linux/fs/btrfs/locking.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2008 Oracle.  All rights reserved.
 */

#include <linux/sched.h>
#include <linux/pagemap.h>
#include <linux/spinlock.h>
#include <linux/page-flags.h>
#include <asm/bug.h>
#include <trace/events/btrfs.h>
#include "misc.h"
#include "ctree.h"
#include "extent_io.h"
#include "locking.h"

/*
 * Lockdep class keys for extent_buffer->lock's in this root.  For a given
 * eb, the lockdep key is determined by the btrfs_root it belongs to and
 * the level the eb occupies in the tree.
 *
 * Different roots are used for different purposes and may nest inside each
 * other and they require separate keysets.  As lockdep keys should be
 * static, assign keysets according to the purpose of the root as indicated
 * by btrfs_root->root_key.objectid.  This ensures that all special purpose
 * roots have separate keysets.
 *
 * Lock-nesting across peer nodes is always done with the immediate parent
 * node locked thus preventing deadlock.  As lockdep doesn't know this, use
 * subclass to avoid triggering lockdep warning in such cases.
 *
 * The key is set by the readpage_end_io_hook after the buffer has passed
 * csum validation but before the pages are unlocked.  It is also set by
 * btrfs_init_new_buffer on freshly allocated blocks.
 *
 * We also add a check to make sure the highest level of the tree is the
 * same as our lockdep setup here.  If BTRFS_MAX_LEVEL changes, this code
 * needs update as well.
 */
#ifdef CONFIG_DEBUG_LOCK_ALLOC
#if BTRFS_MAX_LEVEL != 8
#error
#endif

#define DEFINE_LEVEL …

#define DEFINE_NAME …

static struct btrfs_lockdep_keyset { … } btrfs_lockdep_keysets[] = …;

#undef DEFINE_LEVEL
#undef DEFINE_NAME

void btrfs_set_buffer_lockdep_class(u64 objectid, struct extent_buffer *eb, int level)
{ … }

void btrfs_maybe_reset_lockdep_class(struct btrfs_root *root, struct extent_buffer *eb)
{ … }

#endif

#ifdef CONFIG_BTRFS_DEBUG
static void btrfs_set_eb_lock_owner(struct extent_buffer *eb, pid_t owner)
{ … }
#else
static void btrfs_set_eb_lock_owner(struct extent_buffer *eb, pid_t owner) { }
#endif

/*
 * Extent buffer locking
 * =====================
 *
 * We use a rw_semaphore for tree locking, and the semantics are exactly the
 * same:
 *
 * - reader/writer exclusion
 * - writer/writer exclusion
 * - reader/reader sharing
 * - try-lock semantics for readers and writers
 *
 * The rwsem implementation does opportunistic spinning which reduces number of
 * times the locking task needs to sleep.
 */

/*
 * btrfs_tree_read_lock_nested - lock extent buffer for read
 * @eb:		the eb to be locked
 * @nest:	the nesting level to be used for lockdep
 *
 * This takes the read lock on the extent buffer, using the specified nesting
 * level for lockdep purposes.
 */
void btrfs_tree_read_lock_nested(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
{ … }

/*
 * Try-lock for read.
 *
 * Return 1 if the rwlock has been taken, 0 otherwise
 */
int btrfs_try_tree_read_lock(struct extent_buffer *eb)
{ … }

/*
 * Try-lock for write.
 *
 * Return 1 if the rwlock has been taken, 0 otherwise
 */
int btrfs_try_tree_write_lock(struct extent_buffer *eb)
{ … }

/*
 * Release read lock.
 */
void btrfs_tree_read_unlock(struct extent_buffer *eb)
{ … }

/*
 * Lock eb for write.
 *
 * @eb:		the eb to lock
 * @nest:	the nesting to use for the lock
 *
 * Returns with the eb->lock write locked.
 */
void btrfs_tree_lock_nested(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
	__acquires(&eb->lock)
{ … }

/*
 * Release the write lock.
 */
void btrfs_tree_unlock(struct extent_buffer *eb)
{ … }

/*
 * This releases any locks held in the path starting at level and going all the
 * way up to the root.
 *
 * btrfs_search_slot will keep the lock held on higher nodes in a few corner
 * cases, such as COW of the block at slot zero in the node.  This ignores
 * those rules, and it should only be called when there are no more updates to
 * be done higher up in the tree.
 */
void btrfs_unlock_up_safe(struct btrfs_path *path, int level)
{ … }

/*
 * Loop around taking references on and locking the root node of the tree until
 * we end up with a lock on the root node.
 *
 * Return: root extent buffer with write lock held
 */
struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
{ … }

/*
 * Loop around taking references on and locking the root node of the tree until
 * we end up with a lock on the root node.
 *
 * Return: root extent buffer with read lock held
 */
struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
{ … }

/*
 * Loop around taking references on and locking the root node of the tree in
 * nowait mode until we end up with a lock on the root node or returning to
 * avoid blocking.
 *
 * Return: root extent buffer with read lock held or -EAGAIN.
 */
struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root)
{ … }

/*
 * DREW locks
 * ==========
 *
 * DREW stands for double-reader-writer-exclusion lock. It's used in situation
 * where you want to provide A-B exclusion but not AA or BB.
 *
 * Currently implementation gives more priority to reader. If a reader and a
 * writer both race to acquire their respective sides of the lock the writer
 * would yield its lock as soon as it detects a concurrent reader. Additionally
 * if there are pending readers no new writers would be allowed to come in and
 * acquire the lock.
 */

void btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
{ … }

/* Return true if acquisition is successful, false otherwise */
bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)
{ … }

void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)
{ … }

void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)
{ … }

void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)
{ … }

void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)
{ … }