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

[Steven Rostedt]

On Thu, Apr 04, 2013 at 06:38:36PM +0200, Peter Zijlstra 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.

I'm confused to why the above is a problem. Task-X will eventually
release C, and then Y will release B and O will get to continue. Do we
have to drop them once the owner is blocked? Can't we follow the chain
like the PI code does?

-- Steve