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Date:   Wed, 4 Sep 2019 14:03:37 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        paulmck <paulmck@linux.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        "Eric W. Biederman" <ebiederm@xmission.com>,
        "Russell King, ARM Linux" <linux@armlinux.org.uk>,
        Chris Metcalf <cmetcalf@ezchip.com>,
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        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
Message-ID: <20190904120336.GC24568@redhat.com>
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On 09/04, Peter Zijlstra wrote:
>
> +		struct task_struct *g, *t;
> +
> +		read_lock(&tasklist_lock);
> +		do_each_thread(g, t) {

for_each_process_thread() looks better

> +			if (t->mm == mm) {
> +				atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED,
> +					  &t->membarrier_state);
> +			}

then you also need to change dup_task_struct(), it should clear
->membarrier_state unless CLONE_VM.

And probably unuse_mm() should clear current->membarrier_state too.

Hmm. And it can race with copy_process() anyway, tasklist_lock can't
really help. So copy_process() needs to do

	write_lock_irq(&tasklist_lock);
	...

	if (clone_flags & CLONE_VM)
		p->membarrier_state = current->membarrier_state;
	else
		p->membarrier_state = 0;

Oleg.