Re: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt quiescent states when disabled

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Oct 29, 2018 at 07:27:35AM -0700, Paul E. McKenney wrote:
> On Mon, Oct 29, 2018 at 11:24:42AM +0000, Ran Rozenstein wrote:
> > Hi Paul and all,
> > 
> > > -----Original Message-----
> > > From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-
> > > owner@vger.kernel.org] On Behalf Of Paul E. McKenney
> > > Sent: Thursday, August 30, 2018 01:21
> > > To: linux-kernel@vger.kernel.org
> > > Cc: mingo@kernel.org; jiangshanlai@gmail.com; dipankar@in.ibm.com;
> > > akpm@linux-foundation.org; mathieu.desnoyers@efficios.com;
> > > josh@joshtriplett.org; tglx@linutronix.de; peterz@infradead.org;
> > > rostedt@goodmis.org; dhowells@redhat.com; edumazet@google.com;
> > > fweisbec@gmail.com; oleg@redhat.com; joel@joelfernandes.org; Paul E.
> > > McKenney <paulmck@linux.vnet.ibm.com>
> > > Subject: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt
> > > quiescent states when disabled
> > > 
> > > This commit defers reporting of RCU-preempt quiescent states at
> > > rcu_read_unlock_special() time when any of interrupts, softirq, or
> > > preemption are disabled.  These deferred quiescent states are reported at a
> > > later RCU_SOFTIRQ, context switch, idle entry, or CPU-hotplug offline
> > > operation.  Of course, if another RCU read-side critical section has started in
> > > the meantime, the reporting of the quiescent state will be further deferred.
> > > 
> > > This also means that disabling preemption, interrupts, and/or softirqs will act
> > > as an RCU-preempt read-side critical section.
> > > This is enforced by checking preempt_count() as needed.
> > > 
> > > Some special cases must be handled on an ad-hoc basis, for example,
> > > context switch is a quiescent state even though both the scheduler and
> > > do_exit() disable preemption.  In these cases, additional calls to
> > > rcu_preempt_deferred_qs() override the preemption disabling.  Similar logic
> > > overrides disabled interrupts in rcu_preempt_check_callbacks() because in
> > > this case the quiescent state happened just before the corresponding
> > > scheduling-clock interrupt.
> > > 
> > > In theory, this change lifts a long-standing restriction that required that if
> > > interrupts were disabled across a call to rcu_read_unlock() that the matching
> > > rcu_read_lock() also be contained within that interrupts-disabled region of
> > > code.  Because the reporting of the corresponding RCU-preempt quiescent
> > > state is now deferred until after interrupts have been enabled, it is no longer
> > > possible for this situation to result in deadlocks involving the scheduler's
> > > runqueue and priority-inheritance locks.  This may allow some code
> > > simplification that might reduce interrupt latency a bit.  Unfortunately, in
> > > practice this would also defer deboosting a low-priority task that had been
> > > subjected to RCU priority boosting, so real-time-response considerations
> > > might well force this restriction to remain in place.
> > > 
> > > Because RCU-preempt grace periods are now blocked not only by RCU read-
> > > side critical sections, but also by disabling of interrupts, preemption, and
> > > softirqs, it will be possible to eliminate RCU-bh and RCU-sched in favor of
> > > RCU-preempt in CONFIG_PREEMPT=y kernels.  This may require some
> > > additional plumbing to provide the network denial-of-service guarantees
> > > that have been traditionally provided by RCU-bh.  Once these are in place,
> > > CONFIG_PREEMPT=n kernels will be able to fold RCU-bh into RCU-sched.
> > > This would mean that all kernels would have but one flavor of RCU, which
> > > would open the door to significant code cleanup.
> > > 
> > > Moving to a single flavor of RCU would also have the beneficial effect of
> > > reducing the NOCB kthreads by at least a factor of two.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> [ paulmck:
> > > Apply rcu_read_unlock_special() preempt_count() feedback
> > >   from Joel Fernandes. ]
> > > [ paulmck: Adjust rcu_eqs_enter() call to rcu_preempt_deferred_qs() in
> > >   response to bug reports from kbuild test robot. ] [ paulmck: Fix bug located
> > > by kbuild test robot involving recursion
> > >   via rcu_preempt_deferred_qs(). ]
> > > ---
> > >  .../RCU/Design/Requirements/Requirements.html |  50 +++---
> > >  include/linux/rcutiny.h                       |   5 +
> > >  kernel/rcu/tree.c                             |   9 ++
> > >  kernel/rcu/tree.h                             |   3 +
> > >  kernel/rcu/tree_exp.h                         |  71 +++++++--
> > >  kernel/rcu/tree_plugin.h                      | 144 +++++++++++++-----
> > >  6 files changed, 205 insertions(+), 77 deletions(-)
> > > 
> > 
> > We started seeing the trace below in our regression system, after I bisected I found this is the offending commit.
> > This appears immediately on boot. 
> > Please let me know if you need any additional details.
> 
> Interesting.  Here is the offending function:
> 
> 	static void rcu_preempt_deferred_qs(struct task_struct *t)
> 	{
> 		unsigned long flags;
> 		bool couldrecurse = t->rcu_read_lock_nesting >= 0;
> 
> 		if (!rcu_preempt_need_deferred_qs(t))
> 			return;
> 		if (couldrecurse)
> 			t->rcu_read_lock_nesting -= INT_MIN;
> 		local_irq_save(flags);
> 		rcu_preempt_deferred_qs_irqrestore(t, flags);
> 		if (couldrecurse)
> 			t->rcu_read_lock_nesting += INT_MIN;
> 	}
> 
> Using twos-complement arithmetic (which the kernel build gcc arguments
> enforce, last I checked) this does work.  But as UBSAN says, subtracting
> INT_MIN is unconditionally undefined behavior according to the C standard.
> 
> Good catch!!!
> 
> So how do I make the above code not simply function, but rather meet
> the C standard?
> 
> One approach to add INT_MIN going in, then add INT_MAX and then add 1
> coming out.
> 
> Another approach is to sacrifice the INT_MAX value (should be plenty
> safe), thus subtract INT_MAX going in and add INT_MAX coming out.
> For consistency, I suppose that I should change the INT_MIN in
> __rcu_read_unlock() to -INT_MAX.
> 
> I could also leave __rcu_read_unlock() alone and XOR the top
> bit of t->rcu_read_lock_nesting on entry and exit to/from
> rcu_preempt_deferred_qs().
> 
> Sacrificing the INT_MIN value seems most maintainable, as in the following
> patch.  Thoughts?

