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

Oleg Nesterov
2019-09-04T10:53:57Z

On 09/03, Mathieu Desnoyers wrote:
>
> @@ -1130,6 +1130,10 @@ struct task_struct {
> unsigned long numa_pages_migrated;
> #endif /* CONFIG_NUMA_BALANCING */
>
> +#ifdef CONFIG_MEMBARRIER
> + atomic_t membarrier_state;
> +#endif

...

> +static inline void membarrier_prepare_task_switch(struct task_struct *t)
> +{
> + if (!t->mm)
> + return;
> + atomic_set(&t->membarrier_state,
> + atomic_read(&t->mm->membarrier_state));
> +}

Why not

rq->membarrier_state = next->mm ? t->mm->membarrier_state : 0;

and

if (cpu_rq(cpu)->membarrier_state & MEMBARRIER_STATE_GLOBAL_EXPEDITED) {
...
}

in membarrier_global_expedited() ? (I removed atomic_ to simplify)

IOW, why this new member has to live in task_struct, not in rq?

Oleg.