[RFC PATCH 06/12] lockdep: Prepare IRQ-on state from a hardirq return tail

[Karl Mehltretter <kmehltretter@gmail.com>]

lockdep_hardirqs_on_prepare() rejects any live hardirq context. An ordinary
hardirq return tail needs to retain one synthetic hardirq nesting level
through traceable IRQ-on preparation so callbacks remain classified as
hardirq work.

Factor the common preparation and add a return-tail variant which accepts
exactly one nesting level. Keep the ordinary helper restricted to zero.
Neither helper changes IRQ or hardirq state. The caller must remove the
tail context before lockdep_hardirqs_on().

Assisted-by: LLM
---
 include/linux/irqflags.h |  2 ++
 kernel/locking/lockdep.c | 42 ++++++++++++++++++++++++++++------------
 2 files changed, 32 insertions(+), 12 deletions(-)

diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index e98313a8ad307..c7601a173e55b 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -25,6 +25,7 @@ struct task_struct;
   extern void lockdep_softirqs_on(unsigned long ip);
   extern void lockdep_softirqs_off(unsigned long ip);
   extern void lockdep_hardirqs_on_prepare(void);
+void lockdep_hardirqs_on_prepare_irqtail(void);
   extern void lockdep_hardirqs_on(unsigned long ip);
   extern void lockdep_hardirqs_off(unsigned long ip);
   extern void lockdep_cleanup_dead_cpu(unsigned int cpu,
@@ -33,6 +34,7 @@ struct task_struct;
   static inline void lockdep_softirqs_on(unsigned long ip) { }
   static inline void lockdep_softirqs_off(unsigned long ip) { }
   static inline void lockdep_hardirqs_on_prepare(void) { }
+static inline void lockdep_hardirqs_on_prepare_irqtail(void) { }
   static inline void lockdep_hardirqs_on(unsigned long ip) { }
   static inline void lockdep_hardirqs_off(unsigned long ip) { }
   static inline void lockdep_cleanup_dead_cpu(unsigned int cpu,
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 01dba8fb32a90..457bef3ea1689 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4395,15 +4395,7 @@ static void __trace_hardirqs_on_caller(void)
 		mark_held_locks(curr, LOCK_ENABLED_SOFTIRQ);
 }
 
-/**
- * lockdep_hardirqs_on_prepare - Prepare for enabling interrupts
- *
- * Invoked before a possible transition to RCU idle from exit to user or
- * guest mode. This ensures that all RCU operations are done before RCU
- * stops watching. After the RCU transition lockdep_hardirqs_on() has to be
- * invoked to set the final state.
- */
-void lockdep_hardirqs_on_prepare(void)
+static void __lockdep_hardirqs_on_prepare(unsigned int expected_context)
 {
 	if (unlikely(!debug_locks))
 		return;
@@ -4442,10 +4434,11 @@ void lockdep_hardirqs_on_prepare(void)
 		return;
 
 	/*
-	 * Can't allow enabling interrupts while in an interrupt handler,
-	 * that's general bad form and such. Recursion, limited stack etc..
+	 * Ordinary IRQ enable requires no hardirq context. The interrupt return
+	 * tail variant permits exactly the synthetic context it is about to
+	 * remove.
 	 */
-	if (DEBUG_LOCKS_WARN_ON(lockdep_hardirq_context()))
+	if (DEBUG_LOCKS_WARN_ON(lockdep_hardirq_context() != expected_context))
 		return;
 
 	current->hardirq_chain_key = current->curr_chain_key;
@@ -4454,8 +4447,33 @@ void lockdep_hardirqs_on_prepare(void)
 	__trace_hardirqs_on_caller();
 	lockdep_recursion_finish();
 }
+
+/**
+ * lockdep_hardirqs_on_prepare - Prepare for enabling interrupts
+ *
+ * Invoked before a possible transition to RCU idle from exit to user or
+ * guest mode. This ensures that all RCU operations are done before RCU
+ * stops watching. After the RCU transition lockdep_hardirqs_on() has to be
+ * invoked to set the final state.
+ */
+void lockdep_hardirqs_on_prepare(void)
+{
+	__lockdep_hardirqs_on_prepare(0);
+}
 EXPORT_SYMBOL_GPL(lockdep_hardirqs_on_prepare);
 
+/**
+ * lockdep_hardirqs_on_prepare_irqtail - Prepare an interrupt return tail
+ *
+ * Like lockdep_hardirqs_on_prepare(), but invoked while the single synthetic
+ * lockdep hardirq context covering an interrupt return tail is still active.
+ * The caller must remove that context before lockdep_hardirqs_on().
+ */
+void lockdep_hardirqs_on_prepare_irqtail(void)
+{
+	__lockdep_hardirqs_on_prepare(1);
+}
+
 void noinstr lockdep_hardirqs_on(unsigned long ip)
 {
 	struct irqtrace_events *trace = &current->irqtrace;
-- 
2.39.5 (Apple Git-154)