linux-kernel - Re: [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier

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Message-ID: <1629045844.2645.1568388056947.JavaMail.zimbra@efficios.com>
Date:   Fri, 13 Sep 2019 11:20:56 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...ux.ibm.com>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Chris Lameter <cl@...ux.com>, Kirill Tkhai <tkhai@...dex.ru>,
        Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 4/4] Fix: sched/membarrier: p->mm->membarrier_state
 racy load (v2)

----- On Sep 9, 2019, at 7:00 AM, Oleg Nesterov oleg@...hat.com wrote:

> 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().

Good point! I'll use this_cpu_write() there and within
membarrier_exec_mmap(), which seems to be affected by the same problem.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com