Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

Alan Stern
2009-12-08T19:34:16Z

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