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

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Message-ID: <1294156079.3948.209.camel@gandalf.stny.rr.com>
Date:	Tue, 04 Jan 2011 10:47:59 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	pmorreale@...ell.com
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 Tue, 2011-01-04 at 08:19 -0700, Peter W. Morreale wrote:
> 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?

Why? Preemption doesn't change the task state.

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

Yes it is a preemptive wait, I would not have accepted the patches if it
was anything else. But preemption is not affected by the state of the
task. A task could be TASK_RUNNING or TASK_INTERRUPTIBLE or
TASK_UNINTERRUPTIBLE, and that would not affect how it acts when it is
preempted.

-- Steve


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