linux-kernel - Re: [TIP][RFC 6/7] futex: add requeue

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Date:	Mon, 09 Mar 2009 12:55:11 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	"lkml, " <linux-kernel@...r.kernel.org>,
	Sripathi Kodi <sripathik@...ibm.com>,
	John Stultz <johnstul@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [TIP][RFC 6/7] futex: add requeue_pi calls

Thomas Gleixner wrote:
> On Thu, 5 Mar 2009, Darren Hart wrote:
>> int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
>> 			      struct rt_mutex_waiter *waiter,
>> 			      struct task_struct *task, int detect_deadlock)
>> {
>> 	int ret;
>>
>> 	spin_lock(&lock->wait_lock);
>> 	ret = task_blocks_on_rt_mutex(lock, waiter, task, detect_deadlock);
>>
>>
>> I add the following line to fix the bug.  Question is, should I use this
>> atomic
>> optimization here (under the lock->wait_lock) or should I just do "lock->owner
>> |= RT_MUTEX_HAS_WAITERS" ?
>>
>> =====>	mark_rt_mutex_waiters(lock);
> 
> This is still not enough as I explained in the review of the original
> patch. What you need to do is:
> 
> 	if (try_to_take_rt_mutex(lock, task)) {
> 	   	spin_unlock(&lock->wait_lock);
> 		/* The caller needs to wake up task, as it is now the owner */
> 		return WAKEIT;
> 	}
> 
> 	ret = task_blocks_on_rt_mutex(lock, waiter, task, detect_deadlock);
> 

Right, so I'm testing this out:

	mark_rt_mutex_waiters(lock);

	if (!rt_mutex_owner(lock) || try_to_steal_lock(lock, task)) {
		/* We got the lock for task. */
		debug_rt_mutex_lock(lock);

		rt_mutex_set_owner(lock, task, 0);

		rt_mutex_deadlock_account_lock(lock, task);
		return 1;
	}

Steven, is this the proper use of the debug* routines?  I copied them 
from try_to_take_rt_mutex(), but they are empty routines without 
comments so I wasn't sure exactly how they were intended to be used. 
Does the usage of debug_rt_mutex_lock() assume task=current (the other 
has the task_struct passed int).

Thanks,

Darren


>> 	if (ret && !waiter->task) {
>> 		/*
>> 		 * Reset the return value. We might have
>> 		 * returned with -EDEADLK and the owner
>> 		 * released the lock while we were walking the
>> 		 * pi chain.  Let the waiter sort it out.
>> 		 */
>> 		ret = 0;
>> 	}
>> 	spin_unlock(&lock->wait_lock);
>>
>> 	debug_rt_mutex_print_deadlock(waiter);
>>
>> 	return ret;
>> }
> 
> Thanks,
> 
> 	tglx


-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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