linux-kernel - Re: [patch 4/7] rtmutex: unify state manipulation

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Date:	Fri, 19 Dec 2008 20:40:17 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Linux-rt <linux-rt-users@...r.kernel.org>
Subject: Re: [patch 4/7] rtmutex: unify state manipulation

On Fri, 19 Dec 2008, Steven Rostedt wrote:
> > +	} while (cmpxchg(&current->state, state, block_state) != state);
> 
> Doesn't this break archs that do not have cmpxchg?

We can use xchg. The waiter is protected against the RUNNING_MUTEX
state change via the mutex->lock. It's just some overcautioness when I
started to fix this.

> There might be another way. We could just use your TASK_RUNNING_MUTEX or 
> trick for both mutexes and spinlocks.

The mechanisms should be the same for everything now.
 
> > -	if (mtx)
> > -		set_current_state(TASK_RUNNING);
> > -	else {
> > -		state = xchg(&current->state, saved_state);
> > -		if (unlikely(state == TASK_RUNNING))
> > -			current->state = TASK_RUNNING;
> > -	}
> > +	rt_restore_current_state(saved_state);
> 
> This is a bug. A mutex always leaves in the TASK_RUNNING state.
 
Duh, yes. So this should be:

     rt_restore_current_state(!mtx ? saved_state : TASK_RUNNING);

> What about having the locking spinlocks and mutexes be almost identical. 
> Like the rwlocks are (rwlocks and rwsems share the same code). We can use 
> the RT_MUTEX_RUNNING trick for both. The only difference is that a mutex 
> will always leave in the TASK_RUNNING state.

Good point.

Thanks,

	tglx
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