On Thu, Mar 19, 2026 at 03:02:57AM -0700, Paul E. McKenney wrote:
> 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/[email protected]/
> > [1]
> > Signed-off-by: Boqun Feng <[email protected]>
> > ---
> > 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.
>
I think you're talking about another direction of the dependency ;-)
Joel's example shows both depenedencies clearly:
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
To remove [1] -> [4], i.e. the dependency "&rq->__lock" -> "srcu locks",
yes, you need to bail out earlier on. But to remove [2] -> [6], i.e. the
dependency "srcu locks" -> "&rq->__lock", you just need to make sure
call_srcu() won't call anything that need to acquire the rq lock.
In the old version of call_rcu_tasks_trace(), we were fine also because
[2] -> [6] didn't exist ([6] is rcu node lock in this case). [1] -> [4]
([4] being rcu node lock) existed in the RCU Tasks Trace version:
// Enqueue a callback for the specified flavor of Tasks RCU.
static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t
func,
struct rcu_tasks *rtp)
{
...
if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already
disabled.
raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
...
}
...
rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
...
/* We can't create the thread unless interrupts are enabled. */
if (needwake && READ_ONCE(rtp->kthread_ptr))
irq_work_queue(&rtpcp->rtp_irq_work);
}
Hope it helps.
Regards,
Boqun
> 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)
> >
