From: Nikolay Borisov Date: Wed, 14 Feb 2018 08:53:36 +0000 (+0200) Subject: btrfs: Remove redundant memory barriers around dio_private error status X-Git-Tag: 4.17-rc1SMB3-Fixes~98^2~100 X-Git-Url: http://git.samba.org/samba.git/?p=sfrench%2Fcifs-2.6.git;a=commitdiff_plain;h=de224b7c56baa335bde9afcb4aa68f03c38f5f42;hp=16db5758fef8c71b0353a7b5505496d6f9229820 btrfs: Remove redundant memory barriers around dio_private error status 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 Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index bb5de52cbc09..be167f5ec433 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -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 @@ submit: 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);