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Subject: Re: [PATCH] sched/core: fix illegal RCU from offline CPUs
To:     Qian Cai <cai@lca.pw>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        "Steven Rostedt (VMware)" <rostedt@goodmis.org>,
        bsegall@google.com, mgorman@suse.de, paulmck@kernel.org,
        tglx@linutronix.de, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
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Date:   Mon, 13 Jan 2020 17:20:57 +0900
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On 2020/01/13 15:30, Qian Cai wrote:
>>> -	mmdrop(mm);
>>> +	smp_call_function_single(cpumask_first(cpu_online_mask),
>>> +				(void (*)(void *))mmdrop, mm, 0);
>>
>> mmdrop() might sleep, but
> 
> If that is the case, and then the commit e78a7614f387 (“idle: Prevent
> late-arriving interrupts from disrupting offline”) is incorrect because it
> will disable local irq before calling mmdrop() which will trigger
> the might_sleep() warning. Can you prove it?

Is commit 7283094ec3db318e ("kernel, oom: fix potential pgd_lock deadlock from
__mmdrop") about only softirq? Is it guaranteed that smp_call_function_single()
does not hit such race? Then just my overcareful...