linux-kernel - Re: mm: deadlock between get_online_cpus/pcpu

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Message-ID: <alpine.DEB.2.20.1702090759370.22559@east.gentwo.org>
Date:   Thu, 9 Feb 2017 08:00:57 -0600 (CST)
From:   Christoph Lameter <cl@...ux.com>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mm: deadlock between get_online_cpus/pcpu_alloc

On Thu, 9 Feb 2017, Thomas Gleixner wrote:

> And how does that solve the problem at hand? Not at all:
>
> CPU 0	     	  	    CPU 1
>
> for_each_online_cpu(cpu)
>  ==> cpu = 1
> 			    stop_machine()
> 			    set_cpu_online(1, false)
>  queue_work(cpu1)
>
> Thanks,

Well thats not how I remember stop_machine does work. Doesnt it stop the
processing on all cpus otherwise its not a real usable stop.

The stop_machine would need to ensure that all cpus cease processing
before proceeding.