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 21:35:59 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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, Linus Torvalds wrote:

> 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 agree that it is uglier.  The only advantage is in handling
asynchronous non-tree suspend dependencies, of which we probably won't
have very many.  In fact, I don't know of _any_ offhand.

Interestingly, this non-tree dependency problem does not affect resume.

> 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.

You mean, if A comes before B in the list and A must suspend after B?  
Then A's down_read() on B _can't_ occur before B's down_write() on
itself.  The down_write() on B happens before the
list_for_each_entry_reverse() iteration reaches A; it even happens
before B's async task is launched.

> 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.

Not necessary.

Alan Stern

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