linux-kernel - Re: [PATCH rcu/next 1/3] rcu/tree: Use READ_ONCE() for lockless read of rnp->qsmask

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Date:   Fri, 16 Sep 2022 12:55:23 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH rcu/next 1/3] rcu/tree: Use READ_ONCE() for lockless read
 of rnp->qsmask

On Thu, Sep 15, 2022 at 12:14:17AM +0000, Joel Fernandes (Google) wrote:
> The rnp->qsmask is locklessly accessed from rcutree_dying_cpu(). This
> may help avoid load tearing due to concurrent access, KCSAN
> 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;

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>