Re: [PATCH] Fix RCU race in access of nohz_cpu_mask

[Oleg Nesterov <oleg@tv-sign.ru>]

Srivatsa Vaddagiri wrote:
>
> Accessing  nohz_cpu_mask before incrementing rcp->cur is racy. It can
> cause tickless idle CPUs to be included in rsp->cpumask, which will
> extend graceperiods unnecessarily.
> ...
> @@ -244,15 +244,15 @@ static void rcu_start_batch(struct rcu_c
>
>         if (rcp->next_pending &&
>                         rcp->completed == rcp->cur) {
> -               /* Can't change, since spin lock held. */
> -               cpus_andnot(rsp->cpumask, cpu_online_map, nohz_cpu_mask);
> -
>                 rcp->next_pending = 0;
>                 /* next_pending == 0 must be visible in __rcu_process_callbacks()
>                  * before it can see new value of cur.
>                  */
>                 smp_wmb();
>                 rcp->cur++;
> +               smp_mb();
> +               cpus_andnot(rsp->cpumask, cpu_online_map, nohz_cpu_mask);
> +

Could you explain why this patch makes any difference?

grep shows the only user of nohz_cpu_mask in arch/s390/kernel/time.c,
start_hz_timer() and stop_hz_timer().

I can't see how this change can prevent idle cpus to be included in
->cpumask, cpu can add itself to nohz_cpu_mask right after some other
cpu started new grace period.

I think cpu should call cpu_quiet() after adding itself to nohz mask
to eliminate this race.

No?

Oleg.