linux-kernel - Re: [PATCH v5 03/18] rcu/tree: Use READ_ONCE() for lockless read of rnp->qsmask

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Date:   Tue, 6 Sep 2022 15:26:33 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, paulmck@...nel.org,
        rostedt@...dmis.org, vineeth@...byteword.org
Subject: Re: [PATCH v5 03/18] rcu/tree: Use READ_ONCE() for lockless read of
 rnp->qsmask

On Thu, Sep 01, 2022 at 10:17:05PM +0000, Joel Fernandes (Google) wrote:
> The rnp->qsmask is locklessly accessed from rcutree_dying_cpu(). This
> may help avoid load/store tearing due to concurrent access, KCSAN

Nit: you can avoid only load tearing with READ_ONCE().

Regards,
Boqun

> issues, and preserve sanity of people reading the mask in tracing.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> ---
>  kernel/rcu/tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0ca21ac0f064..5ec97e3f7468 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2106,7 +2106,7 @@ int rcutree_dying_cpu(unsigned int cpu)
>  	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
>  		return 0;
>  
> -	blkd = !!(rnp->qsmask & rdp->grpmask);
> +	blkd = !!(READ_ONCE(rnp->qsmask) & rdp->grpmask);
>  	trace_rcu_grace_period(rcu_state.name, READ_ONCE(rnp->gp_seq),
>  			       blkd ? TPS("cpuofl-bgp") : TPS("cpuofl"));
>  	return 0;
> -- 
> 2.37.2.789.g6183377224-goog
>