Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

[Joel Fernandes]

Hi Paul,

On Fri, Jun 28, 2019 at 01:04:23PM -0700, Paul E. McKenney wrote:
[snip]
> > > > Commit
> > > > - 23634ebc1d946 ("rcu: Check for wakeup-safe conditions in
> > > >    rcu_read_unlock_special()") does not trigger the bug within 94
> > > >    attempts.
> > > > 
> > > > - 48d07c04b4cc1 ("rcu: Enable elimination of Tree-RCU softirq
> > > >   processing") needed 12 attempts to trigger the bug.
> > > 
> > > That matches my belief that 23634ebc1d946 ("rcu: Check for wakeup-safe
> > > conditions in rcu_read_unlock_special()") will at least greatly decrease
> > > the probability of this bug occurring.
> > 
> > I was just typing a reply that I can't reproduce it with:
> >   rcu: Check for wakeup-safe conditions in rcu_read_unlock_special()
> > 
> > I am trying to revert enough of this patch to see what would break things,
> > however I think a better exercise might be to understand more what the patch
> > does why it fixes things in the first place ;-) It is probably the
> > deferred_qs thing.
> 
> The deferred_qs flag is part of it!  Looking forward to hearing what
> you come up with as being the critical piece of this commit.

The new deferred_qs flag indeed saves the machine from the dead-lock.

If we don't want the deferred_qs, then the below patch also fixes the issue.
However, I am more sure than not that it does not handle all cases (such as
what if we previously had an expedited grace period IPI in a previous reader
section and had to to defer processing. Then it seems a similar deadlock
would present. But anyway, the below patch does fix it for me! It is based on
your -rcu tree commit 23634ebc1d946f19eb112d4455c1d84948875e31 (rcu: Check
for wakeup-safe conditions in rcu_read_unlock_special()).

---8<-----------------------

From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
Subject: [PATCH] Fix RCU recursive deadlock

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 include/linux/sched.h    |  2 +-
 kernel/rcu/tree_plugin.h | 17 +++++++++++++----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 942a44c1b8eb..347e6dfcc91b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -565,7 +565,7 @@ union rcu_special {
 		u8			blocked;
 		u8			need_qs;
 		u8			exp_hint; /* Hint for performance. */
-		u8			deferred_qs;
+		u8			pad;
 	} b; /* Bits. */
 	u32 s; /* Set of bits. */
 };
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 75110ea75d01..5b9b12c1ba5c 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -455,7 +455,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 		local_irq_restore(flags);
 		return;
 	}
-	t->rcu_read_unlock_special.b.deferred_qs = false;
 	if (special.b.need_qs) {
 		rcu_qs();
 		t->rcu_read_unlock_special.b.need_qs = false;
@@ -608,13 +607,24 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	if (preempt_bh_were_disabled || irqs_were_disabled) {
 		t->rcu_read_unlock_special.b.exp_hint = false;
 		// Need to defer quiescent state until everything is enabled.
+
+		/* If unlock_special was called in the current reader section
+		 * just because we were blocked in a previous reader section,
+		 * then raising softirqs can deadlock. This is because the
+		 * scheduler executes RCU sections with preemption disabled,
+		 * however it may have previously blocked in a previous
+		 * non-scheduler reader section and .blocked got set.  It is
+		 * never safe to call unlock_special from the scheduler path
+		 * due to recursive wake ups (unless we are in_irq(), so
+		 * prevent this by checking if we were previously blocked.
+		 */
 		if (irqs_were_disabled && use_softirq &&
-		    (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) {
+		    (!t->rcu_read_unlock_special.b.blocked || in_irq())) {
 			// Using softirq, safe to awaken, and we get
 			// no help from enabling irqs, unlike bh/preempt.
 			raise_softirq_irqoff(RCU_SOFTIRQ);
 		} else if (irqs_were_disabled && !use_softirq &&
-			   !t->rcu_read_unlock_special.b.deferred_qs) {
+			   !t->rcu_read_unlock_special.b.blocked) {
 			// Safe to awaken and we get no help from enabling
 			// irqs, unlike bh/preempt.
 			invoke_rcu_core();
@@ -623,7 +633,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
 			set_tsk_need_resched(current);
 			set_preempt_need_resched();
 		}
-		t->rcu_read_unlock_special.b.deferred_qs = true;
 		local_irq_restore(flags);
 		return;
 	}
-- 
2.22.0.410.gd8fdbe21b5-goog