[PATCH v2 2/2] rcusync: make rcu_sync_enter() return "bool"

[Oleg Nesterov <oleg@redhat.com>]

Change rcu_sync_enter() to return "need_sync" to let the caller
know whether we did the FAST -> SLOW transition or not.

This is particularly useful in exclusive mode, for example
percpu_down_write() can avoid clear_fast_ctr() if rcu_sync_enter()
returns F.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 include/linux/rcusync.h |    2 +-
 kernel/rcusync.c        |    4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcusync.h b/include/linux/rcusync.h
index aaea86a..8430921 100644
--- a/include/linux/rcusync.h
+++ b/include/linux/rcusync.h
@@ -31,7 +31,7 @@ static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
 
 extern void rcu_sync_init(struct rcu_sync_struct *,
 				enum rcu_sync_type, bool excl);
-extern void rcu_sync_enter(struct rcu_sync_struct *);
+extern bool rcu_sync_enter(struct rcu_sync_struct *);
 extern void rcu_sync_exit(struct rcu_sync_struct *);
 extern void rcu_sync_dtor(struct rcu_sync_struct *);
 
diff --git a/kernel/rcusync.c b/kernel/rcusync.c
index 03ddc61..21a6695 100644
--- a/kernel/rcusync.c
+++ b/kernel/rcusync.c
@@ -59,7 +59,7 @@ void rcu_sync_init(struct rcu_sync_struct *rss,
 	rss->gp_type = type;
 }
 
-void rcu_sync_enter(struct rcu_sync_struct *rss)
+bool rcu_sync_enter(struct rcu_sync_struct *rss)
 {
 	bool need_wait, need_sync;
 
@@ -90,6 +90,8 @@ void rcu_sync_enter(struct rcu_sync_struct *rss)
 		 */
 		BUG_ON(rss->gp_state != GP_PASSED);
 	}
+
+	return need_sync;
 }
 
 static void rcu_sync_func(struct rcu_head *rcu)
-- 
1.5.5.1