linux-kernel - Re: [RFC][RT][PATCH 3/4] rtmutex: Revert Optimize rt lock wakeup

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Date:	Tue, 04 Jan 2011 08:19:09 -0700
From:	"Peter W. Morreale" <pmorreale@...ell.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Gregory Haskins <ghaskins@...ell.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	ThomasGleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][RT][PATCH 3/4] rtmutex: Revert Optimize rt lock wakeup

On Mon, 2011-01-03 at 15:22 -0500, Steven Rostedt wrote:
> On Mon, 2011-01-03 at 14:06 -0500, Steven Rostedt wrote:
> 
> > 
> > 	if (adaptive_wait(&waiter, orig_owner))
> > 		sleep = 1;
> > 	else
> > 		sleep = 0;
> > 
> > 	if (sleep)
> 
> 
> > 		raw_spin_lock(&lock->wait_lock);
> > 		saved_state = rt_set_current_block_state(saved_state);
> > 		if (!lock->owner && &waiter == rt_mutex_top_waiter(lock))
> > 			sleep = 0;
> > 		raw_spin_unlock(&lock->wait_lock);
> 
> I may be able to remove the above locks and replace it with:
> 
> 	saved_state = rt_set_current_blocked_state(saved_state);
> 	if (orig_owner == rt_mutex_owner(lock))
> 		schedule_rt_mutex(lock);
> 
> -- Steve

Isn't it possible to miss a wakeup here if the waiter becomes preempted?

Recall that adaptive wait is a preemptive wait.  Hence the (I believe)
original reason we did the adaptive spin in a (transitioning)  sleep
state.

-PWM

> 
> 
> > 		if (sleep)
> > 			schedule_rt_mutex(lock);
> > 		saved_state = rt_restore_current_blocked_state(saved_state);
> > 	}
> > 
> > Otherwise we can risk the wakeup_next_waiter() missing the wakeup.
> > 
> > To clarify, we want the adaptive_wait() to run as TASK_RUNNING. Then if
> > we must sleep, then we must set the state to TASK_UNINTERRUPTIBLE, test
> > again if we can still the lock, and if not then sleep. Otherwise, if a
> > wakeup happens just before we set the state to TASK_UNINTERRUPTIBLE,
> > then we miss the wake up all together.
> > 
> > I can do this change, and see what impact it makes.
> > 
> > I'm also curious if this ever worked? If it did not, then are you sure
> > your tests that show the benefit of it was true. I don't have a large
> > scale box at my disposal ATM, so I can only see what this does on 4way
> > machines.
> > 
> > -- Steve
> > 
> 
> 


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