[PATCH] timekeeping: Add a lockdep override in tick_freeze().

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

tick_freeze() acquires a raw_spinlock_t (tick_freeze_lock). Later in the
callchain (timekeeping_suspend() -> mc146818_avoid_UIP()) the RTC driver
can acquire a spinlock_t which becomes a sleeping lock on PREEMPT_RT.
Lockdep complains about this lock nesting.

Add a lockdep override for this special case and a comment explaining
why it is okay.

Reported-by: Borislav Petkov <bp@alien8.de>
Closes: https://lore.kernel.org/all/20250330113202.GAZ-krsjAnurOlTcp-@fat_crate.local/
Reported-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Closes: https://lore.kernel.org/all/CAP-bSRZ0CWyZZsMtx046YV8L28LhY0fson2g4EqcwRAVN1Jk+Q@mail.gmail.com/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/time/tick-common.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index a47bcf71defcf..8fd8e2ee09fa1 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -509,6 +509,7 @@ void tick_resume(void)
 
 #ifdef CONFIG_SUSPEND
 static DEFINE_RAW_SPINLOCK(tick_freeze_lock);
+static DEFINE_WAIT_OVERRIDE_MAP(tick_freeze_map, LD_WAIT_SLEEP);
 static unsigned int tick_freeze_depth;
 
 /**
@@ -528,9 +529,20 @@ void tick_freeze(void)
 	if (tick_freeze_depth == num_online_cpus()) {
 		trace_suspend_resume(TPS("timekeeping_freeze"),
 				     smp_processor_id(), true);
+		/*
+		 * All other CPUs have their interrupts disabled and are
+		 * suspended to idle. Other tasks have been frozen so there is
+		 * no scheduling happening. This means that there is no
+		 * concurrency in the system at this point. Therefore it is okay
+		 * to acquire a sleeping lock on PREEMPT_RT, such as spinlock_t,
+		 * because the lock can not be acquired and can not block.
+		 * Inform lockdep about the situation.
+		 */
+		lock_map_acquire_try(&tick_freeze_map);
 		system_state = SYSTEM_SUSPEND;
 		sched_clock_suspend();
 		timekeeping_suspend();
+		lock_map_release(&tick_freeze_map);
 	} else {
 		tick_suspend_local();
 	}
@@ -552,8 +564,16 @@ void tick_unfreeze(void)
 	raw_spin_lock(&tick_freeze_lock);
 
 	if (tick_freeze_depth == num_online_cpus()) {
+		/*
+		 * Similar to tick_freeze(). On resumption the first CPU may
+		 * acquire uncontended sleeping locks while other CPUs block on
+		 * tick_freeze_lock.
+		 */
+		lock_map_acquire_try(&tick_freeze_map);
 		timekeeping_resume();
 		sched_clock_resume();
+		lock_map_release(&tick_freeze_map);
+
 		system_state = SYSTEM_RUNNING;
 		trace_suspend_resume(TPS("timekeeping_freeze"),
 				     smp_processor_id(), false);
-- 
2.49.0