Date:	Fri, 15 Jul 2011 18:11:55 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Ed Tomlinson <edt@....ca>, Steven Rostedt <rostedt@...dmis.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: INFO: possible circular locking dependency detected

On Fri, 2011-07-15 at 08:59 -0700, Paul E. McKenney wrote:

> > Because we're in irq_exit(), after decrementing preempt_count, so
> > in_irq() returns false.
> 
> Can we delay decrementing preempt_count so that RCU has some chance
> of actually working?

No, softirqs must be ran with in_irq() being false.

> > No, the *BANG* being that we end up calling rcu_read_unlock_special()
> > while holding scheduler locks, which is BAD(tm).
> 
> Well, it certainly is BAD(tm) if you guys continue to deprive
> rcu_read_unlock_special() of the means of determining whether it is
> being invoked from hardware irq handler context.

hard irq handler isn't really the problem here, its the nested softirq
code that is.

> > >  (Which I believe, perhaps
> > > incorrectly, to be prevented by the fact that all modifications to
> > > ->rcu_read_unlock_special are carried out with irqs disabled on the
> > > corresponding CPU, at least given no RCU_BOOST.)  The check for in_irq()
> > > should prevent the from-irq rcu_read_unlock_special() from attempting
> > > to acquire any locks.
> > 
> > Right, so in_irq() simply checks a few bits in preempt_count, which we
> > just cleared due to being in irq_exit().
> 
> Right.  So how about delaying clearing those bits until after you get
> done messing with the scheduler from hardware irq handler context?

Can't do.

> > But in_irq() isn't sufficient for RCU usage after the hardirq ends, see
> > irq_exit(). Also there's all of softirq to consider, that too can run
> > and not get caught by in_irq().
> 
> Change the rules without adjusting the callers can in fact result in some
> breakage.  ;-)

There's no changing the rules here, this is how its worked for a very
long time indeed. Softirqs can run from the hardirq tail.

> The bit about local_irq_save() and local_irq_restore() invoking the
> scheduler is rather surprising -- is there a raw_ version that avoids
> this?

They don't, they might for -rt, but that's a different story. But
looking at the latest version I have its only local_irq_save_rt() and
friends that do that.

> > > 3.	It is possible that the task is preempted after the
> > > 	--rcu_read_lock_nesting, in which case the task won't be queued.
> > > 	Of course the task might already be queued if there was an
> > > 	earlier preemption during this same RCU read-side critical
> > > 	section, in which case #2 applies.
> > > 
> > > 	In other words, a preemption in __rcu_read_unlock() after the
> > > 	--rcu_read_lock_nesting has no effect on RCU state: either the
> > > 	task was already marked RCU_READ_UNLOCK_BLOCKED, or it wasn't.
> > > 	Either way, rcu_note_context_switch() does not see this task as
> > > 	being in an RCU read-side critical section.
> > > 
> > > So what am I missing here?
> > 
> >  $task			IRQ		SoftIRQ
> > 
> >  rcu_read_lock()
> > 
> >  /* do stuff */
> > 
> >  <preempt> |= UNLOCK_BLOCKED
> > 
> >  rcu_read_unlock()
> >    --t->rcu_read_lock_nesting
> > 
> > 			irq_enter();
> > 			/* do stuff, don't use RCU */
> > 			irq_exit();
> > 			  sub_preempt_count(IRQ_EXIT_OFFSET);
> > 			  invoke_softirq()
> 
> Why can't we exchange the order of the above two so that RCU correctly
> avoids messing with the scheduler if called from hardware interrupt
> context?

Because softirqs != hardirq ? This has been so like forever, can't go
change the semantics of this without risking tons of borkage. Every time
we try to change softirq semantics (we tried with -rt, because softirqs
are a massive pain) everything goes tits up fast.

> > 
> > 					ttwu();
> > 					  spin_lock_irq(&pi->lock)
> > 					  rcu_read_lock();
> > 					  /* do stuff */
> > 					  rcu_read_unlock();
> > 					    rcu_read_unlock_special()
> > 					      rcu_report_exp_rnp()
> > 					        ttwu()
> > 					          spin_lock_irq(&pi->lock) /* deadlock */
> > 					      
> > 
> >    rcu_read_unlock_special(t);
> > 
> > Ed can simply trigger this 'easy' because invoke_softirq() immediately
> > does a ttwu() of ksoftirqd/# instead of doing the in-place softirq stuff
> > first, but even without that the above happens.
> 
> An easily reproduced bug is certainly a nice change of pace...
> 
> > Something like the below _might_ fix it..
> 
> Maybe, but how does tglx make PREEMPT_RT work in this case?  The problem
> is that PREEMPT_RT allows ksoftirqd to be preempted, and thus allows it
> to be RCU priority boosted.

RT is mostly easier since it doesn't nest as many contexts, softirqs for
example always run in task context, and the only way to run them in a
random tasks' context is through local_bh_enable() and since there's no
local_bh_enable() call in the middle of __rcu_read_unlock() you're
pretty good there.

I know tglx has some softirq changes he hasn't yet shared with me, but
if the patch I send earlier fixes the problem for mainline, I'm fairly
confident I can cook one up for him as well.


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