Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load

[Peter Zijlstra]

On Wed, Sep 04, 2019 at 01:28:19PM +0200, Peter Zijlstra wrote:
> @@ -196,6 +198,17 @@ static int membarrier_register_global_expedited(void)
>  		 */
>  		smp_mb();
>  	} else {
> +		struct task_struct *g, *t;
> +
> +		read_lock(&tasklist_lock);
> +		do_each_thread(g, t) {
> +			if (t->mm == mm) {
> +				atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED,
> +					  &t->membarrier_state);
> +			}
> +		} while_each_thread(g, t);
> +		read_unlock(&tasklist_lock);
> +
>  		/*
>  		 * For multi-mm user threads, we need to ensure all
>  		 * future scheduler executions will observe the new

Arguably, because this is exposed to unpriv users and a potential
preemption latency issue, we could do it in 3 passes:

	- RCU, mark all found lacking, count
	- RCU, mark all found lacking, count
	- if count of last pass, tasklist_lock

That way, it becomes much harder to trigger the bad case.

Do we worry about that?