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

[Frederic Weisbecker]

Le Mon, Jun 09, 2025 at 12:49:06PM -0700, Boqun Feng a écrit :
> 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?

If !ct_in_irq(), we issue a self-IPI, then preempt_schedule_irq() will
call into schedule() and report a QS (if preempt/bh is not disabled, otherwise
this is delayed to preempt_enable() or local_bh_enable() issuing preempt_schedule())

If ct_in_irq(), we are already in an IRQ, then it's the same as above
eventually.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs