[RFC PATCH 2/3] softirq: Add function to preempt serving softirqs.

[Sebastian Andrzej Siewior]

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.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 include/linux/bottom_half.h |  2 ++
 kernel/softirq.c            | 13 +++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/bottom_half.h b/include/linux/bottom_half.h
index fc53e0ad56d90..448bbef474564 100644
--- a/include/linux/bottom_half.h
+++ b/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 */
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 807b34ccd7973..dd3307a619af7 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -247,6 +247,19 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 }
 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.
-- 
2.40.1