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

["Paul E. McKenney" <paulmck@kernel.org>]

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.

							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)
>