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

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Date:   Tue, 7 Feb 2017 09:46:13 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 Tue, Feb 07, 2017 at 10:23:31AM +0100, Vlastimil Babka wrote:
> 
> > cpu offlining. I have to check the code but my impression was that WQ
> > code will ignore the cpu requested by the work item when the cpu is
> > going offline. If the offline happens while the worker function already
> > executes then it has to wait as we run with preemption disabled so we
> > should be safe here. Or am I missing something obvious?
> 
> Tejun suggested an alternative solution to avoiding get_online_cpus() in
> this thread:
> https://lkml.kernel.org/r/<20170123170329.GA7820@....duckdns.org>

I was hoping it would not really be necessary to sync against the cpu
offline callback if we know we don't really have to guarantee we run on
the correct CPU.

-- 
Mel Gorman
SUSE Labs