finalized:A can be
* different CPUs. However, the desc_update_last_finalized:A CPU
* (which performs the release) must have previously seen
* desc_read:C, which implies desc_reserve:D can be seen.
*/
head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_next_reserve_seq:A) */
d = to_desc(desc_ring, last_finalized_seq);
state_var = &d->state_var;
/* Extract the ID, used to specify the descriptor to read. */
last_finalized_id = DESC_ID(atomic_long_read(state_var));
/* Ensure @last_finalized_id is correct. */
err = desc_read_finalized_seq(desc_ring, last_finalized_id, last_finalized_seq, &desc);
if (err == -EINVAL) {
if (last_finalized_seq == 0) {
/*
* No record has been finalized or even reserved yet.
*
* The @head_id is initialized such that the first
* increment will yield the first record (seq=0).
* Handle it separately to avoid a negative @diff
* below.
*/
if (head_id == DESC0_ID(desc_ring->count_bits))
return 0;
/*
* One or more descriptors are already reserved. Use
* the descriptor ID of the first one (@seq=0) for
* the @diff below.
*/
last_finalized_id = DESC0_ID(desc_ring->count_bits) + 1;
} else {
/* Record must have been overwritten. Try again. */
goto try_again;
}
}
/* Diff of known descriptor IDs to compute related sequence numbers. */
diff = head_id - last_finalized_id;
/*
* @head_id points to the most recently reserved record, but this
* function returns the sequence number that will be assigned to the
* next (not yet reserved) record. Thus +1 is needed.
*/
return (last_finalized_seq + diff + 1);
}
/*
* Non-blocking read of a record.
*
* On success @seq is updated to the record that was read and (if provided)
* @r and @line_count will contain the read/calculated data.
*
* On failure @seq is updated to a record that is not yet available to the
* reader, but it will be the next record available to the reader.
*
* Note: When the current CPU is in panic, this function will skip over any
* non-existent/non-finalized records in order to allow the panic CPU
* to print any and all records that have been finalized.
*/
static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
struct printk_record *r, unsigned int *line_count)
{
u64 tail_seq;
int err;
while ((err = prb_read(rb, *seq, r, line_count))) {
tail_seq = prb_first_seq(rb);
if (*seq < tail_seq) {
/*
* Behind the tail. Catch up and try again. This
* can happen for -ENOENT and -EINVAL cases.
*/
*seq = tail_seq;
} else if (err == -ENOENT) {
/* Record exists, but the data was lost. Skip. */
(*seq)++;
} else {
/*
* Non-existent/non-finalized record. Must stop.
*
* For panic situations it cannot be expected that
* non-finalized records will become finalized. But
* there may be other finalized records beyond that
* need to be printed for a panic situation. If this
* is the panic CPU, skip this
* non-existent/non-finalized record unless it is
* at or beyond the head, in which case it is not
* possible to continue.
*
* Note that new messages printed on panic CPU are
* finalized when we are here. The only exception
* might be the last message without trailing newline.
* But it would have the sequence number returned
* by "prb_next_reserve_seq() - 1".
*/
if (this_cpu_in_panic() && ((*seq + 1) < prb_next_reserve_seq(rb)))
(*seq)++;
else
return false;
}
}
return true;
}
/**
* prb_read_valid() - Non-blocking read of a requested record or (if gone)
* the next available record.
*
* @rb: The ringbuffer to read from.
* @seq: The sequence number of the record to read.
* @r: A record data buffer to store the read record to.
*
* This is the public function available to readers to read a record.
*
* The reader provides the @info and @text_buf buffers of @r to be
* filled in. Any of the buffer pointers can be set to NULL if the reader
* is not interested in that data. To ensure proper initialization of @r,
* prb_rec_init_rd() should be used.
*
* Context: Any context.
* Return: true if a record was read, otherwise false.
*
* On success, the reader must check r->info.seq to see which record was
* actually read. This allows the reader to detect dropped records.
*
* Failure means @seq refers to a record not yet available to the reader.
*/
bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
struct printk_record *r)
{
return _prb_read_valid(rb, &seq, r, NULL);
}
/**
* prb_read_valid_info() - Non-blocking read of meta data for a requested
* record or (if gone) the next available record.
*
* @rb: The ringbuffer to read from.
* @seq: The sequence number of the record to read.
* @info: A buffer to store the read record meta data to.
* @line_count: A buffer to store the number of lines in the record text.
*
* This is the public function available to readers to read only the
* meta data of a record.
*
* The reader provides the @info, @line_count buffers to be filled in.
* Either of the buffer pointers can be set to NULL if the reader is not
* interested in that data.
*
* Context: Any context.
* Return: true if a record's meta data was read, otherwise false.
*
* On success, the reader must check info->seq to see which record meta data
* was actually read. This allows the reader to detect dropped records.
