linux-kernel - Re: [PATCH 2/5] bio: fix improper use of smp_mb__before

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Message-ID: <20190430082158.GA2677@hirez.programming.kicks-ass.net>
Date:   Tue, 30 Apr 2019 10:21:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org
Subject: Re: [PATCH 2/5] bio: fix improper use of smp_mb__before_atomic()

On Mon, Apr 29, 2019 at 10:14:58PM +0200, Andrea Parri wrote:
> 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().

> @@ -224,7 +224,7 @@ 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();

Maybe also add:

		/*
		 * XXX the comment that explain this barrier goes here
		 */
> +		smp_mb();
>  	}
>  	atomic_set(&bio->__bi_cnt, count);
>  }