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

[Boqun Feng]

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.
> 
> irq_exit()
>   __irq_exit_rcu()
>     /* in_hardirq() returns false after this */
>     preempt_count_sub(HARDIRQ_OFFSET)
>     tick_irq_exit()

@Frederic, while we are at it, what's the purpose of in_hardirq() in
tick_irq_exit()? For nested interrupt detection?

Regards,
Boqun

>       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@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/9acd5f9f-6732-7701-6880-4b51190aa070@xxxxxxxxxx/
> Tested-by: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>
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