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Message-Id: <20250708142224.3940851-2-joelagnelf@nvidia.com>
Date: Tue, 8 Jul 2025 10:22:19 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: linux-kernel@...r.kernel.org,
"Paul E. McKenney" <paulmck@...nel.org>,
Frederic Weisbecker <frederic@...nel.org>,
Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
Joel Fernandes <joelagnelf@...dia.com>,
Josh Triplett <josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>,
Uladzislau Rezki <urezki@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Zqiang <qiang.zhang@...ux.dev>
Cc: Xiongfeng Wang <wangxiongfeng2@...wei.com>,
Qi Xi <xiqi2@...wei.com>,
rcu@...r.kernel.org,
bpf@...r.kernel.org
Subject: [PATCH -rcu -next 2/7] rcu: Fix rcu_read_unlock() deadloop due to IRQ work
During rcu_read_unlock_special(), if this happens during irq_exit(), we
can lockup if an IPI is issued. This is because the IPI itself triggers
the irq_exit() path causing a recursive lock up.
This is precisely what Xiongfeng found when invoking a BPF program on
the trace_tick_stop() tracepoint As shown in the trace below. Fix by
managing the irq_work state correctly.
irq_exit()
__irq_exit_rcu()
/* in_hardirq() returns false after this */
preempt_count_sub(HARDIRQ_OFFSET)
tick_irq_exit()
tick_nohz_irq_exit()
tick_nohz_stop_sched_tick()
trace_tick_stop() /* a bpf prog is hooked on this trace point */
__bpf_trace_tick_stop()
bpf_trace_run2()
rcu_read_unlock_special()
/* will send a IPI to itself */
irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
A simple reproducer can also be obtained by doing the following in
tick_irq_exit(). It will hang on boot without the patch:
static inline void tick_irq_exit(void)
{
+ rcu_read_lock();
+ WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
+ rcu_read_unlock();
+
Reported-by: Xiongfeng Wang <wangxiongfeng2@...wei.com>
Closes: https://lore.kernel.org/all/9acd5f9f-6732-7701-6880-4b51190aa070@huawei.com/
Tested-by: Qi Xi <xiqi2@...wei.com>
Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
---
kernel/rcu/tree.h | 11 ++++++++++-
kernel/rcu/tree_plugin.h | 23 +++++++++++++++++++----
2 files changed, 29 insertions(+), 5 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..fa7b0d854833 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;
@@ -628,7 +631,18 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
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);
}
@@ -675,7 +689,8 @@ 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);
}
}
--
2.34.1
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