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Date:   Mon, 9 Sep 2019 13:00:36 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Chris Metcalf <cmetcalf@ezchip.com>,
        Christoph Lameter <cl@linux.com>,
        Kirill Tkhai <tkhai@yandex.ru>, Mike Galbraith <efault@gmx.de>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier_state
 racy load (v2)
Message-ID: <20190909110036.GC6719@redhat.com>
References: <20190906082305.GU2349@hirez.programming.kicks-ass.net>
 <20190908134909.12389-1-mathieu.desnoyers@efficios.com>
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On 09/08, Mathieu Desnoyers wrote:
>
> +static void sync_runqueues_membarrier_state(struct mm_struct *mm)
> +{
> +	int membarrier_state = atomic_read(&mm->membarrier_state);
> +	bool fallback = false;
> +	cpumask_var_t tmpmask;
> +	int cpu;
> +
> +	if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1) {
> +		WRITE_ONCE(this_rq()->membarrier_state, membarrier_state);

This doesn't look safe, this caller can migrate to another CPU after
it calculates the per-cpu ptr.

I think you need do disable preemption or simply use this_cpu_write().

Oleg.