Using any kind of memory barriers around atomic operations which have
a return value is redundant, since those operations themselves are
fully ordered. atomic_t.txt states:

    - RMW operations that have a return value are fully ordered;

    Fully ordered primitives are ordered against everything prior and
    everything subsequent. Therefore a fully ordered primitive is like
    having an smp_mb() before and an smp_mb() after the primitive.

Given this let's replace the extra memory barriers with comments.

Signed-off-by: Nikolay Borisov <nbori...@suse.com>
---
 fs/btrfs/inode.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 8997e39c8e37..b6e4288010fa 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -8321,13 +8321,13 @@ static void btrfs_end_dio_bio(struct bio *bio)
                err = dip->subio_endio(dip->inode, btrfs_io_bio(bio), err);
 
        if (err) {
-               dip->errors = 1;
-
                /*
-                * before atomic variable goto zero, we must make sure
-                * dip->errors is perceived to be set.
+                * We want to perceive the errors flag being set before
+                * decrementing the reference count. We don't need a barrier
+                * since atomic operations with a return value are fully
+                * ordered as per atomic_t.txt
                 */
-               smp_mb__before_atomic();
+               dip->errors = 1;
        }
 
        /* if there are more bios still pending for this dio, just exit */
@@ -8515,10 +8515,11 @@ static int btrfs_submit_direct_hook(struct 
btrfs_dio_private *dip)
 out_err:
        dip->errors = 1;
        /*
-        * before atomic variable goto zero, we must
-        * make sure dip->errors is perceived to be set.
+        * Before atomic variable goto zero, we must  make sure dip->errors is
+        * perceived to be set. This ordering is ensured by the fact that an
+        * atomic operations with a return value are fully ordered as per
+        * atomic_t.txt
         */
-       smp_mb__before_atomic();
        if (atomic_dec_and_test(&dip->pending_bios))
                bio_io_error(dip->orig_bio);
 
-- 
2.7.4

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