Re: [PATCH 2/2] rcu: Fix lockup when RCU reader used while IRQ exiting

[Boqun Feng <boqun.feng@gmail.com>]

Hi Joel,

On Mon, Jun 09, 2025 at 02:01:24PM -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
> using context-tracking to tell us if we're still in an IRQ.
> context-tracking keeps track of the IRQ until after the tracepoint, so
> it cures the issues.
> 

This does fix the issue, but do we know when the CPU will eventually
report a QS after this fix? I believe we still want to report a QS as
early as possible in this case?

Regards,
Boqun

> 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();
>  +
> 
> While at it, add some comments to this code.
> 
> Reported-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
> Closes: https://lore.kernel.org/all/9acd5f9f-6732-7701-6880-4b51190aa070@huawei.com/
> Tested-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
>  kernel/rcu/tree_plugin.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 3c0bbbbb686f..53d8b3415776 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -653,6 +653,9 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  		struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  		struct rcu_node *rnp = rdp->mynode;
>  
> +		// In cases where the RCU-reader is boosted, we'd attempt deboost sooner than
> +		// later to prevent inducing latency to other RT tasks. Also, expedited GPs
> +		// should not be delayed too much. Track both these needs in expboost.
>  		expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
>  			   (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
>  			   (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
> @@ -670,10 +673,15 @@ static void rcu_read_unlock_special(struct task_struct *t)
>  			// Also if no expediting and no possible deboosting,
>  			// slow is OK.  Plus nohz_full CPUs eventually get
>  			// tick enabled.
> +			//
> +			// Also prevent doing this if context-tracking thinks
> +			// we're handling an IRQ (including when we're exiting
> +			// one -- required to prevent self-IPI deadloops).
>  			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 && cpu_online(rdp->cpu) &&
> +			    !ct_in_irq()) {
>  				// Get scheduler to re-evaluate and call hooks.
>  				// If !IRQ_WORK, FQS scan will eventually IPI.
>  				if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
> -- 
> 2.34.1
> 
>