linux-kernel - Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

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Date:	Tue, 8 Dec 2009 14:32:40 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alan Stern <stern@...land.harvard.edu>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>, Zhang Rui <rui.zhang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

On Tue, 8 Dec 2009, Alan Stern wrote:
> 
> Suppose A and B are unrelated devices and we need to impose the
> off-tree constraint that A suspends after B.

Ah. Ok, I can imagine the off-tree constraints, but part of my "keep it 
simple" was to simply not do them. If there are constraints that aren't 
in the topology of the tree, then I simply don't think that async is worth 
it in the first place.

> You misunderstand.  The suspend algorithm will look like this:
> 
> 	dpm_suspend()
> 	{
> 		list_for_each_entry_reverse(dpm_list, dev) {
> 			down_write(dev->lock);
> 			async_schedule(device_suspend, dev);
> 		}
> 	}
> 
> 	device_suspend(dev)
> 	{
> 		device_for_each_child(dev, child) {
> 			down_read(child->lock);
> 			up_read(child->lock);
> 		}
> 		dev->suspend(dev);	/* May do off-tree down+up pairs */
> 		up_write(dev->lock);
> 	}

Ok, so the above I think work (and see my previous email: I think 
completions would be workable there too).

It's just that I think the "looping over children" is ugly, when I think 
that by doing it the other way around you can make the code simpler and 
only depend on the PM device list and a simple parent pointer access.

I also think that you are wrong that the above somehow protects against 
non-topological dependencies. If the device you want to keep delay 
yourself suspending for is after you in the list, the down_read() on that 
may succeed simply because it hasn't even done its down_write() yet and 
you got scheduled early.

But I guess you could do that by walking the list twice (first to lock 
them all, then to actually call the suspend function). That whole 
two-phase thing, except the first phase _only_ locks, and doesn't do any 
callbacks.

			Linus
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