In non-preemptible kernels, an unfortunately timed expedited grace period
can result in the rcu_exp_handler() IPI handler setting the rcu_data
structure's cpu_no_qs.b.exp field just as the target CPU enters idle.
There are situations in which this field will not be checked until after
that CPU exits idle. The resulting grace-period latency does not qualify
as "expedited".
This commit therefore checks this field upon non-preemptible idle entry in
the rcu_preempt_deferred_qs() function. It also qualifies the rcu_core()
preempt_count() check with IS_ENABLED(CONFIG_PREEMPT_COUNT) to prevent
false-positive quiescent states from count-free kernels.
Reported-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
kernel/rcu/tree.c | 2 +-
kernel/rcu/tree_plugin.h | 13 ++++++++++++-
2 files changed, 13 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ef8d36f580fc3..a8d1fe35f4820 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2714,7 +2714,7 @@ static __latent_entropy void rcu_core(void)
WARN_ON_ONCE(!rdp->beenonline);
/* Report any deferred quiescent states if preemption enabled. */
- if (!(preempt_count() & PREEMPT_MASK)) {
+ if (IS_ENABLED(CONFIG_PREEMPT_COUNT) && (!(preempt_count() & PREEMPT_MASK))) {
rcu_preempt_deferred_qs(current);
} else if (rcu_preempt_need_deferred_qs(current)) {
set_tsk_need_resched(current);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 972acb114ce3c..6d715575226f0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -927,7 +927,18 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
return false;
}
-static void rcu_preempt_deferred_qs(struct task_struct *t) { }
+
+// Except that we do need to respond to a request by an expedited grace
+// period for a quiescent state from this CPU. Note that requests from
+// tasks are handled when removing the task from the blocked-tasks list
+// below.
+static void rcu_preempt_deferred_qs(struct task_struct *t)
+{
+ struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
+
+ if (rdp->cpu_no_qs.b.exp)
+ rcu_report_exp_rdp(rdp);
+}
/*
* Because there is no preemptible RCU, there can be no readers blocked,
--
2.31.1.189.g2e36527f23
