linux-kernel - Re: [PATCH] sched/core: fix illegal RCU from offline CPUs

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Date:   Mon, 13 Jan 2020 17:20:57 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Qian Cai <cai@....pw>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        bsegall@...gle.com, mgorman@...e.de, paulmck@...nel.org,
        tglx@...utronix.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/core: fix illegal RCU from offline CPUs

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...