linux-kernel - Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier

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Date:   Wed, 4 Sep 2019 12:53:49 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state
 racy load

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.