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

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Date:   Thu, 9 Feb 2017 11:22:49 -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:

> You are just not getting it, really.
>
> The problem is that this for_each_online_cpu() is racy against a concurrent
> hot unplug and therefor can queue stuff for a not longer online cpu. That's
> what the mm folks tried to avoid by preventing a CPU hotplug operation
> before entering that loop.

With a stop machine action it is NOT racy because the machine goes into a
special kernel state that guarantees that key operating system structures
are not touched. See mm/page_alloc.c's use of that characteristic to build
zonelists. Thus it cannot be executing for_each_online_cpu and related
tasks (unless one does not disable preempt .... but that is a given if a
spinlock has been taken)..

> > Lets get rid of get_online_cpus() etc.
>
> And that solves what?

It gets rid of future issues with serialization in paths were we need to
lock and still do for_each_online_cpu().

> Can you please start to understand the scope of the whole hotplug machinery
> including the requirements for get_online_cpus() before you waste
> everybodys time with your uninformed and halfbaken proposals?

Its an obvious solution to the issues that have arisen multiple times with
get_online_cpus() within the slab allocators. The hotplug machinery should
make things as easy as possible for other people and having these
get_online_cpus() everywhere does complicate things.