On Tue, Jul 08, 2025 at 10:22:19AM -0400, Joel Fernandes wrote:
> 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 <wangxiongfe...@huawei.com>
> Closes:
> https://lore.kernel.org/all/9acd5f9f-6732-7701-6880-4b51190aa...@huawei.com/
> Tested-by: Qi Xi <xi...@huawei.com>
> Signed-off-by: Joel Fernandes <joelagn...@nvidia.com>
Reviewed-by: Paul E. McKenney <paul...@kernel.org>
> ---
> 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
>
