linux-kernel - Re: [PATCH 0/6] PM: Asynchronous suspend and resume of devices

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Message-Id: <200908272318.19868.rjw@sisk.pl>
Date:	Thu, 27 Aug 2009 23:18:19 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"linux-pm" <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Len Brown <lenb@...nel.org>,
	Pavel Machek <pavel@....cz>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Zhang Rui <rui.zhang@...el.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Linux PCI <linux-pci@...r.kernel.org>
Subject: Re: [PATCH 0/6] PM: Asynchronous suspend and resume of devices

On Thursday 27 August 2009, Alan Stern wrote:
> On Wed, 26 Aug 2009, Rafael J. Wysocki wrote:
> 
> > Hi,
> > 
> > The following patches introduce a mechanism allowing us to execute device
> > drivers' suspend and resume callbacks asynchronously during system sleep
> > transitions, such as suspend to RAM.
> > 
> > The idea is explained in the changelogs of the first two patches.
> > 
> > Comments welcome.
> 
> I've been terribly busy and haven't had a chance to look at this.  The
> earlier version seemed to have a bunch of mutual-exclusion issues; are
> they resolved now?

Please be more specific.

> There were also some problems involving unsafe iteration over the dpm_list
> -- remember that devices can be unregistered at any time, not just while
> they are suspending or resuming.

Not at the time we're holding dpm_list_mtx, though.

I don't think there are any unsafe iterations over dpm_list in the patches.

> When a device finishes, instead of having the async thread look for 
> another device to work on, I think it would be better to have the 
> thread check the dependents of the current device.  Those which are 
> now ready can be added to a "device-ready" list, which is used to 
> direct the actions of new async threads.

Actually, I wanted to avoid adding such a list, because that would be
duplicating of the async framework's actions, to some extent.  Yes, we can
duplicate the async framework just for the suspend/resume needs.  No, I don't
think it's worth it.

BTW, the patches have been tested on a dual-core box and I haven't seen any
problems with them so far.  Also, the testing shows that the waiting for
devices with unsatisfied dependencies is almost eliminated (on my test box
it happens 4-8 times during the entire suspend-resume cycle).

Thanks,
Rafael
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