linux-kernel - Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu rwsem

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Date:   Tue, 4 Jul 2017 14:49:47 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu
 rwsem

On Tue, 4 Jul 2017, Vlastimil Babka wrote:

> On 07/04/2017 11:32 AM, Thomas Gleixner wrote:
> > Andrey reported a potential deadlock with the memory hotplug lock and the
> > cpu hotplug lock.
> > 
> > The reason is that memory hotplug takes the memory hotplug lock and then
> > calls stop_machine() which calls get_online_cpus(). That's the reverse lock
> > order to get_online_cpus(); get_online_mems(); in mm/slub_common.c
> > 
> > The problem has been there forever. The reason why this was never reported
> > is that the cpu hotplug locking had this homebrewn recursive reader writer
> > semaphore construct which due to the recursion evaded the full lock dep
> > coverage. The memory hotplug code copied that construct verbatim and
> > therefor has similar issues.
> > 
> > Three steps to fix this:
> > 
> > 1) Convert the memory hotplug locking to a per cpu rwsem so the potential
> >    issues get reported proper by lockdep.
> > 
> > 2) Lock the online cpus in mem_hotplug_begin() before taking the memory
> >    hotplug rwsem and use stop_machine_cpuslocked() in the page_alloc code
> >    and use to avoid recursive locking.
> 
>      ^ s/and use // ?

Ooops, yes.

Thanks,

	tglx