[PATCH 5.10 158/236] PM: cpu: Make notifier chain use a raw_spinlock_t

Greg Kroah-Hartman
2021-09-13T13:45:09Z

From: Valentin Schneider <valentin.schneider@arm.com>

[ Upstream commit b2f6662ac08d0e7c25574ce53623c71bdae9dd78 ]

Invoking atomic_notifier_chain_notify() requires acquiring a spinlock_t,
which can block under CONFIG_PREEMPT_RT. Notifications for members of the
cpu_pm notification chain will be issued by the idle task, which can never
block.

Making *all* atomic_notifiers use a raw_spinlock is too big of a hammer, as
only notifications issued by the idle task are problematic.

Special-case cpu_pm_notifier_chain by kludging a raw_notifier and
raw_spinlock_t together, matching the atomic_notifier behavior with a
raw_spinlock_t.

Fixes: 70d932985757 ("notifier: Fix broken error handling pattern")
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/cpu_pm.c | 50 +++++++++++++++++++++++++++++++++++++------------
1 file changed, 38 insertions(+), 12 deletions(-)

diff --git a/kernel/cpu_pm.c b/kernel/cpu_pm.c
index f7e1d0eccdbc..246efc74e3f3 100644
--- a/kernel/cpu_pm.c
+++ b/kernel/cpu_pm.c
@@ -13,19 +13,32 @@
#include <linux/spinlock.h>
#include <linux/syscore_ops.h>

-static ATOMIC_NOTIFIER_HEAD(cpu_pm_notifier_chain);
+/*
+ * atomic_notifiers use a spinlock_t, which can block under PREEMPT_RT.
+ * Notifications for cpu_pm will be issued by the idle task itself, which can
+ * never block, IOW it requires using a raw_spinlock_t.
+ */
+static struct {
+ struct raw_notifier_head chain;
+ raw_spinlock_t lock;
+} cpu_pm_notifier = {
+ .chain = RAW_NOTIFIER_INIT(cpu_pm_notifier.chain),
+ .lock = __RAW_SPIN_LOCK_UNLOCKED(cpu_pm_notifier.lock),
+};

static int cpu_pm_notify(enum cpu_pm_event event)
{
int ret;

/*
- * atomic_notifier_call_chain has a RCU read critical section, which
- * could be disfunctional in cpu idle. Copy RCU_NONIDLE code to let
- * RCU know this.
+ * This introduces a RCU read critical section, which could be
+ * disfunctional in cpu idle. Copy RCU_NONIDLE code to let RCU know
+ * this.
*/
rcu_irq_enter_irqson();
- ret = atomic_notifier_call_chain(&cpu_pm_notifier_chain, event, NULL);
+ rcu_read_lock();
+ ret = raw_notifier_call_chain(&cpu_pm_notifier.chain, event, NULL);
+ rcu_read_unlock();
rcu_irq_exit_irqson();

return notifier_to_errno(ret);
@@ -33,10 +46,13 @@ static int cpu_pm_notify(enum cpu_pm_event event)

static int cpu_pm_notify_robust(enum cpu_pm_event event_up, enum cpu_pm_event event_down)
{
+ unsigned long flags;
int ret;

rcu_irq_enter_irqson();
- ret = atomic_notifier_call_chain_robust(&cpu_pm_notifier_chain, event_up, event_down, NULL);
+ raw_spin_lock_irqsave(&cpu_pm_notifier.lock, flags);
+ ret = raw_notifier_call_chain_robust(&cpu_pm_notifier.chain, event_up, event_down, NULL);
+ raw_spin_unlock_irqrestore(&cpu_pm_notifier.lock, flags);
rcu_irq_exit_irqson();

return notifier_to_errno(ret);
@@ -49,12 +65,17 @@ static int cpu_pm_notify_robust(enum cpu_pm_event event_up, enum cpu_pm_event ev
* Add a driver to a list of drivers that are notified about
* CPU and CPU cluster low power entry and exit.
*
- * This function may sleep, and has the same return conditions as
- * raw_notifier_chain_register.
+ * This function has the same return conditions as raw_notifier_chain_register.
*/
int cpu_pm_register_notifier(struct notifier_block *nb)
{
- return atomic_notifier_chain_register(&cpu_pm_notifier_chain, nb);
+ unsigned long flags;
+ int ret;
+
+ raw_spin_lock_irqsave(&cpu_pm_notifier.lock, flags);
+ ret = raw_notifier_chain_register(&cpu_pm_notifier.chain, nb);
+ raw_spin_unlock_irqrestore(&cpu_pm_notifier.lock, flags);
+ return ret;
}
EXPORT_SYMBOL_GPL(cpu_pm_register_notifier);

@@ -64,12 +85,17 @@ EXPORT_SYMBOL_GPL(cpu_pm_register_notifier);
*
* Remove a driver from the CPU PM notifier list.
*
- * This function may sleep, and has the same return conditions as
- * raw_notifier_chain_unregister.
+ * This function has the same return conditions as raw_notifier_chain_unregister.
*/
int cpu_pm_unregister_notifier(struct notifier_block *nb)
{
- return atomic_notifier_chain_unregister(&cpu_pm_notifier_chain, nb);
+ unsigned long flags;
+ int ret;
+
+ raw_spin_lock_irqsave(&cpu_pm_notifier.lock, flags);
+ ret = raw_notifier_chain_unregister(&cpu_pm_notifier.chain, nb);
+ raw_spin_unlock_irqrestore(&cpu_pm_notifier.lock, flags);
+ return ret;
}
EXPORT_SYMBOL_GPL(cpu_pm_unregister_notifier);

--
2.30.2