Date:	Mon, 07 Sep 2015 23:33:21 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Jiang Liu <jiang.liu@...ux.intel.com>, mingo@...nel.org,
	Peter Zijlstra <peterz@...radead.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>, Tejun Heo <tj@...nel.org>,
	linux hotplug mailing <linux-hotplug@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: Possible deadlock related to CPU hotplug and kernfs

On Monday, September 07, 2015 11:11:19 AM Jiang Liu wrote:
> On 2015/9/4 22:16, Rafael J. Wysocki wrote:
> > Hi,
> > 
> > On Fri, Sep 4, 2015 at 9:20 AM, Jiang Liu <jiang.liu@...ux.intel.com> wrote:
> >> On 2015/9/4 4:08, Rafael J. Wysocki wrote:
> >>> Hi Tejun,
> >>>
> >>> On Thu, Sep 3, 2015 at 6:19 PM, Tejun Heo <tj@...nel.org> wrote:
> >>>> Hello, Rafael.
> >>>>
> >>>> On Thu, Sep 03, 2015 at 02:58:16AM +0200, Rafael J. Wysocki wrote:
> >>>>> So acpi_device_hotplug() calls lock_device_hotplug() which simply
> >>>>> acquires device_hotplug_lock.  It is held throughout the entire
> >>>>> hot-add/hot-remove code path.
> >>>>>
> >>>>> Witing anything to /sys/devices/system/cpu/cpux/online goes through
> >>>>> online_store() in drivers/base/core.c and that does
> >>>>> lock_device_hotplug_sysfs() which then attempts to acquire
> >>>>> device_hotplug_lock using mutex_trylock().  And it only calls
> >>>>> either device_online() or device_offline() if it ends up with the
> >>>>> lock held.
> >>>>>
> >>>>> Quite frankly, I don't see how these particular two code paths can
> >>>>> deadlock in any way.
> >>>>>
> >>>>> So either a third code path is involved which is not executed
> >>>>> under device_hotplug_lock, or lockdep needs to be told to actually
> >>>>> take device_hotplug_lock into account in this case IMO.
> >>>>
> >>>> Hmm... all sysfs rw functions are protected from removal.  ie. by
> >>>> default, removal of a sysfs file drains in-flight rw operations, so
> >>>> the hot plug path grabs a lock and then tries to remove a file and
> >>>> writing to the online file makes the file's write method to try to
> >>>> grab the same lock.  It deadlocks if the hotunplug path already has
> >>>> the lock and trying to drain the online file for removal.
> >>>
> >>> My point is that you cannot get into that situation.  If hotplug
> >>> already holds device_hotplug_lock, the write to "online" will end up
> >>> doing restart_syscall().
> >>>
> >>> If the "online" code path is holding the lock, hotplug cannot acquire
> >>> it and cannot proceed.
> >>>
> >>> Am I missing anything?
> >> Hi Rafael,
> >>         I think your are right. The lock_device_hotplug_sysfs() has
> >> already provided a solution for such a deadlock scenario. And there's
> >> another related code path at boot as:
> >> smp_init()
> >>         ->cpu_up()
> >>                 ->cpu_hotplug_begin()
> >>         So it seems to be a false alarm. Any way to teach lockdep
> >> about this to get rid of the false alarm?
> > 
> > Well, maybe we could call lock_device_hotplug() from that code path too?
> Hi Rafael,
> 	Adding lock_device_hotplug() to smp_init() doesn't solve the
> issue. So it seems to be an false alarm of lockdep, and I don't know
> how to get rid of such an lockdep false alarm:(

Peter, Ingo, some help from lockdep expert is needed.

We have a splat that almost certainly is a false positive (the original report
is here http://marc.info/?l=linux-kernel&m=144109156901959&w=4) and no ideas
how to make it go away.  Can you please have a look and advise?

Rafael

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