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.
thanks,
--
Joel Fernandes
