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

[Boqun Feng <boqun@kernel.org>]

On Thu, Mar 19, 2026 at 12:26:38AM +0000, Zqiang wrote:
> > 
> > On Wed, 18 Mar 2026 at 23:15, Boqun Feng <boqun@kernel.org> 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.
> > > 
> > Unfortunately it can be, there is at least one tracepoint that is
> > invoked with hrtimer base lock held.
> > Alexei ended up fixing this in the recent past [0]. So I think this
> > would cause trouble too.
> > 
> > hrtimer_start_range_ns() -> __hrtimer_start_range_ns() ->
> > remove_timer() -> __remove_hrtimer() -> debug_deactivate() ->
> > trace_hrtimer_cancel().
> > BPF can attach to such a tracepoint.
> 
> Is it possible to use irq_work_queue() to trigger queue_delay_work()
> by checking if ssp == &rcu_tasks_trace_srcu_struct ?
> 

Good call! I didn't do the exact == check, but close:

	https://lore.kernel.org/rcu/abtMhd_LVp3uL_pA@tardis.local/

;-)

Regards,
Boqun

> Thanks
> Zqiang
> 
> 
> > 
> >  [0]: https://lore.kernel.org/bpf/20260204040834.22263-2-alexei.starovoitov@gmail.com
> > 
> > > 
> > > 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);
> > >  else if (list_empty(&sup->work.work.entry))
> > >  list_add(&sup->work.work.entry, &srcu_boot_list);
> > >  }
> > >
> >