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

[Boqun Feng]

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@xxxxxxxxxxxxxxxxx>
> ---
>  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
>