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btrfs: Relax memory barrier in btrfs_tree_unlock

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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 <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Nikolay Borisov 2018-02-14 14:37:26 +02:00 committed by David Sterba
parent 7c829b722d
commit 2e32ef87b0
1 changed files with 1 additions and 1 deletions
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2
fs/btrfs/locking.c
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@ -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))
wake_up(&eb->write_lock_wq);
} else {