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btrfs: Remove redundant memory barrier in dev stats 

As per atomic_t.txt documentation :
 - RMW operations that have a return value are fully ordered;

atomic_xchg is one such operation so it already includes everything it
needs w.r.t memory ordering and add a comment to be more explicit about
that.

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 2017-10-20 18:10:58 +03:00 committed by David Sterba
parent 9deae96892
commit 4660c49f9b
1 changed files with 7 additions and 1 deletions
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8
fs/btrfs/volumes.h
View File

@ -519,7 +519,13 @@ static inline int btrfs_dev_stat_read_and_reset(struct btrfs_device *dev,
int ret;
ret = atomic_xchg(dev->dev_stat_values + index, 0);
smp_mb__before_atomic();
/*
* atomic_xchg implies a full memory barriers as per atomic_t.txt:
* - RMW operations that have a return value are fully ordered;
*
* This implicit memory barriers is paired with the smp_rmb in
* btrfs_run_dev_stats
*/
atomic_inc(&dev->dev_stats_ccnt);
return ret;
}