The INT_MAX naming could be very confusing for nesting levels, could we not
instead just define something like:
#define RCU_NESTING_MIN (INT_MIN - 1)
#define RCU_NESTING_MAX (INT_MAX)

and just use that? also one more comment below:

> 
> ------------------------------------------------------------------------
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index bd8186d0f4a7..f1b40c6d36e4 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -424,7 +424,7 @@ void __rcu_read_unlock(void)
>  		--t->rcu_read_lock_nesting;
>  	} else {
>  		barrier();  /* critical section before exit code. */
> -		t->rcu_read_lock_nesting = INT_MIN;
> +		t->rcu_read_lock_nesting = -INT_MAX;
>  		barrier();  /* assign before ->rcu_read_unlock_special load */
>  		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
>  			rcu_read_unlock_special(t);
> @@ -617,11 +617,11 @@ static void rcu_preempt_deferred_qs(struct task_struct *t)
>  	if (!rcu_preempt_need_deferred_qs(t))
>  		return;
>  	if (couldrecurse)
> -		t->rcu_read_lock_nesting -= INT_MIN;
> +		t->rcu_read_lock_nesting -= INT_MAX;

Shouldn't this be:  t->rcu_read_lock_nesting -= -INT_MAX; ?

>  	local_irq_save(flags);
>  	rcu_preempt_deferred_qs_irqrestore(t, flags);
>  	if (couldrecurse)
> -		t->rcu_read_lock_nesting += INT_MIN;
> +		t->rcu_read_lock_nesting += INT_MAX;

And t->rcu_read_lock_nesting += -INT_MAX; ?

But apologies if I missed something, thanks,

 - Joel