Date:	Tue, 17 May 2011 00:53:49 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL rcu/next] rcu commits for 2.6.40

On Tue, May 17, 2011 at 04:40:03AM +0200, Frederic Weisbecker wrote:
> On Mon, May 16, 2011 at 02:24:49PM -0700, Paul E. McKenney wrote:
> > On Mon, May 16, 2011 at 02:23:29PM +0200, Ingo Molnar wrote:
> > > 
> > > * Ingo Molnar <mingo@...e.hu> wrote:
> > > 
> > > > > In the meantime, would you be willing to try out the patch at 
> > > > > https://lkml.org/lkml/2011/5/14/89?  This patch helped out Yinghai in 
> > > > > several configurations.
> > > > 
> > > > Wasn't this the one i tested - or is it a new iteration?
> > > > 
> > > > I'll try it in any case.
> > > 
> > > oh, this was a new iteration, mea culpa!
> > > 
> > > And yes, it solves all problems for me as well. Mind pushing it as a fix? :-)
> > 
> > ;-)
> > 
> > Unfortunately, the only reason I can see that it works is (1) there
> > is some obscure bug in my code or (2) someone somewhere is failing to
> > call irq_exit() on some interrupt-exit path.  Much as I might be tempted
> > to paper this one over, I believe that we do need to find whatever the
> > underlying bug is.
> > 
> > Oh, yes, there is option (3) as well: maybe if an interrupt deschedules
> > a process, the final irq_exit() is omitted in favor of rcu_enter_nohz()?
> > But I couldn't see any evidence of this in my admittedly cursory scan
> > of the x86 interrupt-handling code.
> > 
> > So until I learn differently, I am assuming that each and every
> > irq_enter() has a matching call to irq_exit(), and that rcu_enter_nohz()
> > is called after the final irq_exit() of a given burst of interrupts.
> > 
> > If my assumptions are mistaken, please do let me know!
> 
> So it would be nice to have a trace of the calls to rcu_irq_*() / rcu_*_nohz()
> before the unpairing happened.
> 
> I have tried to reproduce it but couldn't trigger anything.
> 
> So it would be nice if Yinghai can test the patch below, since he was able
> to trigger the warning.
> 
> This is essentially Paul's patch but with stacktrace of the calls recorded.
> Then the whole trace is dumped on the console when one of the WARN_ON_ONCE
> sanity check is positive. Beware as the trace will be dumped everytime
> WARN_ON_ONCE() is positive. So the first dump is enough, you can ignore the
> rest.
> 
> This requires CONFIG_TRACING. May be a good thing to boot with
> "ftrace=nop" parameter, so that ftrace will set up a long enough buffer
> to have an interesting trace.

Very cool, thank you!!!  I was going to do something like this next,
but given my lack of familiarity with tracing, your patch looks much
nicer than mine would have been.

It applies fine on top of tip/core/rcu and builds OK.  I cannot reproduce
the problem, either, so I am hoping that either Yinghai or Ingo can
run this, and hopefully doing so will provide some enlightenment.

I have pushed this as:

git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git diag.2011.05.16b

I also #ifdefed out the bodies of rcu_nmi_enter() and rcu_nmi_exit()
to match the earlier patches.

> PS: the first check in rcu_nmi_enter() doesn't seem to make sense.

Here is what it is doing:

o	rdtp->dynticks_nmi_nesting == 0:

	Is this is the first-level NMI?  In theory this should always
	be true, but I don't trust NMIs to mask each other.  I have seen
	many systems where NMIs could interrupt other NMIs.

	The idea is that if we already recorded one level of NMI, we
	had better record them all so we can figure out when we exit
	the last level of NMI handler.

o	atomic_read(&rdtp->dynticks) & 0x1):

	Did the NMI interrupt a non-dyntick code segment?  If we did,
	then there is no need to tell RCU anything -- RCU is already
	paying attention to this CPU anyway due to the fact that the
	interrupted code segment was not in dyntick mode.

As for the check on rcu_nmi_exit():

o	rdtp->dynticks_nmi_nesting == 0:

	We didn't record the corresponding entry to the NMI handler,
	so we had better not record the exit.

o	--rdtp->dynticks_nmi_nesting != 0:

	We are still nested in a NMI handler, so decrement the nesting
	count, but don't mess with rdtp->dynticks.

And yes, I do need some comments.  Or maybe to rewrite these in a
less-confusing form.  LIke you, I decided that these were incorrect
several times over the past few days.  So even if my analysis above turns
out to be correct, some change is needed, a comment at the very least.

But I also know that there is at least one problem outside of
rcu_nmi_enter() and rcu_nmi_exit() because Ingo verified that he hits
the problem even with the bodies of rcu_nmi_enter() and rcu_nmi_exit()
ifdefed out.

Again, thank you for adding the tracing!

> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 5616b17..a4e6d1b 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -316,24 +316,23 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
>   * critical sections can occur in irq handlers in nohz mode, a possibility
>   * handled by rcu_irq_enter() and rcu_irq_exit()).
>   */
> -void rcu_enter_nohz(void)
> +void __rcu_enter_nohz(void)
>  {
> -	unsigned long flags;
>  	struct rcu_dynticks *rdtp;
> 
> -	local_irq_save(flags);
>  	rdtp = &__get_cpu_var(rcu_dynticks);
> +	WARN_ON_ONCE(rdtp->dynticks_nesting <= 0);
>  	if (--rdtp->dynticks_nesting) {
> -		local_irq_restore(flags);
> +		if (WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)))
> +			ftrace_dump(DUMP_ORIG);
>  		return;
>  	}
>  	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
>  	smp_mb__before_atomic_inc();  /* See above. */
>  	atomic_inc(&rdtp->dynticks);
>  	smp_mb__after_atomic_inc();  /* Force ordering with next sojourn. */
> -	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
> -	local_irq_restore(flags);
> -
> +	if (WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1))
> +		ftrace_dump(DUMP_ORIG);
>  	/* If the interrupt queued a callback, get out of dyntick mode. */
>  	if (in_irq() &&
>  	    (__get_cpu_var(rcu_sched_data).nxtlist ||
> @@ -342,28 +341,57 @@ void rcu_enter_nohz(void)
>  		set_need_resched();
>  }
> 
> +void rcu_enter_nohz(void)
> +{
> +	unsigned long flags;
> +	struct rcu_dynticks *rdtp;
> +
> +	local_irq_save(flags);
> +	trace_dump_stack();
> +	rdtp = &__get_cpu_var(rcu_dynticks);
> +	if (WARN_ON_ONCE(rdtp->dynticks_nesting != 1))
> +		ftrace_dump(DUMP_ORIG);
> +	rdtp->dynticks_nesting = 1;
> +	__rcu_enter_nohz();
> +	local_irq_restore(flags);
> +}
> +
>  /*
>   * rcu_exit_nohz - inform RCU that current CPU is leaving nohz
>   *
>   * Exit nohz mode, in other words, -enter- the mode in which RCU
>   * read-side critical sections normally occur.
>   */
> -void rcu_exit_nohz(void)
> +void __rcu_exit_nohz(void)
>  {
> -	unsigned long flags;
>  	struct rcu_dynticks *rdtp;
> 
> -	local_irq_save(flags);
>  	rdtp = &__get_cpu_var(rcu_dynticks);
>  	if (rdtp->dynticks_nesting++) {
> -		local_irq_restore(flags);
> +		if (WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)))
> +			ftrace_dump(DUMP_ORIG);
>  		return;
>  	}
>  	smp_mb__before_atomic_inc();  /* Force ordering w/previous sojourn. */
>  	atomic_inc(&rdtp->dynticks);
>  	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
>  	smp_mb__after_atomic_inc();  /* See above. */
> -	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> +	if (WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)))
> +		ftrace_dump(DUMP_ORIG);
> +}
> +
> +void rcu_exit_nohz(void)
> +{
> +	unsigned long flags;
> +	struct rcu_dynticks *rdtp;
> +
> +	local_irq_save(flags);
> +	trace_dump_stack();
> +	rdtp = &__get_cpu_var(rcu_dynticks);
> +	if (WARN_ON_ONCE(rdtp->dynticks_nesting != 0))
> +		ftrace_dump(DUMP_ORIG);
> +	rdtp->dynticks_nesting = 0;
> +	__rcu_exit_nohz();
>  	local_irq_restore(flags);
>  }
> 
> @@ -378,15 +406,21 @@ void rcu_nmi_enter(void)
>  {
>  	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
> 
> +	trace_dump_stack();
> +
>  	if (rdtp->dynticks_nmi_nesting == 0 &&
> -	    (atomic_read(&rdtp->dynticks) & 0x1))
> +	    (atomic_read(&rdtp->dynticks) & 0x1)) {
> +		if (WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)))
> +			ftrace_dump(DUMP_ORIG);
>  		return;
> +	}
>  	rdtp->dynticks_nmi_nesting++;
>  	smp_mb__before_atomic_inc();  /* Force delay from prior write. */
>  	atomic_inc(&rdtp->dynticks);
>  	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
>  	smp_mb__after_atomic_inc();  /* See above. */
> -	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> +	if (WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)))
> +		ftrace_dump(DUMP_ORIG);
>  }
> 
>  /**
> @@ -400,14 +434,20 @@ void rcu_nmi_exit(void)
>  {
>  	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
> 
> +	trace_dump_stack();
> +
>  	if (rdtp->dynticks_nmi_nesting == 0 ||
> -	    --rdtp->dynticks_nmi_nesting != 0)
> +	    --rdtp->dynticks_nmi_nesting != 0) {
> +		if (WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1)))
> +			ftrace_dump(DUMP_ORIG);
>  		return;
> +	}
>  	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
>  	smp_mb__before_atomic_inc();  /* See above. */
>  	atomic_inc(&rdtp->dynticks);
>  	smp_mb__after_atomic_inc();  /* Force delay to next write. */
> -	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
> +	if (WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1))
> +		ftrace_dump(DUMP_ORIG);
>  }
> 
>  /**
> @@ -418,7 +458,12 @@ void rcu_nmi_exit(void)
>   */
>  void rcu_irq_enter(void)
>  {
> -	rcu_exit_nohz();
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	trace_dump_stack();
> +	__rcu_exit_nohz();
> +	local_irq_restore(flags);
>  }
> 
>  /**
> @@ -430,7 +475,12 @@ void rcu_irq_enter(void)
>   */
>  void rcu_irq_exit(void)
>  {
> -	rcu_enter_nohz();
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
> +	trace_dump_stack();
> +	__rcu_enter_nohz();
> +	local_irq_restore(flags);
>  }
> 
>  #ifdef CONFIG_SMP
> 
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