Re: allow preemption in check_task_state

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 10 Feb 2014 18:17:12 +0100
Nicholas Mc Guire <der.herr@hofr.at> wrote:


> > 
> > In rtmutex.c we have:
> > 
> > 	pi_lock(&self->pi_lock);
> > 	__set_current_state(self->saved_state);
> > 	self->saved_state = TASK_RUNNING;
> > 	pi_unlock(&self->pi_lock);
> > 
> > As there is no wmb() here, it can be very possible that another CPU
> > will see saved_state as TASK_RUNNING, and current state as
> > TASK_RUNNING, and miss the update completely.
> > 
> > I would not want to add a wmb() unless there is a real bug with the
> > check state, as the above is in a very fast path and the check state is
> > in a slower path.
> >
> maybe I'm missing/missunderstanding something here but
> pi_unlock -> arch_spin_unlock is a full mb() 
> so once any task did an update of the state the loop should be catching
> this update ? if the loop exits before the updat takes effect (pi_unlock)
> would that be ncorrect ?

Even if the spin locks were full memory barriers, it is still buggy.
The fact that we set current_state to saved_state, and then saved_state
to TASK_RUNNING without any memory barriers in between those two
statements, means that the reader (even with a rmb()) can still see
both as TASK_RUNNING.

-- Steve