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From: Peter Zijlstra <peterz@infradead.org>
To: Qian Cai <cai@lca.pw>
Cc: Marco Elver <elver@google.com>, Will Deacon <will@kernel.org>,
	mingo@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next
Date: Thu, 23 Jan 2020 10:39:05 +0100	[thread overview]
Message-ID: <20200123093905.GU14914@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <A90E2B85-77CB-4743-AEC3-90D7836C4D47@lca.pw>

On Wed, Jan 22, 2020 at 06:54:43PM -0500, Qian Cai wrote:
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 1f7734949ac8..832e87966dcf 100644
> --- a/kernel/locking/osq_lock.c
> +++ b/kernel/locking/osq_lock.c
> @@ -75,7 +75,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
>                  * wait for either @lock to point to us, through its Step-B, or
>                  * wait for a new @node->next from its Step-C.
>                  */
> -               if (node->next) {
> +               if (READ_ONCE(node->next)) {
>                         next = xchg(&node->next, NULL);
>                         if (next)
>                                 break;

This could possibly trigger the warning, but is a false positive. The
above doesn't fix anything in that even if that load is shattered the
code will function correctly -- it checks for any !0 value, any byte
composite that is !0 is sufficient.

This is in fact something KCSAN compiler infrastructure could deduce.

  reply	other threads:[~2020-01-23  9:39 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-22 16:38 [PATCH] locking/osq_lock: fix a data race in osq_wait_next Qian Cai
2020-01-22 16:59 ` Will Deacon
2020-01-22 17:08   ` Qian Cai
2020-01-22 22:38     ` Marco Elver
2020-01-22 23:54       ` Qian Cai
2020-01-23  9:39         ` Peter Zijlstra [this message]
2020-01-28  3:11           ` Qian Cai
2020-01-28 11:46             ` Marco Elver
2020-01-28 12:53               ` Qian Cai
2020-01-28 16:52               ` Peter Zijlstra
2020-01-28 16:56               ` Peter Zijlstra
2020-01-29  0:22                 ` Paul E. McKenney
2020-01-29 15:29                   ` Marco Elver
2020-01-29 18:40                     ` Peter Zijlstra
2020-01-30 13:39                       ` Marco Elver
2020-01-30 13:48                         ` Peter Zijlstra
2020-01-31  3:32                           ` Qian Cai
2020-01-29 18:49                   ` Peter Zijlstra
2020-01-29 19:26                     ` Paul E. McKenney
2020-01-23  9:36       ` Peter Zijlstra
2020-01-28  3:12         ` Qian Cai
2020-01-28  8:18           ` Marco Elver
2020-01-28 10:10             ` Qian Cai
2020-01-28 10:29               ` Marco Elver
2020-01-22 17:09 ` Peter Zijlstra

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