Re: [PATCH] rcu: Use an intermediate irq_work to start process_srcu()

[Boqun Feng <boqun@kernel.org>]

On Sat, Mar 21, 2026 at 10:41:47AM -0700, Paul E. McKenney wrote:
[...]
> > > +	raw_spin_lock_rcu_node(ssp->srcu_sup);
> > > +	delay = srcu_get_delay(ssp);
> > > +	raw_spin_unlock_rcu_node(ssp->srcu_sup);
> > > 
> > 
> > It was fixed differently in v2:
> > 
> > 	https://lore.kernel.org/rcu/20260320222916.19987-1-boqun@kernel.org/
> > 
> > I used _irqsave/_irqrestore just in case. Given it's an urgent fix,
> > overly careful code is probably fine ;-)
> > 
> > Thanks for the testing and feedback.
> 
> OK, I will try that one, thank you!
> 
> FYI, with my change on your earlier version, SRCU-T got deadlocks between
> the pi-lock and the workqueue pool lock.  Which might or might not be
> particularly urgent.
> 

I just checked my run yesterday, I also hit it. It's probably what
Zqiang has found:

	https://lore.kernel.org/rcu/4c23c66f86a2aff8f2d7b759f9dd257b82147a17@linux.dev/

We have a queue_work_on() in srcu_schedule_cbs_sdp(), so

    srcu_torture_deferred_free():
      raw_spin_lock_irqsave(->pi_lock,...);
      call_srcu():
        if (snp == snp_leaf && snp_seq != s) {
           srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0):
	     if (!delay)
               queue_work_on(...)

I was about to reply to Zqiang, fixing that could be a touch design
decision. Since it's a per srcu_data work ;-) NR_CPUS x irq_work
incoming.

Regards,
Boqun

> 							Thanx, Paul
>