Re: [PATCH tip/core/rcu 4/4] srcu: Add READ_ONCE() to srcu_struct ->srcu_gp_seq load

[Peter Zijlstra]

On Fri, Feb 14, 2020 at 04:29:32PM -0800, paulmck@xxxxxxxxxx wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> 
> The load of the srcu_struct structure's ->srcu_gp_seq field in
> srcu_funnel_gp_start() is lockless, so this commit adds the requisite
> READ_ONCE().
> 
> This data race was reported by KCSAN.

But is there in actual fact a data-race? AFAICT this code was just fine.

> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>  kernel/rcu/srcutree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 119a373..90ab475 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -678,7 +678,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
>  
>  	/* If grace period not already done and none in progress, start it. */
>  	if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
> -	    rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
> +	    rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_IDLE) {
>  		WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
>  		srcu_gp_start(ssp);
>  		if (likely(srcu_init_done))
> -- 
> 2.9.5
>