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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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