When performing an unlock on an extent buffer we'd like to order the
decrement of extent_buffer::blocking_writers with waking up any
waiters. In such situations it's sufficient to use smp_mb__after_atomic
rather than the heavy smp_mb. On architectures where atomic operations
are fully ordered (such as x86 or s390) unconditionally executing
a heavyweight smp_mb instruction causes a severe hit to performance
while bringin no improvements in terms of correctness.

The better thing is to use the appropriate smp_mb__after_atomic routine
which will do the correct thing (invoke a full smp_mb or in the case
of ordered atomics insert a compiler barrier). Put another way,
an RMW atomic op + smp_load__after_atomic equals, in terms of
semantics, to a full smp_mb. This ensures that none of the problems
described in the accompanying comment of waitqueue_active occur.
No functional changes.

Signed-off-by: Nikolay Borisov <nbori...@suse.com>
 fs/btrfs/locking.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index d13128c70ddd..621083f8932c 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -290,7 +290,7 @@ void btrfs_tree_unlock(struct extent_buffer *eb)
                 * Make sure counter is updated before we wake up waiters.
-               smp_mb();
+               smp_mb__after_atomic();
                if (waitqueue_active(&eb->write_lock_wq))
        } else {

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