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

Joel Fernandes
2019-06-28T21:40:25Z

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@joelfernandes.org>
Subject: [PATCH] Fix RCU recursive deadlock

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
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