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SubjectRe: 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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