linux-kernel - Re: allow preemption in check_task

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Date:	Mon, 10 Feb 2014 19:13:00 +0100
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-rt-users@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Carsten Emde <C.Emde@...dl.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Platschek <platschek@....tuwien.ac.at>
Subject: Re: allow preemption in check_task_state

On Mon, 10 Feb 2014, Steven Rostedt wrote:

> On Mon, 10 Feb 2014 18:17:12 +0100
> Nicholas Mc Guire <der.herr@...r.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.
>
ok - thanks - I think now I got it.

thx!
hofrat 
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