Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

Chris Mason
2009-01-08T19:17:53Z

On Thu, 2009-01-08 at 13:14 -0500, Steven Rostedt wrote:
>
>
> On Thu, 8 Jan 2009, Steven Rostedt wrote:
> > > In fact, you might not even need a process C: all you need is for B to be
> > > on the same runqueue as A, and having enough load on the other CPU's that
> > > A never gets migrated away. So "C" might be in user space.
>
> You're right about not needing process C.
>
> > >
> > > I dunno. There are probably variations on the above.
> >
> > Ouch! I think you are on to something:
> >
> > for (;;) {
> > struct thread_info *owner;
> >
> > old_val = atomic_cmpxchg(&lock->count, 1, 0);
> > if (old_val == 1) {
> > lock_acquired(&lock->dep_map, ip);
> > mutex_set_owner(lock);
> > return 0;
> > }
> >
> > if (old_val < 0 && !list_empty(&lock->wait_list))
> > break;
> >
> > /* See who owns it, and spin on him if anybody */
> > owner = ACCESS_ONCE(lock->owner);
> >
> > The owner was preempted before assigning lock->owner (as you stated).
>
> If it was the current process that preempted the owner and these are RT
> tasks pinned to the same CPU and the owner is of lower priority than the
> spinner, we have a deadlock!
>
> Hmm, I do not think the need_sched here will even fix that :-/

RT tasks could go directly to sleeping. The spinner would see them on
the list and break out.

-chris