Re: [QUESTION] problems report: rcu_read_unlock_special() called in irq_exit() causes dead loop

[Joel Fernandes <joelagnelf@nvidia.com>]

On Thu, Jul 03, 2025 at 09:04:31AM +0800, Xiongfeng Wang wrote:
> 
> 
> On 2025/7/3 1:24, Joel Fernandes wrote:
> > 
> > 
> > On 7/2/2025 6:59 AM, Joel Fernandes wrote:
> >>
> >>
> >> On 7/2/2025 5:14 AM, Qi Xi wrote:
> >>> Hi Joel,
> >>>
> >>> After applying the 2 patches, the problem still exists. Compared to the previous
> >>> fixes which did solve the problem, the difference is ct_in_irq() in the first
> >>> patch.
> >>>
> >>> I am wondering why "nesting != CT_NESTING_IRQ_NONIDLE" is added?
> >>>
> >>>
> >>> (previous fix: problem is solved)
> >>>
> >>> +bool ct_in_irq(void)
> >>> +{
> >>> +    return ct_nmi_nesting() != 0;
> >>> +}
> >>>
> >>> (current fix: problem still exists)
> >>>
> >>> +bool ct_in_irq(void)
> >>> +{
> >>> +    long nesting = ct_nmi_nesting();
> >>> +
> >>> +    return (nesting && nesting != CT_NESTING_IRQ_NONIDLE);
> >>> +}
> >>
> >> Oh gosh, thanks for spotting that! Indeed,  I had changed it to != 0 in the last
> >> version but applied an older patch. I will fix it in the tree. Thank you again!
> >>
> >> Neeraj, would you like this as a separate commit that you can then squash? Or
> >> could you fix it up in your tree?
> >>
> > Qi, Xiongfeng, I am currently working on alternative fix after discussing with
> > the crew. I will keep you posted with the fix, and would it to be good to get
> > your testing on it once I have it (hopefully in couple of days), thanks for the
> > report!
> 
> Sure, we are glad to help test once we get the fix patch.

Could you try the following patch? I tested it and it fixes the issue for me.

If you prefer git, then cherry-pick the HEAD commit from:

https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/log/?h=rcu/irq-exit-v2-no-task
(cherry-pick sha a58cc91fdca766cb719fb8beee3bb10ffe8e9d58)

---8<---

From: Joel Fernandes <joelagnelf@nvidia.com>
Subject: [PATCH] rcu: Fix rcu_read_unlock() deadloop due to IRQ work

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/rcu/tree.h        | 11 ++++++++++-
 kernel/rcu/tree_plugin.h | 29 ++++++++++++++++++++++-------
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 3830c19cf2f6..f8f612269e6e 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -174,6 +174,15 @@ struct rcu_snap_record {
 	unsigned long   jiffies;	/* Track jiffies value */
 };
 
+/*
+ * The IRQ work (deferred_qs_iw) is used by RCU to get scheduler's attention.
+ * It can be in one of the following states:
+ * - DEFER_QS_IDLE: An IRQ work was never scheduled.
+ * - DEFER_QS_PENDING: An IRQ work was scheduler but never run.
+ */
+#define DEFER_QS_IDLE		0
+#define DEFER_QS_PENDING	1
+
 /* Per-CPU data for read-copy update. */
 struct rcu_data {
 	/* 1) quiescent-state and grace-period handling : */
@@ -192,7 +201,7 @@ struct rcu_data {
 					/*  during and after the last grace */
 					/* period it is aware of. */
 	struct irq_work defer_qs_iw;	/* Obtain later scheduler attention. */
-	bool defer_qs_iw_pending;	/* Scheduler attention pending? */
+	int defer_qs_iw_pending;	/* Scheduler attention pending? */
 	struct work_struct strict_work;	/* Schedule readers for strict GPs. */
 
 	/* 2) batch handling */
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index dd1c156c1759..baf57745b42f 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -486,13 +486,16 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
 	struct rcu_node *rnp;
 	union rcu_special special;
 
+	rdp = this_cpu_ptr(&rcu_data);
+	if (rdp->defer_qs_iw_pending == DEFER_QS_PENDING)
+		rdp->defer_qs_iw_pending = DEFER_QS_IDLE;
+
 	/*
 	 * If RCU core is waiting for this CPU to exit its critical section,
 	 * report the fact that it has exited.  Because irqs are disabled,
 	 * t->rcu_read_unlock_special cannot change.
 	 */
 	special = t->rcu_read_unlock_special;
-	rdp = this_cpu_ptr(&rcu_data);
 	if (!special.s && !rdp->cpu_no_qs.b.exp) {
 		local_irq_restore(flags);
 		return;
@@ -623,12 +626,24 @@ notrace void rcu_preempt_deferred_qs(struct task_struct *t)
  */
 static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
 {
-	unsigned long flags;
-	struct rcu_data *rdp;
+	volatile unsigned long flags;
+	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
-	rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
 	local_irq_save(flags);
-	rdp->defer_qs_iw_pending = false;
+
+	/*
+	 * Requeue the IRQ work on next unlock in following situation:
+	 * 1. rcu_read_unlock() queues IRQ work (state -> DEFER_QS_PENDING)
+	 * 2. CPU enters new rcu_read_lock()
+	 * 3. IRQ work runs but cannot report QS due to rcu_preempt_depth() > 0
+	 * 4. rcu_read_unlock() does not re-queue work (state still PENDING)
+	 * 5. Deferred QS reporting does not happen.
+	 */
+	if (rcu_preempt_depth() > 0) {
+		WRITE_ONCE(rdp->defer_qs_iw_pending, DEFER_QS_IDLE);
+		local_irq_restore(flags);
+		return;
+	}
 	local_irq_restore(flags);
 }
 
@@ -675,7 +690,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 			set_tsk_need_resched(current);
 			set_preempt_need_resched();
 			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
-			    expboost && !rdp->defer_qs_iw_pending && cpu_online(rdp->cpu)) {
+			    expboost && rdp->defer_qs_iw_pending != DEFER_QS_PENDING && cpu_online(rdp->cpu)) {
 				// Get scheduler to re-evaluate and call hooks.
 				// If !IRQ_WORK, FQS scan will eventually IPI.
 				if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
@@ -685,7 +700,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 				else
 					init_irq_work(&rdp->defer_qs_iw,
 						      rcu_preempt_deferred_qs_handler);
-				rdp->defer_qs_iw_pending = true;
+				rdp->defer_qs_iw_pending = DEFER_QS_PENDING;
 				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
 			}
 		}
-- 
2.43.0