bio: fix improper use of smp_mb__before_atomic() [Linux 5.2]

bio: fix improper use of smp_mb__before_atomic() [Linux 5.2]

This Linux kernel change "bio: fix improper use of smp_mb__before_atomic()" is included in the Linux 5.2 release. This change is authored by Andrea Parri <andrea.parri [at]> on Mon May 20 19:23:56 2019 +0200. The commit for this change in Linux stable tree is f381c6a (patch).

bio: fix improper use of smp_mb__before_atomic()

This barrier only applies to the read-modify-write operations; in
particular, it does not apply to the atomic_set() primitive.

Replace the barrier with an smp_mb().

Fixes: dac56212e8127 ("bio: skip atomic inc/dec of ->bi_cnt for most use cases")
Reported-by: "Paul E. McKenney" <>
Reported-by: Peter Zijlstra <>
Signed-off-by: Andrea Parri <>
Reviewed-by: Ming Lei <>
Cc: Jens Axboe <>
Cc: Ming Lei <>
Cc: "Paul E. McKenney" <>
Cc: Peter Zijlstra <>
Signed-off-by: Jens Axboe <>

There are 2 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 include/linux/bio.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index ea73df3..0f23b56 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -210,7 +210,7 @@ static inline void bio_cnt_set(struct bio *bio, unsigned int count)
    if (count != 1) {
        bio->bi_flags |= (1 << BIO_REFFED);
-       smp_mb__before_atomic();
+       smp_mb();
    atomic_set(&bio->__bi_cnt, count);

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