[RFC PATCH v2 06/13] entry: Skip inactive IRQ-return reschedule calls

[Karl Mehltretter <kmehltretter@gmail.com>]

irqentry_exit_to_kernel_mode_preempt() calls the reschedule helper when
preemption is enabled even if the preempt count is nonzero. The helper
returns immediately, but its call and return remain traceable after hardirq
context has been removed.

Check the preempt count in the inline caller and avoid the inactive call.
Keep the existing check in the helper for its other callers. This does not
change whether IRQ return schedules.

Arm64 invokes this caller from noinstr code. Force its preempt_count()
implementation inline so Clang cannot outline the new check into
instrumentable text.

Assisted-by: LLM
---
 arch/arm64/include/asm/preempt.h | 2 +-
 include/linux/irq-entry-common.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
index f3a5c2cd1936d..3e9bcc6e76625 100644
--- a/arch/arm64/include/asm/preempt.h
+++ b/arch/arm64/include/asm/preempt.h
@@ -7,7 +7,7 @@
 #define PREEMPT_NEED_RESCHED	BIT(32)
 #define PREEMPT_ENABLED	(PREEMPT_NEED_RESCHED)
 
-static inline int preempt_count(void)
+static __always_inline int preempt_count(void)
 {
 	return READ_ONCE(current_thread_info()->preempt.count);
 }
diff --git a/include/linux/irq-entry-common.h b/include/linux/irq-entry-common.h
index 289e308d2d46c..e1fd9155410c1 100644
--- a/include/linux/irq-entry-common.h
+++ b/include/linux/irq-entry-common.h
@@ -503,7 +503,7 @@ static inline void irqentry_exit_to_kernel_mode_preempt(struct pt_regs *regs,
 	if (regs_irqs_disabled(regs) || state.exit_rcu)
 		return;
 
-	if (IS_ENABLED(CONFIG_PREEMPTION))
+	if (IS_ENABLED(CONFIG_PREEMPTION) && !preempt_count())
 		irqentry_exit_cond_resched();
 }
 
-- 
2.39.5 (Apple Git-154)