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 <nborisov@xxxxxxxx>
---
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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