linux-kernel - Re: [PATCH 3/3] PM: Limit race conditions between runtime PM and system sleep

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Date:	Wed, 29 Jun 2011 10:11:33 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Ming Lei <tom.leiming@...il.com>,
	Linux PM mailing list <linux-pm@...ts.linux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Greg KH <greg@...ah.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>,
	Kevin Hilman <khilman@...com>, <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH 3/3] PM: Limit race conditions between runtime PM and
 system sleep

On Tue, 28 Jun 2011, Rafael J. Wysocki wrote:

> On Tuesday, June 28, 2011, Ming Lei wrote:
> > 
> > Hi Rafael,
> > 
> > On Sun, 26 Jun 2011 00:56:31 +0200
> > "Rafael J. Wysocki" <rjw@...k.pl> wrote:
> > 
> > > Index: linux-2.6/Documentation/power/runtime_pm.txt
> > > ===================================================================
> > > --- linux-2.6.orig/Documentation/power/runtime_pm.txt
> > > +++ linux-2.6/Documentation/power/runtime_pm.txt
> > > @@ -567,6 +567,11 @@ this is:
> > >  	pm_runtime_set_active(dev);
> > >  	pm_runtime_enable(dev);
> > >  
> > > +The PM core always increments the run-time usage counter before calling the
> > > +->suspend() callback and decrements it after calling the ->resume() callback.
> > > +Hence disabling run-time PM temporarily like this will not cause any run-time
> > > +suspend callbacks to be lost.
> > 
> > Could you explain why the above is that "this will not cause any run-time suspend
> > callbacks to be lost"? 
> > 
> > Looks like it should be "this will not cause any run-time suspend callbacks to
> > be called", but not sure.
> 
> You're right the wording is not perfect.  The problem is that if it's done
> this way without incrementing the usage counter beforehand, the status may
> change to "suspended" right before the pm_runtime_set_active() and then the
> new status will not reflect the actual state of the device.
> 
> So, it may be better to say "Hence disabling runtime PM temporarily like this
> will not cause the runtime PM status of the device to conflict with the actual
> device state".
> 
> Alan, what do you think?

How about: "... will not cause any runtime suspend attempts to be 
permanently lost.  If the usage count goes to zero following the return 
of the ->resume() callback, the ->runtime_idle() callback will be 
invoked as usual."

Alan Stern

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