[PATCH 6.6 007/283] rcu: Refactor expedited handling check in rcu_read_unlock_special()

Sasha Levin
2026-02-28T18:07:16Z

From: Joel Fernandes <joelagnelf@nvidia.com>

[ Upstream commit 908a97eba8c8b510996bf5d77d1e3070d59caa6d ]

Extract the complex expedited handling condition in rcu_read_unlock_special()
into a separate function rcu_unlock_needs_exp_handling() with detailed
comments explaining each condition.

This improves code readability. No functional change intended.

Reviewed-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
Stable-dep-of: d41e37f26b31 ("rcu: Fix rcu_read_unlock() deadloop due to softirq")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/rcu/tree_plugin.h | 83 +++++++++++++++++++++++++++++++++++-----
1 file changed, 74 insertions(+), 9 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index f178a8bb83650..18a8fd116fdff 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -640,6 +640,75 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
local_irq_restore(flags);
}

+/*
+ * Check if expedited grace period processing during unlock is needed.
+ *
+ * This function determines whether expedited handling is required based on:
+ * 1. Task blocking an expedited grace period (based on a heuristic, could be
+ * false-positive, see below.)
+ * 2. CPU participating in an expedited grace period
+ * 3. Strict grace period mode requiring expedited handling
+ * 4. RCU priority deboosting needs when interrupts were disabled
+ *
+ * @t: The task being checked
+ * @rdp: The per-CPU RCU data
+ * @rnp: The RCU node for this CPU
+ * @irqs_were_disabled: Whether interrupts were disabled before rcu_read_unlock()
+ *
+ * Returns true if expedited processing of the rcu_read_unlock() is needed.
+ */
+static bool rcu_unlock_needs_exp_handling(struct task_struct *t,
+ struct rcu_data *rdp,
+ struct rcu_node *rnp,
+ bool irqs_were_disabled)
+{
+ /*
+ * Check if this task is blocking an expedited grace period. If the
+ * task was preempted within an RCU read-side critical section and is
+ * on the expedited grace period blockers list (exp_tasks), we need
+ * expedited handling to unblock the expedited GP. This is not an exact
+ * check because 't' might not be on the exp_tasks list at all - its
+ * just a fast heuristic that can be false-positive sometimes.
+ */
+ if (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks))
+ return true;
+
+ /*
+ * Check if this CPU is participating in an expedited grace period.
+ * The expmask bitmap tracks which CPUs need to check in for the
+ * current expedited GP. If our CPU's bit is set, we need expedited
+ * handling to help complete the expedited GP.
+ */
+ if (rdp->grpmask & READ_ONCE(rnp->expmask))
+ return true;
+
+ /*
+ * In CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels, all grace periods
+ * are treated as short for testing purposes even if that means
+ * disturbing the system more. Check if either:
+ * - This CPU has not yet reported a quiescent state, or
+ * - This task was preempted within an RCU critical section
+ * In either case, require expedited handling for strict GP mode.
+ */
+ if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
+ ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node))
+ return true;
+
+ /*
+ * RCU priority boosting case: If a task is subject to RCU priority
+ * boosting and exits an RCU read-side critical section with interrupts
+ * disabled, we need expedited handling to ensure timely deboosting.
+ * Without this, a low-priority task could incorrectly run at high
+ * real-time priority for an extended period degrading real-time
+ * responsiveness. This applies to all CONFIG_RCU_BOOST=y kernels,
+ * not just to PREEMPT_RT.
+ */
+ if (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled && t->rcu_blocked_node)
+ return true;
+
+ return false;
+}
+
/*
* Handle special cases during rcu_read_unlock(), such as needing to
* notify RCU core processing or task having blocked during the RCU
@@ -659,18 +728,14 @@ static void rcu_read_unlock_special(struct task_struct *t)
local_irq_save(flags);
irqs_were_disabled = irqs_disabled_flags(flags);
if (preempt_bh_were_disabled || irqs_were_disabled) {
- bool expboost; // Expedited GP in flight or possible boosting.
+ bool needs_exp; // Expedited handling needed.
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
struct rcu_node *rnp = rdp->mynode;

- expboost = (t->rcu_blocked_node && READ_ONCE(t->rcu_blocked_node->exp_tasks)) ||
- (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
- (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
- ((rdp->grpmask & READ_ONCE(rnp->qsmask)) || t->rcu_blocked_node)) ||
- (IS_ENABLED(CONFIG_RCU_BOOST) && irqs_were_disabled &&
- t->rcu_blocked_node);
+ needs_exp = rcu_unlock_needs_exp_handling(t, rdp, rnp, irqs_were_disabled);
+
// Need to defer quiescent state until everything is enabled.
- if (use_softirq && (in_hardirq() || (expboost && !irqs_were_disabled))) {
+ if (use_softirq && (in_hardirq() || (needs_exp && !irqs_were_disabled))) {
// Using softirq, safe to awaken, and either the
// wakeup is free or there is either an expedited
// GP in flight or a potential need to deboost.
@@ -683,7 +748,7 @@ 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 != DEFER_QS_PENDING &&
+ needs_exp && 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.
--
2.51.0