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

[Boqun Feng <boqun@kernel.org>]

On Thu, Mar 19, 2026 at 03:02:57AM -0700, Paul E. McKenney wrote:
> On Wed, Mar 18, 2026 at 06:08:21PM -0700, Boqun Feng wrote:
> > On Wed, Mar 18, 2026 at 04:27:23PM -0700, Boqun Feng wrote:
> > > On Wed, Mar 18, 2026 at 06:52:53PM -0400, Joel Fernandes wrote:
> > > > 
> > > > 
> > > > On 3/18/2026 6:15 PM, Boqun Feng wrote:
> > > > > On Wed, Mar 18, 2026 at 02:55:48PM -0700, Boqun Feng wrote:
> > > > >> On Wed, Mar 18, 2026 at 02:52:48PM -0700, Boqun Feng wrote:
> > > > >> [...]
> > > > >>>> 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
> > > > >>>
> > > > >>> I see, we can also have a self deadlock even without CPU B, when CPU A
> > > > >>> is going to try_to_wake_up() the a worker on the same CPU.
> > > > >>>
> > > > >>> An interesting observation is that the deadlock can be avoided in
> > > > >>> queue_delayed_work() uses a non-zero delay, that means a timer will be
> > > > >>> armed instead of acquiring the rq lock.
> > > > >>>
> > > > > 
> > > > > If my observation is correct, then this can probably fix the deadlock
> > > > > issue with runqueue lock (untested though), but it won't work if BPF
> > > > > tracepoint can happen with timer base lock held.
> > > > > 
> > > > > Regards,
> > > > > Boqun
> > > > > 
> > > > > ------>
> > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > > > index 2328827f8775..a5d67264acb5 100644
> > > > > --- a/kernel/rcu/srcutree.c
> > > > > +++ b/kernel/rcu/srcutree.c
> > > > > @@ -1061,6 +1061,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> > > > >         struct srcu_node *snp_leaf;
> > > > >         unsigned long snp_seq;
> > > > >         struct srcu_usage *sup = ssp->srcu_sup;
> > > > > +       bool irqs_were_disabled;
> > > > > 
> > > > >         /* Ensure that snp node tree is fully initialized before traversing it */
> > > > >         if (smp_load_acquire(&sup->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER)
> > > > > @@ -1098,6 +1099,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> > > > > 
> > > > >         /* Top of tree, must ensure the grace period will be started. */
> > > > >         raw_spin_lock_irqsave_ssp_contention(ssp, &flags);
> > > > > +       irqs_were_disabled = irqs_disabled_flags(flags);
> > > > >         if (ULONG_CMP_LT(sup->srcu_gp_seq_needed, s)) {
> > > > >                 /*
> > > > >                  * Record need for grace period s.  Pair with load
> > > > > @@ -1118,9 +1120,16 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> > > > >                 // it isn't.  And it does not have to be.  After all, it
> > > > >                 // can only be executed during early boot when there is only
> > > > >                 // the one boot CPU running with interrupts still disabled.
> > > > > +               //
> > > > > +               // If irq was disabled when call_srcu() is called, then we
> > > > > +               // could be in the scheduler path with a runqueue lock held,
> > > > > +               // delay the process_srcu() work 1 more jiffies so we don't go
> > > > > +               // through the kick_pool() -> wake_up_process() path below, and
> > > > > +               // we could avoid deadlock with runqueue lock.
> > > > >                 if (likely(srcu_init_done))
> > > > >                         queue_delayed_work(rcu_gp_wq, &sup->work,
> > > > > -                                          !!srcu_get_delay(ssp));
> > > > > +                                          !!srcu_get_delay(ssp) +
> > > > > +                                          !!irqs_were_disabled);
> > > > Nice, I wonder if it is better to do this in __queue_delayed_work() itself.
> > > > Do we have queue_delayed_work() with zero delays that are in irq-disabled
> > > > regions, and they depend on that zero-delay for correctness? Even with
> > > > delay of 0 though, the work item doesn't execute right away anyway, the
> > > > worker thread has to also be scheduler right?
> > > > 
> > > > Also if IRQ is disabled, I'd think this is a critical path that is not
> > > > wanting to run the work item right-away anyway since workqueue is more a
> > > > bottom-half mechanism, than "run this immediately".
> > > > 
> > > > IOW, would be good to make the workqueue-layer more resilient to waking up
> > > > the scheduler when a delay would have been totally ok. But maybe +Tejun can
> > > > yell if that sounds insane.
> > > > 
> > > 
> > > I think all of these are probably a good point. However my fix is not
> > > complete :( It's missing the ABBA case in your example (it obviously
> > > could solve the self deadlock if my observation is correct), because we
> > > will still build rcu_node::lock -> runqueue::lock in some conditions,
> > > and BPF contributes the runqueue::lock -> rcu_node::lock dependency.
> > > Hence we still have ABBA deadlock.
> > > 
> > > To remove the rcu_node::lock -> runqueue::lock entirely, we need to
> > > always delay 1+ jiffies:
> > > 
> > 
> > Hmm.. or I can do as the old call_rcu_tasks_trace() does: using an
> > irq_work. I also pushed it at:
> > 
> > 	https://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git/ srcu-fix
> > 
> > (based on Paul's fix on spinlock already, but only lightly build test).
> > 
> > Regards,
> > Boqun
> > 
> > -------------------------->8
> > Subject: [PATCH] rcu: Use an intermediate irq_work to start process_srcu()
> > 
> > Since commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms
> > of SRCU-fast") we switched to SRCU in BPF. However as BPF instrument can
> > happen basically everywhere (including where a scheduler lock is held),
> > call_srcu() now needs to avoid acquiring scheduler lock because
> > otherwise it could cause deadlock [1]. Fix this by following what the
> > previous RCU Tasks Trace did: using an irq_work to delay the queuing of
> > the work to start process_srcu().
> > 
> > Fixes: commit c27cea4416a3 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast")
> > Link: https://lore.kernel.org/rcu/3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com/ [1]
> > Signed-off-by: Boqun Feng <boqun@kernel.org>
> > ---
> >  include/linux/srcutree.h |  1 +
> >  kernel/rcu/srcutree.c    | 22 ++++++++++++++++++++--
> >  2 files changed, 21 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> > index b122c560a59c..fd1a9270cb9a 100644
> > --- a/include/linux/srcutree.h
> > +++ b/include/linux/srcutree.h
> > @@ -95,6 +95,7 @@ struct srcu_usage {
> >  	unsigned long reschedule_jiffies;
> >  	unsigned long reschedule_count;
> >  	struct delayed_work work;
> > +	struct irq_work irq_work;
> >  	struct srcu_struct *srcu_ssp;
> >  };
> >  
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 2328827f8775..57116635e72d 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -19,6 +19,7 @@
> >  #include <linux/mutex.h>
> >  #include <linux/percpu.h>
> >  #include <linux/preempt.h>
> > +#include <linux/irq_work.h>
> >  #include <linux/rcupdate_wait.h>
> >  #include <linux/sched.h>
> >  #include <linux/smp.h>
> > @@ -75,6 +76,7 @@ static bool __read_mostly srcu_init_done;
> >  static void srcu_invoke_callbacks(struct work_struct *work);
> >  static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay);
> >  static void process_srcu(struct work_struct *work);
> > +static void srcu_irq_work(struct irq_work *work);
> >  static void srcu_delay_timer(struct timer_list *t);
> >  
> >  /*
> > @@ -216,6 +218,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
> >  	mutex_init(&ssp->srcu_sup->srcu_barrier_mutex);
> >  	atomic_set(&ssp->srcu_sup->srcu_barrier_cpu_cnt, 0);
> >  	INIT_DELAYED_WORK(&ssp->srcu_sup->work, process_srcu);
> > +	init_irq_work(&ssp->srcu_sup->irq_work, srcu_irq_work);
> >  	ssp->srcu_sup->sda_is_static = is_static;
> >  	if (!is_static) {
> >  		ssp->sda = alloc_percpu(struct srcu_data);
> > @@ -1118,9 +1121,13 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> >  		// it isn't.  And it does not have to be.  After all, it
> >  		// can only be executed during early boot when there is only
> >  		// the one boot CPU running with interrupts still disabled.
> > +		//
> > +		// Use an irq_work here to avoid acquiring runqueue lock with
> > +		// srcu rcu_node::lock held. BPF instrument could introduce the
> > +		// opposite dependency, hence we need to break the possible
> > +		// locking dependency here.
> 
> If I understand the lockdep splat, you need to bail out earlier on,
> prior to the first lock acquisition.
> 

