linux-kernel - Re: rcu,sched: spinlock recursion on 3.5-rc2

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Date:	Tue, 12 Jun 2012 17:07:37 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Sasha Levin <levinsasha928@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: rcu,sched: spinlock recursion on 3.5-rc2

On Tue, 12 Jun 2012, Paul E. McKenney wrote:
> On Tue, Jun 12, 2012 at 03:40:13PM +0200, Thomas Gleixner wrote:
> > The torture thread got preempted. rcu_preempt_note_context_switch()
> > tries to unlock the boosting rt mutex.
> > 
> > Though rcu_preempt_note_context_switch() is called with rq lock
> > held. So it's not a surprise that the code will dead lock.
> > 
> > My brain hurts already from looking, so Paul to the rescue!
> 
> My brain hurts from beating my head on my desk.  It seems that attempts
> to enhance PREEMPT_RCU's read-side performance require even more paranoia
> than I normally bring to bear.  :-/
> 
> Please see below for what I expect is the relevant revert.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> Revert "rcu: Move PREEMPT_RCU preemption to switch_to() invocation"
> 
> This reverts commit 616c310e83b872024271c915c1b9ab505b9efad9
> (Move PREEMPT_RCU preemption to switch_to() invocation) which can
> result in runqueue deadlock.

Hmm, not sure. The deadlock was not triggered in switch_to. It was
just at the beginning of __schedule()

need_resched:
	preempt_disable();
        cpu = smp_processor_id();
	rq = cpu_rq(cpu);
        rcu_note_context_switch(cpu);
	
	Which ends up in rcu_read_unlock_special() which tries to
	unlock the rtmutex.

So that code is still there ....

Thanks,

	tglx
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