*
* Failure means @seq refers to a record not yet available to the reader.
*/
bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
struct printk_info *info, unsigned int *line_count)
{
struct printk_record r;
prb_rec_init_rd(&r, info, NULL, 0);
return _prb_read_valid(rb, &seq, &r, line_count);
}
/**
* prb_first_valid_seq() - Get the sequence number of the oldest available
* record.
*
* @rb: The ringbuffer to get the sequence number from.
*
* This is the public function available to readers to see what the
* first/oldest valid sequence number is.
*
* This provides readers a starting point to begin iterating the ringbuffer.
*
* Context: Any context.
* Return: The sequence number of the first/oldest record or, if the
* ringbuffer is empty, 0 is returned.
*/
u64 prb_first_valid_seq(struct printk_ringbuffer *rb)
{
u64 seq = 0;
if (!_prb_read_valid(rb, &seq, NULL, NULL))
return 0;
return seq;
}
/**
* prb_next_seq() - Get the sequence number after the last available record.
*
* @rb: The ringbuffer to get the sequence number from.
*
* This is the public function available to readers to see what the next
* newest sequence number available to readers will be.
*
* This provides readers a sequence number to jump to if all currently
* available records should be skipped. It is guaranteed that all records
* previous to the returned value have been finalized and are (or were)
* available to the reader.
*
* Context: Any context.
* Return: The sequence number of the next newest (not yet available) record
* for readers.
*/
u64 prb_next_seq(struct printk_ringbuffer *rb)
{
u64 seq;
seq = desc_last_finalized_seq(rb);
/*
* Begin searching after the last finalized record.
*
* On 0, the search must begin at 0 because of hack#2
* of the bootstrapping phase it is not known if a
* record at index 0 exists.
*/
if (seq != 0)
seq++;
/*
* The information about the last finalized @seq might be inaccurate.
* Search forward to find the current one.
*/
while (_prb_read_valid(rb, &seq, NULL, NULL))
seq++;
return seq;
}
/**
* prb_init() - Initialize a ringbuffer to use provided external buffers.
*
* @rb: The ringbuffer to initialize.
* @text_buf: The data buffer for text data.
* @textbits: The size of @text_buf as a power-of-2 value.
* @descs: The descriptor buffer for ringbuffer records.
* @descbits: The count of @descs items as a power-of-2 value.
* @infos: The printk_info buffer for ringbuffer records.
*
* This is the public function available to writers to setup a ringbuffer
* during runtime using provided buffers.
*
* This must match the initialization of DEFINE_PRINTKRB().
*
* Context: Any context.
*/
void prb_init(struct printk_ringbuffer *rb,
char *text_buf, unsigned int textbits,
struct prb_desc *descs, unsigned int descbits,
struct printk_info *infos)
{
memset(descs, 0, _DESCS_COUNT(descbits) * sizeof(descs[0]));
memset(infos, 0, _DESCS_COUNT(descbits) * sizeof(infos[0]));
rb->desc_ring.count_bits = descbits;
rb->desc_ring.descs = descs;
rb->desc_ring.infos = infos;
atomic_long_set(&rb->desc_ring.head_id, DESC0_ID(descbits));
atomic_long_set(&rb->desc_ring.tail_id, DESC0_ID(descbits));
atomic_long_set(&rb->desc_ring.last_finalized_seq, 0);
rb->text_data_ring.size_bits = textbits;
rb->text_data_ring.data = text_buf;
atomic_long_set(&rb->text_data_ring.head_lpos, BLK0_LPOS(textbits));
atomic_long_set(&rb->text_data_ring.tail_lpos, BLK0_LPOS(textbits));
atomic_long_set(&rb->fail, 0);
atomic_long_set(&(descs[_DESCS_COUNT(descbits) - 1].state_var), DESC0_SV(descbits));
descs[_DESCS_COUNT(descbits) - 1].text_blk_lpos.begin = FAILED_LPOS;
descs[_DESCS_COUNT(descbits) - 1].text_blk_lpos.next = FAILED_LPOS;
infos[0].seq = -(u64)_DESCS_COUNT(descbits);
infos[_DESCS_COUNT(descbits) - 1].seq = 0;
}
/**
* prb_record_text_space() - Query the full actual used ringbuffer space for
* the text data of a reserved entry.
*
* @e: The successfully reserved entry to query.
*
* This is the public function available to writers to see how much actual
* space is used in the ringbuffer to store the text data of the specified
* entry.
*
* This function is only valid if @e has been successfully reserved using
* prb_reserve().
*
* Context: Any context.
* Return: The size in bytes used by the text data of the associated record.
*/
unsigned int prb_record_text_space(struct prb_reserved_entry *e)
{
return e->text_space;
}
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