linux-kernel - Re: [PATCH 3/3] PM / hibernate / memory hotplug: Rework mutual exclusion

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Date:	Sat, 31 Aug 2013 02:39:28 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Toshi Kani <toshi.kani@...com>
Cc:	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	linux-mm@...ck.org
Subject: Re: [PATCH 3/3] PM / hibernate / memory hotplug: Rework mutual exclusion

On Friday, August 30, 2013 06:23:19 PM Toshi Kani wrote:
> On Thu, 2013-08-29 at 23:18 +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > 
> > Since all of the memory hotplug operations have to be carried out
> > under device_hotplug_lock, they won't need to acquire pm_mutex if
> > device_hotplug_lock is held around hibernation.
> > 
> > For this reason, make the hibernation code acquire
> > device_hotplug_lock after freezing user space processes and
> > release it before thawing them.  At the same tim drop the
> > lock_system_sleep() and unlock_system_sleep() calls from
> > lock_memory_hotplug() and unlock_memory_hotplug(), respectively.
> > 
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  kernel/power/hibernate.c |    4 ++++
> >  kernel/power/user.c      |    2 ++
> >  mm/memory_hotplug.c      |    4 ----
> >  3 files changed, 6 insertions(+), 4 deletions(-)
> > 
> > Index: linux-pm/kernel/power/hibernate.c
> > ===================================================================
> > --- linux-pm.orig/kernel/power/hibernate.c
> > +++ linux-pm/kernel/power/hibernate.c
> > @@ -652,6 +652,7 @@ int hibernate(void)
> >  	if (error)
> >  		goto Exit;
> >  
> > +	lock_device_hotplug();
> 
> Since hibernate() can be called from sysfs, do you think the tool may
> see this as a circular dependency with p_active again?  This shouldn't
> be a problem in practice, though.

/sys/power/state isn't a device attribute even and is never removed, so it
would be very sad and disappointing if lockdep reported that as a circular
dependency.  The deadlock is surely not possible here anyway.

Thanks,
Rafael

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