On Tue, 8 Dec 2009, Linus Torvalds wrote: > Side note: if this was a real lock, you'd also needed an smp_wmb() in the > 'wait_lock()' path after the atomic_inc(), to make sure that others see > the atomic lock was seen by other people before the suspend started. > > In your usage scenario, I don't think it would ever be noticeable, since > the other users are always going to start running from the same thread > that did the wait_lock(), so even if they run on other CPU's, we'll have > scheduled _to_ those other CPU's and done enough memory ordering to > guarantee that they will see the thing. > > So it would be ok in this situation, simply because it acts as an > initializer and never sees any real SMP issues. Yes. I would have brought this up, but you made the point for me. > But it's an example of how you now don't just depend on the locking > primitives themselves doing the right thing, you end up depending very > subtly on exactly how the lock is used. The standard locks do have the > same kind of issue for initializers, but we avoid it elsewhere because > it's so risky. No doubt there are other reasons why the "wait-lock" pattern doesn't get used enough to be noticed. Alan Stern -- 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
