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

[Paul E. McKenney]

On Mon, Oct 29, 2018 at 11:24:42AM +0000, Ran Rozenstein wrote:
> Hi Paul and all,
> 
> > -----Original Message-----
> > From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-
> > owner@xxxxxxxxxxxxxxx] On Behalf Of Paul E. McKenney
> > Sent: Thursday, August 30, 2018 01:21
> > To: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: mingo@xxxxxxxxxx; jiangshanlai@xxxxxxxxx; dipankar@xxxxxxxxxx;
> > akpm@xxxxxxxxxxxxxxxxxxxx; mathieu.desnoyers@xxxxxxxxxxxx;
> > josh@xxxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; peterz@xxxxxxxxxxxxx;
> > rostedt@xxxxxxxxxxx; dhowells@xxxxxxxxxx; edumazet@xxxxxxxxxx;
> > fweisbec@xxxxxxxxx; oleg@xxxxxxxxxx; joel@xxxxxxxxxxxxxxxxx; Paul E.
> > McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxxxxxxxx> [ 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?

							Thanx, Paul

------------------------------------------------------------------------

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;
 	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;
 }
 
 /*