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

From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: paulmck <paulmck@linux.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Oleg Nesterov <oleg@redhat.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Russell King, ARM Linux" <linux@armlinux.org.uk>,
	Chris Metcalf <cmetcalf@ezchip.com>, Chris Lameter <cl@linux.com>,
	Kirill Tkhai <tkhai@yandex.ru>, Mike Galbraith <efault@gmx.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load
Date: Wed, 4 Sep 2019 13:49:29 +0200	[thread overview]
Message-ID: <20190904114929.GV2386@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190904112819.GD2349@hirez.programming.kicks-ass.net>

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?