On Tuesday 08 December 2009, Alan Stern wrote: > On Tue, 8 Dec 2009, Linus Torvalds wrote: > > > On Tue, 8 Dec 2009, Alan Stern wrote: > > > > > > That's not the way it should be done. Linus had children taking their > > > parents' locks during suspend, which is simple but leads to > > > difficulties. > > > > No it doesn't. Name them. > > Well, one difficulty. It arises only because we are contemplating > having the PM core fire up the async tasks, rather than having the > drivers' suspend routines launch them (the way your original proposal > did -- the difficulty does not arise there). > > Suppose A and B are unrelated devices and we need to impose the > off-tree constraint that A suspends after B. With children taking > their parent's lock, the way to prevent A from suspending too soon is > by having B's suspend routine acquire A's lock. > > But B's suspend routine runs entirely in an async task, because that > task is started by the PM core and it does the method call. Hence by > the time B's suspend routine is called, A may already have begun > suspending -- it's too late to take A's lock. To make the locking > work, B would have to acquire A's lock _before_ B's async task starts. > Since the PM core is unaware of the off-tree dependency, there's no > simple way to make it work. Do not set async_suspend for B and instead start your own async thread from its suspend callback. The parent-children synchronization is done by the core anyway (at least I'd do it that way), so the only thing you need to worry about is the extra dependency. > > That just complicates things. Compare to my simple locking scheme I've > > quoted several times. > > It is a little more complicated in that it involves explicitly > iterating over children. But it is simpler in that it can use > completions instead of rwsems and it avoids the off-tree dependency > problem described above. I would be slightly more comfortable using completions, but the rwsem-based approach is fine with me as well. Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
