Subject: [PATCH 2/3] softirq: Add function to preempt serving softirqs.
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Fri, 4 Aug 2023 13:30:38 +0200

Add a functionality for the softirq handler to preempt its current work
if needed. The softirq core has no particular state. It reads and resets
the pending softirq bits and then processes one after the other.
It can already be preempted while it invokes a certain softirq handler.

By enabling the BH the softirq core releases the per-CPU bh lock which
serializes all softirq handler. It is safe to do as long as the code
does not expect any serialisation in between. A typical scenarion would
after the invocation of callback where no state needs to be preserved
before the next callback is invoked.

Add functionaliry to preempt the serving softirqs.

Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 include/linux/bottom_half.h |    2 ++
 kernel/softirq.c            |   13 +++++++++++++
 2 files changed, 15 insertions(+)

Index: linux-6.8.2-rt10/include/linux/bottom_half.h
===================================================================
--- linux-6.8.2-rt10.orig/include/linux/bottom_half.h
+++ linux-6.8.2-rt10/include/linux/bottom_half.h
@@ -35,8 +35,10 @@ static inline void local_bh_enable(void)
 
 #ifdef CONFIG_PREEMPT_RT
 extern bool local_bh_blocked(void);
+extern void softirq_preempt(void);
 #else
 static inline bool local_bh_blocked(void) { return false; }
+static inline void softirq_preempt(void) { }
 #endif
 
 #endif /* _LINUX_BH_H */
Index: linux-6.8.2-rt10/kernel/softirq.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/softirq.c
+++ linux-6.8.2-rt10/kernel/softirq.c
@@ -247,6 +247,19 @@ out:
 }
 EXPORT_SYMBOL(__local_bh_enable_ip);
 
+void softirq_preempt(void)
+{
+	if (WARN_ON_ONCE(!preemptible()))
+		return;
+
+	if (WARN_ON_ONCE(__this_cpu_read(softirq_ctrl.cnt) != SOFTIRQ_OFFSET))
+		return;
+
+	__local_bh_enable(SOFTIRQ_OFFSET, true);
+	/* preemption point */
+	__local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
+}
+
 /*
  * Invoked from ksoftirqd_run() outside of the interrupt disabled section
  * to acquire the per CPU local lock for reentrancy protection.