linux-kernel - [PATCH] rcu: Remove superfluous versions of rcu_read_lock_sched

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Date:	Wed, 23 Mar 2016 23:11:48 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: [PATCH] rcu: Remove superfluous versions of rcu_read_lock_sched_held()

Currently, we have four versions of rcu_read_lock_sched_held(),
depending on the combined choices on PREEMPT_COUNT and DEBUG_LOCK_ALLOC.
But we actually don't need to specialize those for PREEMPT_COUNT=n
kernel. Because:

1.	For the implementations in DEBUG_LOCK_ALLOC=n kernel, we can use
	preemptible() to implement one rcu_read_lock_sched_held(), which
	gives us the same behavior as the current two.

2.	For the implementations in DEBUG_LOCK_ALLOC=y kernel, even when
	PREEMPT_COUNT=n, one CPU may block no grace period because of
	the same reason for the PREEMPT_COUNT=y and !PREEMPT kernel.
	(e.g. dynticks or cpu hotplug)

So unify the implementations of rcu_read_lock_sched_held() by using
macro preemptible() and tightening up the lock-held checking for
PREEMPT_COUNT=n kernel. And this will improve the readability, make the
debug checking as expected and save several lines of code.

Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 include/linux/rcupdate.h | 17 +----------------
 kernel/rcu/update.c      |  4 ++--
 2 files changed, 3 insertions(+), 18 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index b5d48bd..e3f845b 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -510,14 +510,7 @@ int rcu_read_lock_bh_held(void);
  * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
  * critical section unless it can prove otherwise.
  */
-#ifdef CONFIG_PREEMPT_COUNT
 int rcu_read_lock_sched_held(void);
-#else /* #ifdef CONFIG_PREEMPT_COUNT */
-static inline int rcu_read_lock_sched_held(void)
-{
-	return 1;
-}
-#endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
 
 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
@@ -534,18 +527,10 @@ static inline int rcu_read_lock_bh_held(void)
 	return 1;
 }
 
-#ifdef CONFIG_PREEMPT_COUNT
 static inline int rcu_read_lock_sched_held(void)
 {
-	return preempt_count() != 0 || irqs_disabled();
+	return !preemptible();
 }
-#else /* #ifdef CONFIG_PREEMPT_COUNT */
-static inline int rcu_read_lock_sched_held(void)
-{
-	return 1;
-}
-#endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
-
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 #ifdef CONFIG_PROVE_RCU
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index ca828b4..3ccdc8e 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -67,7 +67,7 @@ static int rcu_normal_after_boot;
 module_param(rcu_normal_after_boot, int, 0);
 #endif /* #ifndef CONFIG_TINY_RCU */
 
-#if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
 /**
  * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
  *
@@ -111,7 +111,7 @@ int rcu_read_lock_sched_held(void)
 		return 0;
 	if (debug_locks)
 		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
-	return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
+	return lockdep_opinion || !preemptible();
 }
 EXPORT_SYMBOL(rcu_read_lock_sched_held);
 #endif
-- 
2.7.3