linux-kernel - Re: [PATCH [RT] 05/14] rearrange rt_spin

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Message-ID: <47BECE30.8030100@gmail.com>
Date:	Fri, 22 Feb 2008 08:29:20 -0500
From:	Gregory Haskins <gregory.haskins.ml@...il.com>
To:	Gregory Haskins <ghaskins@...ell.com>
CC:	mingo@...e.hu, a.p.zijlstra@...llo.nl, tglx@...utronix.de,
	rostedt@...dmis.org, linux-rt-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, bill.huey@...il.com,
	kevin@...man.org, cminyard@...sta.com, dsingleton@...sta.com,
	dwalker@...sta.com, npiggin@...e.de, dsaxena@...xity.net,
	ak@...e.de, gregkh@...e.de, sdietrich@...ell.com,
	pmorreale@...ell.com, mkohari@...ell.com
Subject: Re: [PATCH [RT] 05/14] rearrange rt_spin_lock sleep

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();


> +		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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