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btrfs: Remove redundant memory barriers around dio_private error status

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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@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 10:53:36 +02:00 committed by David Sterba
parent 16db5758fe
commit de224b7c56
1 changed files with 9 additions and 8 deletions
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17
fs/btrfs/inode.c
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@ -8322,13 +8322,13 @@ 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 */
@ -8516,10 +8516,11 @@ 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);