> > On Wed, 18 Mar 2026 at 23:15, Boqun Feng <[email protected]> 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 ? Thanks Zqiang > > [0]: > https://lore.kernel.org/bpf/[email protected] > > > > > 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); > > } > > >
