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SubjectRe: [PATCH [RT] 05/14] rearrange rt_spin_lock sleep

On Fri, 22 Feb 2008, Gregory Haskins wrote:

> Gregory Haskins wrote:
> > @@ -732,14 +741,15 @@ rt_spin_lock_slowlock(struct rt_mutex *lock)
> >
> > debug_rt_mutex_print_deadlock(&waiter);
> >
> > - schedule_rt_mutex(lock);
> > + update_current(TASK_UNINTERRUPTIBLE, &saved_state);
>
> I have a question for everyone out there about this particular part of
> the code. Patch 6/14 adds an optimization that is predicated on the
> order in which we modify the state==TASK_UNINTERRUPTIBLE vs reading the
> waiter.task below.
>
> My assumption is that the xchg() (inside update_current()) acts as an
> effective wmb(). If xchg() does not have this property, then this code
> is broken and patch 6/14 should also add a:
>
>
> + smp_wmb();

I believe that the wmb would be needed. I doubt that xchg on all archs
would force any ordering of reads and writes. It only needs to guarantee the
atomic nature of the data exchange. I don't see any reason that it would
imply any type of memory barrier.

-- Steve


>
>
> > + if (waiter.task)
> > + schedule_rt_mutex(lock);
> > + else
> > + update_current(TASK_RUNNING_MUTEX, &saved_state);
> >
> > spin_lock_irqsave(&lock->wait_lock, flags);
> > current->flags |= saved_flags;
> > current->lock_depth = saved_lock_depth;
> > - state = xchg(&current->state, TASK_UNINTERRUPTIBLE);
> > - if (unlikely(state == TASK_RUNNING))
> > - saved_state = TASK_RUNNING;
>
>
> Does anyone know the answer to this?
>
> Regards,
> -Greg
>


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