I think you're talking about another direction of the dependency ;-)

Joel's example shows both depenedencies clearly:

  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

To remove [1] -> [4], i.e. the dependency "&rq->__lock" -> "srcu locks",
yes, you need to bail out earlier on. But to remove [2] -> [6], i.e. the
dependency "srcu locks" -> "&rq->__lock", you just need to make sure
call_srcu() won't call anything that need to acquire the rq lock.

In the old version of call_rcu_tasks_trace(), we were fine also because
[2] -> [6] didn't exist ([6] is rcu node lock in this case). [1] -> [4]
([4] being rcu node lock) existed in the RCU Tasks Trace version:


	// Enqueue a callback for the specified flavor of Tasks RCU.
	static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
					   struct rcu_tasks *rtp)
	{
		...
		if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
			raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
			...
		}
		...
		rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
		...
		/* We can't create the thread unless interrupts are enabled. */
		if (needwake && READ_ONCE(rtp->kthread_ptr))
			irq_work_queue(&rtpcp->rtp_irq_work);
	}

Hope it helps.

Regards,
Boqun

> 							Thanx, Paul
> 
> >  		if (likely(srcu_init_done))
> > -			queue_delayed_work(rcu_gp_wq, &sup->work,
> > -					   !!srcu_get_delay(ssp));
> > +			irq_work_queue(&sup->irq_work);
> >  		else if (list_empty(&sup->work.work.entry))
> >  			list_add(&sup->work.work.entry, &srcu_boot_list);
> >  	}
> > @@ -1979,6 +1986,17 @@ static void process_srcu(struct work_struct *work)
> >  	srcu_reschedule(ssp, curdelay);
> >  }
> >  
> > +static void srcu_irq_work(struct irq_work *work)
> > +{
> > +	struct srcu_struct *ssp;
> > +	struct srcu_usage *sup;
> > +
> > +	sup = container_of(work, struct srcu_usage, irq_work);
> > +	ssp = sup->srcu_ssp;
> > +
> > +	queue_delayed_work(rcu_gp_wq, &sup->work, !!srcu_get_delay(ssp));
> > +}
> > +
> >  void srcutorture_get_gp_data(struct srcu_struct *ssp, int *flags,
> >  			     unsigned long *gp_seq)
> >  {
> > -- 
> > 2.50.1 (Apple Git-155)
> >