[RFC PATCH] sched: avoid unnecessary atomic_read when sync_core_before_usermode() is empty

[Tianchen Ding]

On archs except x86, CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE is not
defined. We found membarrier_mm_sync_core_before_usermode() looks like
this when compiled by gcc10:

	if (current->mm != mm)
		return;
	atomic_read(&mm->membarrier_state);

This memory access is unnecessary. Remove it to improve performance.

Signed-off-by: Tianchen Ding <dtcccc@xxxxxxxxxxxxxxxxx>
---
 include/linux/sched/mm.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index a80356e9dc69..3ded68d9f913 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -401,6 +401,7 @@ enum {
 #include <asm/membarrier.h>
 #endif
 
+#ifdef CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
 static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
 {
 	if (current->mm != mm)
@@ -410,6 +411,11 @@ static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
 		return;
 	sync_core_before_usermode();
 }
+#else
+static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
+{
+}
+#endif
 
 extern void membarrier_exec_mmap(struct mm_struct *mm);
 
-- 
2.33.0