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btrfs: locking: use atomic for DREW lock writers

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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>
This commit is contained in:
David Sterba 2023-03-01 21:47:08 +01:00
parent ce4cf3793e
commit 0b5485391d
3 changed files with 9 additions and 33 deletions
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12
fs/btrfs/disk-io.c
View file

@ -1341,17 +1341,8 @@ struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
static int btrfs_init_fs_root(struct btrfs_root *root, dev_t anon_dev)
{
int ret;
unsigned int nofs_flag;
/*
* We might be called under a transaction (e.g. indirect backref
* resolution) which could deadlock if it triggers memory reclaim
*/
nofs_flag = memalloc_nofs_save();
ret = btrfs_drew_lock_init(&root->snapshot_lock);
memalloc_nofs_restore(nofs_flag);
if (ret)
goto fail;
btrfs_drew_lock_init(&root->snapshot_lock);
if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID &&
!btrfs_is_data_reloc_root(root)) {
@ -2065,7 +2056,6 @@ void btrfs_put_root(struct btrfs_root *root)
WARN_ON(test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state));
if (root->anon_dev)
free_anon_bdev(root->anon_dev);
btrfs_drew_lock_destroy(&root->snapshot_lock);
free_root_extent_buffers(root);
#ifdef CONFIG_BTRFS_DEBUG
spin_lock(&root->fs_info->fs_roots_radix_lock);

25
fs/btrfs/locking.c
View file

@ -325,24 +325,12 @@ struct extent_buffer *btrfs_try_read_lock_root_node(struct btrfs_root *root)
* acquire the lock.
*/
int btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
void btrfs_drew_lock_init(struct btrfs_drew_lock *lock)
{
int ret;
ret = percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
if (ret)
return ret;
atomic_set(&lock->readers, 0);
atomic_set(&lock->writers, 0);
init_waitqueue_head(&lock->pending_readers);
init_waitqueue_head(&lock->pending_writers);
return 0;
}
void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock)
{
percpu_counter_destroy(&lock->writers);
}
/* Return true if acquisition is successful, false otherwise */
@ -351,10 +339,10 @@ bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock)
if (atomic_read(&lock->readers))
return false;
percpu_counter_inc(&lock->writers);
atomic_inc(&lock->writers);
/* Ensure writers count is updated before we check for pending readers */
smp_mb();
smp_mb__after_atomic();
if (atomic_read(&lock->readers)) {
btrfs_drew_write_unlock(lock);
return false;
@ -374,7 +362,7 @@ void btrfs_drew_write_lock(struct btrfs_drew_lock *lock)
void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock)
{
percpu_counter_dec(&lock->writers);
atomic_dec(&lock->writers);
cond_wake_up(&lock->pending_readers);
}
@ -390,8 +378,7 @@ void btrfs_drew_read_lock(struct btrfs_drew_lock *lock)
*/
smp_mb__after_atomic();
wait_event(lock->pending_readers,
percpu_counter_sum(&lock->writers) == 0);
wait_event(lock->pending_readers, atomic_read(&lock->writers) == 0);
}
void btrfs_drew_read_unlock(struct btrfs_drew_lock *lock)

5
fs/btrfs/locking.h
View file

@ -195,13 +195,12 @@ static inline void btrfs_tree_unlock_rw(struct extent_buffer *eb, int rw)
struct btrfs_drew_lock {
atomic_t readers;
struct percpu_counter writers;
atomic_t writers;
wait_queue_head_t pending_writers;
wait_queue_head_t pending_readers;
};
int btrfs_drew_lock_init(struct btrfs_drew_lock *lock);
void btrfs_drew_lock_destroy(struct btrfs_drew_lock *lock);
void btrfs_drew_lock_init(struct btrfs_drew_lock *lock);
void btrfs_drew_write_lock(struct btrfs_drew_lock *lock);
bool btrfs_drew_try_write_lock(struct btrfs_drew_lock *lock);
void btrfs_drew_write_unlock(struct btrfs_drew_lock *lock);