Re: [PATCH v2 2/3] mutex: add support for reservation style locks, v2

Daniel Vetter
2013-04-04T16:59:14Z

On Thu, Apr 4, 2013 at 6:38 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, 2013-04-04 at 15:31 +0200, Daniel Vetter wrote:
>> Hm, I guess your aim with the TASK_DEADLOCK wakeup is to bound the
>> wait
>> times of older task.
>
> No, imagine the following:
>
> struct ww_mutex A, B;
> struct mutex C;
>
> task-O task-Y task-X
> A
> B
> C
> C
> B
>
> At this point O finds that Y owns B and thus we want to make Y 'yield'
> B to make allow B progress. Since Y is blocked, we'll send a wakeup.
> However Y is blocked on a different locking primitive; one that doesn't
> collaborate in the -EDEADLK scheme therefore we don't want the wakeup to
> succeed.

Yeah, I've thought about this some more and the special sleep state
seems to be only required to ensure we don't cause spurious wakeups
for other any other reasons a task blocks. But I think we can use that
kick-current-holder approach to ensure that older tasks get the lock
in a more timely fashion than the current code. I've tried to detail
it a bit with another 3 task example - that seems to be the point
where the fun starts ;-)
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch