linux-kernel - Re: [PATCH] mm, add_memory_resource: hold device_hotplug lock over mem_hotplug

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Date:   Mon, 6 Mar 2017 09:22:21 +0100
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Sebastian Ott <sebott@...ux.vnet.ibm.com>,
        Linux MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Ben Hutchings <ben@...adent.org.uk>
Subject: Re: [PATCH] mm, add_memory_resource: hold device_hotplug lock over
 mem_hotplug_{begin, done}

Hello Dan,

> > If you look at commit 5e33bc4165f3 ("driver core / ACPI: Avoid device hot
> > remove locking issues") then lock_device_hotplug_sysfs() was introduced to
> > avoid a different subtle deadlock, but it also sleeps uninterruptible, but
> > not for more than 5ms ;)
> >
> > However I'm not sure if the device hotplug lock should also be used to fix
> > an unrelated bug that was introduced with the get_online_mems() /
> > put_online_mems() interface. Should it?
> 
> No, I don't think it should.
> 
> I like your proposed direction of creating a new lock internal to
> mem_hotplug_begin() to protect active_writer, and stop relying on
> lock_device_hotplug to serve this purpose.
> 
> > If so, we need to sprinkle around a couple of lock_device_hotplug() calls
> > near mem_hotplug_begin() calls, like Sebastian already started, and give it
> > additional semantics (protecting mem_hotplug.active_writer), and hope it
> > doesn't lead to deadlocks anywhere.
> 
> I'll put your proposed patch through some testing.

On s390 it _seems_ to work. Did it pass your testing too?
If so I would send a patch with proper patch description for inclusion.

Thanks,
Heiko