On 3/18/2026 2:42 PM, Paul E. McKenney wrote:
> On Wed, Mar 18, 2026 at 08:51:16AM -0700, Boqun Feng wrote:
>> 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.
>
> The issue is that call_rcu_tasks() (which is call_srcu() now) is
> also invoked with a scheduler pi/rq lock held, which results in a
> deadlock cycle. So the srcu_gp_start_if_needed() function's call to
> raw_spin_lock_irqsave_sdp_contention() must be deferred to the workqueue
> handler, not just the wake-up. And that in turn means that the callback
> point also needs to be passed to this handler.
>
> See this email thread:
>
> https://lore.kernel.org/all/cap01t75ekpvw+95nqnwg9p-1+kzvzojpn0nlat+28sf1b9w...@mail.gmail.com/
>
>> (but Paul, we are not talking about calling call_srcu(), that requires
>> some more work to get it work)
>
> Agreed, splitting srcu_gp_start_if_needed() and using a workqueue if
> interrupts were already disabled on entry. Otherwise, directly invoking
> the split-out portion of srcu_gp_start_if_needed().
>
> But we might be talking past each other.
>
Ah so it is an ABBA deadlock, not a ABA self-deadlock. I guess this is a
different issue, from the NMI issue? It is more of an issue of calling
call_srcu API with scheduler locks held.
Something like below I think:
CPU A (BPF tracepoint) CPU B (concurrent call_srcu)
---------------------------- ------------------------------------
[1] holds &rq->__lock
[2]
-> call_srcu
-> srcu_gp_start_if_needed
-> srcu_funnel_gp_start
-> spin_lock_irqsave_ssp_content...
-> holds srcu locks
[4] calls call_rcu_tasks_trace() [5] srcu_funnel_gp_start (cont..)
-> queue_delayed_work
-> call_srcu() -> __queue_work()
-> srcu_gp_start_if_needed() -> wake_up_worker()
-> srcu_funnel_gp_start() -> try_to_wake_up()
-> spin_lock_irqsave_ssp_contention() [6] WANTS rq->__lock
-> WANTS srcu locks
If I understand this, this looks like an issue that can happen independent
of the conversion of the spin locks.
thanks,
--
Joel Fernandes
