linux-kernel - Re: [RFC][PATCH] PM: Introduce new top level suspend and hibernation callbacks (rev. 3)

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Message-ID: <Pine.LNX.4.44L0.0803251044590.4838-100000@iolanthe.rowland.org>
Date:	Tue, 25 Mar 2008 11:06:44 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Greg KH <greg@...ah.com>, Len Brown <lenb@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexey Starikovskiy <astarikovskiy@...e.de>,
	David Brownell <david-b@...bell.net>,
	Pavel Machek <pavel@....cz>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC][PATCH] PM: Introduce new top level suspend and hibernation
 callbacks (rev. 3)

On Mon, 24 Mar 2008, Rafael J. Wysocki wrote:

> > Can we also have a DPM_PREPARING state, set when ->prepare() is about
> > to be called?  The PM core wouldn't make use of it but some drivers
> > would.  (I can't think of any use at all for the analogous
> > DPM_COMPLETING state, however.)
> 
> Hmm.  dev->power.status is protected by dpm_list_mtx.  Do you think it would be
> useful to have an accessor function for reading it under the lock?

I don't think so.  What I have in mind is situations where there 
accessed has already been synchronized by other means, while the 
prepare() method is running.  For example:

	Task 0				Task 1
	------				------
	->prepare() is called
	Waits for currently-running
	  registration in task 1
	  to finish
	Does other stuff
					Receives a request to register
					  a new child under dev
					Sees that dev->power.state is
					  still DPM_ON, so goes ahead
					  with the child's registration
	->prepare() returns
	dev->power.state is set to
	  DPM_SUSPENDING
					device_pm_add() checks
					  dev->power.state and fails
					  the registration

If dev->power.state had been set to DPM_PREPARING before ->prepare() 
was called, then task 1 would have avoided trying to register the 
child.

> > > +			dev->power.status = DPM_RESUMING;
> > > +			get_device(dev);
> > > +			mutex_unlock(&dpm_list_mtx);
> > > +
> > > +			resume_device(dev, state);
> > > +
> > > +			mutex_lock(&dpm_list_mtx);
> > > +			put_device(dev);
> > > +		}
> > > +		if (!list_empty(&dev->power.entry))
> > > +			list_move_tail(&dev->power.entry, &list);
> > 
> > A little problem here: You refer to dev after calling put_device().
> 
> The device can't be removed at this point, because we hold dpm_list_mtx, which
> is needed by device_del().

True, it can't be removed at this point.  But it _can_ be removed
between the calls to resume_device() and mutex_lock().

> > >  	}
> > > -	if (!error)
> > > -		all_sleeping = true;
> > > +	list_splice(&list, &dpm_list);
> > 
> > Instead you could eliminate the list_splice_init() above and put here:
> > 
> > 	list_splice(&list, dpm_list->prev);
> > 
> > This will move the entries from list to the end of dpm_list.
> 
> dpm_list may be empty at this point.  Wouldn't that cause any trouble?

It will still work correctly.  If dpm_list is empty then dpm_list->prev
is equal to &dpm_list, so it will do the same thing as your current
code does.


I just thought of another problem.  At the point where
local_irq_disable() is called, in between device_suspend() and
device_power_down(), it is possible in a preemptible kernel that
another task is holding dpm_list_mtx and is in the middle of updating
the list pointers.  This would mess up the traversal in
device_power_down().

I'm not sure about the best way to prevent this.  Is it legal to call
unlock_mutex() while interrupts or preemption are disabled?

Alan Stern

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