linux-kernel - Re: [PATCH] locking/osq_lock: fix a data race in osq_wait

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Date:   Thu, 23 Jan 2020 10:39:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qian Cai <cai@....pw>
Cc:     Marco Elver <elver@...gle.com>, Will Deacon <will@...nel.org>,
        mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next

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.