Possible bug with mutex adaptative spinning

[Peter Zijlstra]

On Wed, 2010-04-14 at 12:56 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2010-04-14 at 12:35 +1000, Benjamin Herrenschmidt wrote:
> > Hi Peter !
> > 
> > I -may- have found a bug with mutex adaptative spinning. We hit it when
> > torture testing CPU unplug.
> 
>  .../...
> 
> In fact, I wonder if there's another potential problem:
> 
> If the owner is actually running, it may do so for a very long time. It
> looks to me that everybody trying to take the mutex will thus spin and
> never get out of the spin loop until the owner stops running.

The inner-most spin loop breaks out on need_resched():

                if (task_thread_info(rq->curr) != owner || need_resched())
                        return 0;