Messages in this thread | | | Date | Tue, 8 Dec 2009 21:35:59 -0500 (EST) | From | Alan Stern <> | Subject | Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33) |
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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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