linux-kernel - Re: [PATCH] driver core: Ensure proper suspend/resume ordering

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Date:	Wed, 16 Sep 2015 03:28:59 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] driver core: Ensure proper suspend/resume ordering

On Tuesday, September 15, 2015 03:18:19 PM Alan Stern wrote:
> On Tue, 15 Sep 2015, Thierry Reding wrote:
> 
> > > There are a few things to watch out for.  Since the dpm_list gets
> > > modified during system sleep transitions, we would have to make sure
> > > that nothing gets probed during those times.  In principle, that's what
> > > the "prepare" stage is meant for, but there's still a race.  As long as
> > > no other kernel thread (such as the deferred probing mechanism) tries
> > > to probe a device once everything has been frozen, we should be okay.
> > > But if not, there will be trouble -- after the ->prepare callback runs, 
> > > the device is no longer on the dpm_list and so we don't want this patch 
> > > to put it back on that list.
> > 
> > Perhaps moving to the end of the list needs to be a little smarter. That
> > is it could check whether the device has been prepared for suspension or
> > not and only move when it hasn't?
> 
> Maybe.  But doesn't that mean it won't solve your problem completely?
> 
> > Then again, shouldn't the core even prohibit new probes once the suspend
> > has been triggered? Sounds like asking for a lot of trouble if it didn't
> > ...
> 
> The core prohibits new devices from being registered.  It does not 
> prohibit probes of existing devices, because they currently do not 
> affect the dpm_list.

Which may be a mistake, because it does affect callbacks executed during
suspend/resume (after successful probe the device potentially has a different
set of PM callbacks than before).

> In general, we rely on subsystems not to do any probing once a device 
> is suspended.  It's probably reasonable to ask them not to do any 
> probing once a device has gone through the "prepare" stage.

Right.

Question is when it should be allowed to probe again.  I guess at the same
time we allow registrations to to take place again?

Thanks,
Rafael

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