Re: Next-level bug in SRCU implementation of RCU Tasks Trace + PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-03-18 08:43:32 [-0700], Paul E. McKenney wrote:
> > Your patch just s/spinlock_t/raw_spinlock_t so we get the locking/
> > nesting right. The wakeup problem remains, right?
> > But looking at the code, there is just srcu_funnel_gp_start(). If its
> > srcu_schedule_cbs_sdp() / queue_delayed_work() usage is always delayed
> > then there will be always a timer and never a direct wake up of the
> > worker. Wouldn't that work?
> 
> Right, that patch fixes one lockdep problem, but another remains.

What remains?

> > > It would be nice, but your point about needing to worry about spinlocks
> > > is compelling.
> > > 
> > > But couldn't lockdep scan the current task's list of held locks and see
> > > whether only raw spinlocks are held (including when no spinlocks of any
> > > type are held), and complain in that case?  Or would that scanning be
> > > too high of overhead?  (But we need that scan anyway to check deadlock,
> > > don't we?)
> > 
> > PeterZ didn't like it and the nesting thing identified most of the
> > problem cases. It should also catch _this_ one.
> > 
> > Thinking about it further, you don't need to worry about
> > local_bh_disable() but RCU will becomes another corner case. You would
> > have to exclude "rcu_read_lock(); spin_lock();" on a !preempt kernel
> > which would otherwise lead to false positives.
> > But as I said, this case as explained is a nesting problem and should be
> > reported by lockdep with its current features.
> 
> With a raw spinlock held, agreed.
> 
> Not a big deal, just working out what to put in rcutorture to avoid
> regressions that would otherwise result in being unable to invoke
> call_srcu() from non-preemptible contexts.

Okay. So take this as _no_ more work items ;)

> > > > > 						Thanx, Paul [2]
> > > > > 
> > > > > [1] The exceptions to this rule being handled by the call to
> > > > >     invoke_rcu_core() when rcu_is_watching() returns false.
> > > > > 
> > > > > [2] Ah, and should vanilla RCU's call_rcu() be invokable from NMI
> > > > >     handlers?  Or should there be a call_rcu_nmi() for this purpose?
> > > > >     Or should we continue to have its callers check in_nmi() when needed?
> > > > 
> > > > Did someone ask for this?
> > > 
> > > Yes.  The BPF guys need to invoke call_srcu() from interrupts-disabled
> > > regions of code.  I am way to old and lazy to do this sort of thing
> > > spontaneously.  ;-)
> > 
> > IRQ disabled should work but you asked about call_rcu_nmi() and NMI is
> > already complicated because "most" other things don't work and you would
> > need irq_work to let the remaining kernel know that you did something in
> > NMI and this needs to be integrated now. I don't think regular RCU has
> > call_rcu() from NMI. But I guess wrapping it via irq_work would be one
> > way of dealing with it.
> 
> Agreed, and as long as there is only a few call_rcu() call sites within
> NMI handlers, it is best to let the caller deal with it.  But if this
> becomes popular enough, it would be better to have a call_rcu_nmi() or
> some such.

Popular? Okay. Keep me posted, please.

> 							Thanx, Paul

Sebastian