[ Added Frederic to the cc, since he's touched this file/area most ]

On Wed, Feb 11, 2015 at 11:59 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So the caller has a really hard time guaranteeing that CSD_LOCK isn't
> set. And if the call is done in interrupt context, for all we know it
> is interrupting the code that is going to clear CSD_LOCK, so CSD_LOCK
> will never be cleared at all, and csd_lock() will wait forever.
>
> So I actually think that for the async case, we really *should* unlock
> before doing the callback (which is what Thomas' old patch did).
>
> And we migth well be better off doing something like
>
>         WARN_ON_ONCE(csd->flags & CSD_LOCK);
>
> in smp_call_function_single_async(), because that really is a hard requirement.
>
> And it strikes me that hrtick_csd is one of these cases that do this
> with interrupts disabled, and use the callback for serialization. So I
> really wonder if this is part of the problem..
>
> Thomas? Am I missing something?

Ok, this is a more involved patch than I'd like, but making the
*caller* do all the CSD maintenance actually cleans things up.

And this is still completely untested, and may be entirely buggy. What
do you guys think?

                             Linus