Subject: [PATCH 3/3] time: Allow to preempt after a callback.
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Fri, 4 Aug 2023 13:30:39 +0200

The TIMER_SOFTIRQ handler invokes timer callbacks of the expired timers.
Before each invocation the timer_base::lock is dropped. The only lock
that is still held is the timer_base::expiry_lock and the per-CPU
bh-lock as part of local_bh_disable(). The former is released as part
of lock up prevention if the timer is preempted by the caller which is
waiting for its completion.

Both locks are already released as part of timer_sync_wait_running().
This can be extended by also releasing in bh-lock. The timer core does
not rely on any state that is serialized by the bh-lock. The timer
callback expects the bh-state to be serialized by the lock but there is
no need to keep state synchronized while invoking multiple callbacks.

Preempt handling softirqs and release all locks after a timer invocation
if the current has inherited priority.

Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/time/timer.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Index: linux-6.8.2-rt10/kernel/time/timer.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/time/timer.c
+++ linux-6.8.2-rt10/kernel/time/timer.c
@@ -1470,9 +1470,16 @@ static inline void timer_base_unlock_exp
  */
 static void timer_sync_wait_running(struct timer_base *base)
 {
-	if (atomic_read(&base->timer_waiters)) {
+	bool need_preempt;
+
+	need_preempt = task_is_pi_boosted(current);
+	if (need_preempt || atomic_read(&base->timer_waiters)) {
 		raw_spin_unlock_irq(&base->lock);
 		spin_unlock(&base->expiry_lock);
+
+		if (need_preempt)
+			softirq_preempt();
+
 		spin_lock(&base->expiry_lock);
 		raw_spin_lock_irq(&base->lock);
 	}