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Btrfs: reduce calls to wake_up on uncontended locks 

The btrfs locks were unconditionally calling wake_up as the
locks were released.  This lead to extra thrashing on the waitqueue,
especially for locks that were dominated by readers.

Signed-off-by: Chris Mason <chris.mason@fusionio.com>
This commit is contained in:
Chris Mason 2012-07-23 15:25:05 -04:00
parent e39e64ac0c
commit cbea5ac1ee
1 changed files with 9 additions and 5 deletions
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14
fs/btrfs/locking.c
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@ -78,13 +78,15 @@ void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
write_lock(&eb->lock);
WARN_ON(atomic_read(&eb->spinning_writers));
atomic_inc(&eb->spinning_writers);
if (atomic_dec_and_test(&eb->blocking_writers))
if (atomic_dec_and_test(&eb->blocking_writers) &&
waitqueue_active(&eb->write_lock_wq))
wake_up(&eb->write_lock_wq);
} else if (rw == BTRFS_READ_LOCK_BLOCKING) {
BUG_ON(atomic_read(&eb->blocking_readers) == 0);
read_lock(&eb->lock);
atomic_inc(&eb->spinning_readers);
if (atomic_dec_and_test(&eb->blocking_readers))
if (atomic_dec_and_test(&eb->blocking_readers) &&
waitqueue_active(&eb->read_lock_wq))
wake_up(&eb->read_lock_wq);
}
return;
@ -199,7 +201,8 @@ void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
}
btrfs_assert_tree_read_locked(eb);
WARN_ON(atomic_read(&eb->blocking_readers) == 0);
if (atomic_dec_and_test(&eb->blocking_readers))
if (atomic_dec_and_test(&eb->blocking_readers) &&
waitqueue_active(&eb->read_lock_wq))
wake_up(&eb->read_lock_wq);
atomic_dec(&eb->read_locks);
}
@ -247,8 +250,9 @@ void btrfs_tree_unlock(struct extent_buffer *eb)
if (blockers) {
WARN_ON(atomic_read(&eb->spinning_writers));
atomic_dec(&eb->blocking_writers);
smp_wmb();
wake_up(&eb->write_lock_wq);
smp_mb();
if (waitqueue_active(&eb->write_lock_wq))
wake_up(&eb->write_lock_wq);
} else {
WARN_ON(atomic_read(&eb->spinning_writers) != 1);
atomic_dec(&eb->spinning_writers);