linux-kernel - Re: [PATCH 0/4] [GIT PULL] tracing: recursion and compile fixes

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Date:	Mon, 20 Apr 2009 17:14:14 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/4] [GIT PULL] tracing: recursion and compile fixes



On Mon, 20 Apr 2009, Frederic Weisbecker wrote:

> On Mon, Apr 20, 2009 at 04:44:46PM -0400, Steven Rostedt wrote:
> > 
> > On Mon, 20 Apr 2009, Frederic Weisbecker wrote:
> > > > > 
> > > > 
> > > > Hmm, doesn't the trace wakeup test if the runqueue lock is locked or not?
> > > > 
> > > > -- Steve
> > > > 
> > > 
> > > 
> > > Hmm, yes it does but that's not the first time we meet this problem
> > > (sched switch event tracing recursions by the past). So either the
> > > test doesn't work well or this is about another lock that
> > > wake_up_common takes...
> > 
> > Ug, it is the task's rq lock. Not the current rq lock. wakeup takes the 
> > runqueue lock of the task. The "runqueue_is_locked" only tests the lock of 
> > current CPU, which is not what we can have.
> 
> 
> You mean the lock held on the wait_queue for wake_up_trace() ?
> 
> void __wake_up(wait_queue_head_t *q, unsigned int mode,
> 			int nr_exclusive, void *key)
> {
> 	unsigned long flags;
> 
> 	spin_lock_irqsave(&q->lock, flags);
> 	__wake_up_common(q, mode, nr_exclusive, 0, key);
> 	spin_unlock_irqrestore(&q->lock, flags);
> }
> 

No, I mean that we check the runqueue lock on the current CPU (there's a 
runqueue for each CPU). The runqueue_is_locked tests only the lock for the 
current cpu. But the wake up of a task grabs the runqueue of the lock the 
task is on.

If we are on CPU0 and hold the runqueue lock of CPU1, and we wake up a 
task on CPU1, we will deadlock. Even thought runqueue_is_locked passed. 
Because the current runqueue lock is not part of this equation.

-- Steve

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