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 (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)
