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

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Message-ID: <20170208152106.GP5686@dhcp22.suse.cz>
Date:   Wed, 8 Feb 2017 16:21:07 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Christoph Lameter <cl@...ux.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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 Wed 08-02-17 09:11:06, Cristopher Lameter wrote:
> On Wed, 8 Feb 2017, Michal Hocko wrote:
> 
> > > Huch? stop_machine() is horrible and heavy weight. Don't go there, there
> > > must be simpler solutions than that.
> >
> > Absolutely agreed. We are in the page allocator path so using the
> > stop_machine* is just ridiculous. And, in fact, there is a much simpler
> > solution [1]
> 
> That is nonsense. stop_machine would be used when adding removing a
> processor. There would be no need to synchronize when looping over active
> cpus anymore. get_online_cpus() etc would be removed from the hot
> path since the cpu masks are guaranteed to be stable.

I have no idea what you are trying to say and how this is related to the
deadlock we are discussing here. We certainly do not need to add
stop_machine the problem. And yeah, dropping get_online_cpus was
possible after considering all fallouts.

-- 
Michal Hocko
SUSE Labs