linux-kernel - Re: possible deadlock in lru_add_drain

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Message-ID: <20171031163058.2byfed2i36fcum3g@hirez.programming.kicks-ass.net>
Date:   Tue, 31 Oct 2017 17:30:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Byungchul Park <byungchul.park@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot 
        <bot+e7353c7141ff7cbb718e4c888a14fa92de41ebaa@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        jglisse@...hat.com, LKML <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, shli@...com, syzkaller-bugs@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, ying.huang@...el.com,
        kernel-team@....com, David Herrmann <dh.herrmann@...il.com>
Subject: Re: possible deadlock in lru_add_drain_all

On Tue, Oct 31, 2017 at 04:45:46PM +0100, Michal Hocko wrote:
> Anyway, this lock dependecy is subtle as hell and I am worried that we
> might have way too many of those. We have so many callers of
> get_online_cpus that dependecies like this are just waiting to blow up.

Yes, the filesystem locks inside hotplug thing is totally annoying. I've
got a few other splats that contain a similar theme and I've no real
clue what to do about.

See for instance this one:

  https://lkml.kernel.org/r/20171027151137.GC3165@worktop.lehotels.local

splice from devtmpfs is another common theme and links it do the
pipe->mutex, which then makes another other splice op invert against
that hotplug crap :/


I'm sure I've suggested simply creating possible_cpus devtmpfs files up
front to get around this... maybe we should just do that.