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 "misc.h"
#include "ctree.h"
#include "extent_io.h"
#include "locking.h"
#include "accessors.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(stem, level)					\
	.names[level] = "btrfs-" stem "-0" #level,

#define DEFINE_NAME(stem)						\
	DEFINE_LEVEL(stem, 0)						\
	DEFINE_LEVEL(stem, 1)						\
	DEFINE_LEVEL(stem, 2)						\
	DEFINE_LEVEL(stem, 3)						\
	DEFINE_LEVEL(stem, 4)						\
	DEFINE_LEVEL(stem, 5)						\
	DEFINE_LEVEL(stem, 6)						\
	DEFINE_LEVEL(stem, 7)

static struct btrfs_lockdep_keyset {
	u64			id;		/* root objectid */
	/* Longest entry: btrfs-block-group-00 */
	char			names[BTRFS_MAX_LEVEL][24];
	struct lock_class_key	keys[BTRFS_MAX_LEVEL];
} btrfs_lockdep_keysets[] = {
	{ .id = BTRFS_ROOT_TREE_OBJECTID,	DEFINE_NAME("root")	},
	{ .id = BTRFS_EXTENT_TREE_OBJECTID,	DEFINE_NAME("extent")	},
	{ .id = BTRFS_CHUNK_TREE_OBJECTID,	DEFINE_NAME("chunk")	},
	{ .id = BTRFS_DEV_TREE_OBJECTID,	DEFINE_NAME("dev")	},
	{ .id = BTRFS_CSUM_TREE_OBJECTID,	DEFINE_NAME("csum")	},
	{ .id = BTRFS_QUOTA_TREE_OBJECTID,	DEFINE_NAME("quota")	},
	{ .id = BTRFS_TREE_LOG_OBJECTID,	DEFINE_NAME("log")	},
	{ .id = BTRFS_TREE_RELOC_OBJECTID,	DEFINE_NAME("treloc")	},
	{ .id = BTRFS_DATA_RELOC_TREE_OBJECTID,	DEFINE_NAME("dreloc")	},
	{ .id = BTRFS_UUID_TREE_OBJECTID,	DEFINE_NAME("uuid")	},
	{ .id = BTRFS_FREE_SPACE_TREE_OBJECTID,	DEFINE_NAME("free-space") },
	{ .id = BTRFS_BLOCK_GROUP_TREE_OBJECTID, DEFINE_NAME("block-group") },
	{ .id = 0,				DEFINE_NAME("tree")	},
};

#undef DEFINE_LEVEL
#undef DEFINE_NAME

void btrfs_set_buffer_lockdep_class(u64 objectid, struct extent_buffer *eb, int level)
{
	struct btrfs_lockdep_keyset *ks;

	BUG_ON(level >= ARRAY_SIZE(ks->keys));

	/* Find the matching keyset, id 0 is the default entry */
	for (ks = btrfs_lockdep_keysets; ks->id; ks++)
		if (ks->id == objectid)
			break;

	lockdep_set_class_and_name(&eb->lock, &ks->keys[level], ks->names[level]);
}

void btrfs_maybe_reset_lockdep_class(struct btrfs_root *root, struct extent_buffer *eb)
{
	if (test_bit(BTRFS_ROOT_RESET_LOCKDEP_CLASS, &root->state))
		btrfs_set_buffer_lockdep_class(root->root_key.objectid,
					       eb, btrfs_header_level(eb));
}

#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 - 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(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
{
	u64 start_ns = 0;

	if (trace_btrfs_tree_read_lock_enabled())
		start_ns = ktime_get_ns();

	down_read_nested(&eb->lock, nest);
	trace_btrfs_tree_read_lock(eb, start_ns);
}

void btrfs_tree_read_lock(struct extent_buffer *eb)
{
	__btrfs_tree_read_lock(eb, BTRFS_NESTING_NORMAL);
}

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

/*
 * Try-lock for write.
 *
 * Return 1 if the rwlock has been taken, 0 otherwise
 */
int btrfs_try_tree_write_lock(struct extent_buffer *eb)
{
	if (down_write_trylock(&eb->lock)) {
		eb->lock_owner = current->pid;
		trace_btrfs_try_tree_write_lock(eb);
		return 1;
	}
	return 0;
}

