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

[Boqun Feng <boqun@kernel.org>]

On Wed, Mar 18, 2026 at 03:43:05PM +0100, Sebastian Andrzej Siewior wrote:
[..]
> > > > way that vanilla RCU's call_rcu_core() function takes an early exit if
> > > > interrupts are disabled.  Of course, vanilla RCU can rely on things like
> > > > the scheduling-clock interrupt to start any needed grace periods [1],
> > > > but SRCU will instead need to manually defer this work, perhaps using
> > > > workqueues or IRQ work.
> > > > 
> > > > In addition, rcutorture needs to be upgraded to sometimes invoke
> > > > ->call() with the scheduler pi lock held, but this change is not fixing
> > > > a regression, so could be deferred.  (There is already code in rcutorture
> > > > that invokes the readers while holding a scheduler pi lock.)
> > > > 
> > > > Given that RCU for this week through the end of March belongs to you guys,
> > > > if one of you can get this done by end of day Thursday, London time,
> > > > very good!  Otherwise, I can put something together.
> > > > 
> > > > Please let me know!
> > > 
> > > Given that the current locking does allow it and lockdep should have
> > > complained, I am curious if we could rule that out ;)
> 
> 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?
> 

Late to the party, so just make sure I understand the problem. The
problem is the wakeup in call_srcu() when it's called with scheduler
lock held, right? If so I think the current code works as what you
already explain, we defer the wakeup into a workqueue.

(but Paul, we are not talking about calling call_srcu(), that requires
some more work to get it work)

> > 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.
> 

Right, otherwise there is a lockdep bug ;-)

Regards,
Boqun