Bleh, my commit message got lost, but hey this is an RFC ;-) For context, it repairs this issue: https://lore.kernel.org/all/[email protected]/
Will add commit message when sending non-RFC later. thanks, - Joel On 7/5/2025 4:39 PM, Joel Fernandes wrote: > Signed-off-by: Joel Fernandes <[email protected]> > --- > kernel/rcu/tree.h | 11 ++++++++++- > kernel/rcu/tree_plugin.h | 29 ++++++++++++++++++++++------- > 2 files changed, 32 insertions(+), 8 deletions(-) > > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > index 3830c19cf2f6..f8f612269e6e 100644 > --- a/kernel/rcu/tree.h > +++ b/kernel/rcu/tree.h > @@ -174,6 +174,15 @@ struct rcu_snap_record { > unsigned long jiffies; /* Track jiffies value */ > }; > > +/* > + * The IRQ work (deferred_qs_iw) is used by RCU to get scheduler's attention. > + * It can be in one of the following states: > + * - DEFER_QS_IDLE: An IRQ work was never scheduled. > + * - DEFER_QS_PENDING: An IRQ work was scheduler but never run. > + */ > +#define DEFER_QS_IDLE 0 > +#define DEFER_QS_PENDING 1 > + > /* Per-CPU data for read-copy update. */ > struct rcu_data { > /* 1) quiescent-state and grace-period handling : */ > @@ -192,7 +201,7 @@ struct rcu_data { > /* during and after the last grace */ > /* period it is aware of. */ > struct irq_work defer_qs_iw; /* Obtain later scheduler attention. */ > - bool defer_qs_iw_pending; /* Scheduler attention pending? */ > + int defer_qs_iw_pending; /* Scheduler attention pending? */ > struct work_struct strict_work; /* Schedule readers for strict GPs. */ > > /* 2) batch handling */ > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index dd1c156c1759..baf57745b42f 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -486,13 +486,16 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct > *t, unsigned long flags) > struct rcu_node *rnp; > union rcu_special special; > > + rdp = this_cpu_ptr(&rcu_data); > + if (rdp->defer_qs_iw_pending == DEFER_QS_PENDING) > + rdp->defer_qs_iw_pending = DEFER_QS_IDLE; > + > /* > * If RCU core is waiting for this CPU to exit its critical section, > * report the fact that it has exited. Because irqs are disabled, > * t->rcu_read_unlock_special cannot change. > */ > special = t->rcu_read_unlock_special; > - rdp = this_cpu_ptr(&rcu_data); > if (!special.s && !rdp->cpu_no_qs.b.exp) { > local_irq_restore(flags); > return; > @@ -623,12 +626,24 @@ notrace void rcu_preempt_deferred_qs(struct task_struct > *t) > */ > static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp) > { > - unsigned long flags; > - struct rcu_data *rdp; > + volatile unsigned long flags; > + struct rcu_data *rdp = this_cpu_ptr(&rcu_data); > > - rdp = container_of(iwp, struct rcu_data, defer_qs_iw); > local_irq_save(flags); > - rdp->defer_qs_iw_pending = false; > + > + /* > + * Requeue the IRQ work on next unlock in following situation: > + * 1. rcu_read_unlock() queues IRQ work (state -> DEFER_QS_PENDING) > + * 2. CPU enters new rcu_read_lock() > + * 3. IRQ work runs but cannot report QS due to rcu_preempt_depth() > 0 > + * 4. rcu_read_unlock() does not re-queue work (state still PENDING) > + * 5. Deferred QS reporting does not happen. > + */ > + if (rcu_preempt_depth() > 0) { > + WRITE_ONCE(rdp->defer_qs_iw_pending, DEFER_QS_IDLE); > + local_irq_restore(flags); > + return; > + } > local_irq_restore(flags); > } > > @@ -675,7 +690,7 @@ static void rcu_read_unlock_special(struct task_struct *t) > set_tsk_need_resched(current); > set_preempt_need_resched(); > if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled && > - expboost && !rdp->defer_qs_iw_pending && > cpu_online(rdp->cpu)) { > + expboost && rdp->defer_qs_iw_pending != > DEFER_QS_PENDING && cpu_online(rdp->cpu)) { > // Get scheduler to re-evaluate and call hooks. > // If !IRQ_WORK, FQS scan will eventually IPI. > if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) > && > @@ -685,7 +700,7 @@ static void rcu_read_unlock_special(struct task_struct *t) > else > init_irq_work(&rdp->defer_qs_iw, > > rcu_preempt_deferred_qs_handler); > - rdp->defer_qs_iw_pending = true; > + rdp->defer_qs_iw_pending = DEFER_QS_PENDING; > irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu); > } > }
