linux/fs/btrfs/locking.c
David Sterba 0b5485391d btrfs: locking: use atomic for DREW lock writers
The DREW lock uses percpu variable to track lock counters and for that
it needs to allocate the structure. In btrfs_read_tree_root() or
btrfs_init_fs_root() this may add another error case or requires the
NOFS scope protection.

One way is to preallocate the structure as was suggested in
https://lore.kernel.org/linux-btrfs/20221214021125.28289-1-robbieko@synology.com/

We may avoid the allocation altogether if we don't use the percpu
variables but an atomic for the writer counter. This should not make any
difference, the DREW lock is used for truncate and NOCOW writes along
with other IO operations.

The percpu counter for writers has been there since the original commit
8257b2dc3c "Btrfs: introduce btrfs_{start, end}_nocow_write() for
each subvolume". The reason could be to avoid hammering the same
cacheline from all the readers but then the writers do that anyway.

Signed-off-by: David Sterba <dsterba@suse.com>
2023-04-17 18:01:17 +02:00

393 lines
10 KiB
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-free-space-00 */
char names[BTRFS_MAX_LEVEL][20];
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 = 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);
}