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

[Boqun Feng <boqun@kernel.org>]

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:

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 2328827f8775..86733f7bf637 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1118,9 +1118,13 @@ 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.
+               //
+               // 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) + 1);
                else if (list_empty(&sup->work.work.entry))
                        list_add(&sup->work.work.entry, &srcu_boot_list);
        }


Paul's suggestion at [1] is basically breaking another dependecy
runqueue::lock -> rcu_node::lock, I'm investigating how we can do that.

[1]: https://lore.kernel.org/rcu/214fb140-041d-4fd1-8694-658547209b84@paulmck-laptop/

Regards,
Boqun

> thanks,
> 
> --
> Joel Fernandes
>