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

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Date:   Wed, 1 Mar 2017 18:04:29 +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}

On Wed, Mar 01, 2017 at 07:52:18AM -0800, Dan Williams wrote:
> On Wed, Mar 1, 2017 at 4:51 AM, Heiko Carstens
> <heiko.carstens@...ibm.com> wrote:
> > Since it is anything but obvious why Dan wrote in changelog of b5d24fda9c3d
> > ("mm, devm_memremap_pages: hold device_hotplug lock over
> > mem_hotplug_{begin, done}") that write accesses to
> > mem_hotplug.active_writer are coordinated via lock_device_hotplug() I'd
> > rather propose a new private memory_add_remove_lock which has similar
> > semantics like the cpu_add_remove_lock for cpu hotplug (see patch below).
> >
> > However instead of sprinkling locking/unlocking of that new lock around all
> > calls of mem_hotplug_begin() and mem_hotplug_end() simply include locking
> > and unlocking into these two functions.
> >
> > This still allows get_online_mems() and put_online_mems() to work, while at
> > the same time preventing mem_hotplug.active_writer corruption.
> >
> > Any opinions?
> 
> Sorry, yes, I didn't make it clear that I derived that locking
> requirement from store_mem_state() and its usage of
> lock_device_hotplug_sysfs().
> 
> That routine is trying very hard not trip the soft-lockup detector. It
> seems like that wants to be an interruptible wait.

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?

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.