/*
 * Release read lock.
 */
void btrfs_tree_read_unlock(struct extent_buffer *eb)
{
	trace_btrfs_tree_read_unlock(eb);
	up_read(&eb->lock);
}

/*
 * __btrfs_tree_lock - 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(struct extent_buffer *eb, enum btrfs_lock_nesting nest)
	__acquires(&eb->lock)
{
	u64 start_ns = 0;

	if (trace_btrfs_tree_lock_enabled())
		start_ns = ktime_get_ns();

	down_write_nested(&eb->lock, nest);
	eb->lock_owner = current->pid;
	trace_btrfs_tree_lock(eb, start_ns);
}

void btrfs_tree_lock(struct extent_buffer *eb)
{
	__btrfs_tree_lock(eb, BTRFS_NESTING_NORMAL);
}

/*
 * Release the write lock.
 */
void btrfs_tree_unlock(struct extent_buffer *eb)
{
	trace_btrfs_tree_unlock(eb);
	eb->lock_owner = 0;
	up_write(&eb->lock);
}

/*
 * 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)
{
	int i;

	if (path->keep_locks)
		return;

	for (i = level; i < BTRFS_MAX_LEVEL; i++) {
		if (!path->nodes[i])
			continue;
		if (!path->locks[i])
			continue;
		btrfs_tree_unlock_rw(path->nodes[i], path->locks[i]);
		path->locks[i] = 0;
	}
}

/*
 * 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)
{
	struct extent_buffer *eb;

	while (1) {
		eb = btrfs_root_node(root);

		btrfs_maybe_reset_lockdep_class(root, eb);
		btrfs_tree_lock(eb);
		if (eb == root->node)
			break;
		btrfs_tree_unlock(eb);
		free_extent_buffer(eb);
	}
	return eb;
}

/*
 * 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)
{
	struct extent_buffer *eb;

	while (1) {
		eb = btrfs_root_node(root);

		btrfs_maybe_reset_lockdep_class(root, eb);
		btrfs_tree_read_lock(eb);
		if (eb == root->node)
			break;
		btrfs_tree_read_unlock(eb);
		free_extent_buffer(eb);
	}
	return eb;
}

/*
 * 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)
{
	struct extent_buffer *eb;

	while (1) {
		eb = btrfs_root_node(root);
		if (!btrfs_try_tree_read_lock(eb)) {
			free_extent_buffer(eb);
			return ERR_PTR(-EAGAIN);
		}
		if (eb == root->node)
			break;
		btrfs_tree_read_unlock(eb);
		free_extent_buffer(eb);
	}
	return eb;
}

/*
 * 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)
{
	atomic_set(&lock->readers, 0);
	atomic_set(&lock->writers, 0);
	init_waitqueue_head(&lock->pending_readers);
	init_waitqueue_head(&lock->pending_writers);
}

/* Return true if acquisition is successful, false otherwise */
bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)
{
	if (atomic_read(&lock->readers))
		return false;

	atomic_inc(&lock->writers);

	/* Ensure writers count is updated before we check for pending readers */
	smp_mb__after_atomic();
	if (atomic_read(&lock->readers)) {
		btrfs_drew_write_unlock(lock);
		return false;
	}

	return true;
}

void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)
{
	while (true) {
		if (btrfs_drew_try_write_lock(lock))
			return;
		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
	}
}

void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)
{
	atomic_dec(&lock->writers);
	cond_wake_up(&lock->pending_readers);
}

void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)
{
	atomic_inc(&lock->readers);

	/*
	 * Ensure the pending reader count is perceieved BEFORE this reader
	 * goes to sleep in case of active writers. This guarantees new writers
	 * won't be allowed and that the current reader will be woken up when
	 * the last active writer finishes its jobs.
	 */
	smp_mb__after_atomic();

	wait_event(lock->pending_readers, atomic_read(&lock->writers) == 0);
}

void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)
{
	/*
	 * atomic_dec_and_test implies a full barrier, so woken up writers
	 * are guaranteed to see the decrement
	 */
	if (atomic_dec_and_test(&lock->readers))
		wake_up(&lock->pending_writers);
}