spinlock: lockbreak cleanup

The break_lock data structure and code for spinlocks is quite nasty.
Not only does it double the size of a spinlock but it changes locking to
a potentially less optimal trylock.

Put all of that under CONFIG_GENERIC_LOCKBREAK, and introduce a
__raw_spin_is_contended that uses the lock data itself to determine whether
there are waiters on the lock, to be used if CONFIG_GENERIC_LOCKBREAK is
not set.

Rename need_lockbreak to spin_needbreak, make it use spin_is_contended to
decouple it from the spinlock implementation, and make it typesafe (rwlocks
do not have any need_lockbreak sites -- why do they even get bloated up
with that break_lock then?).

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

4 files changed, 17 insertions, 16 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2d0546e884e..9d4797609aa 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1922,23 +1922,16 @@ extern int cond_resched_softirq(void);
 
 /*
  * Does a critical section need to be broken due to another
- * task waiting?:
+ * task waiting?: (technically does not depend on CONFIG_PREEMPT,
+ * but a general need for low latency)
  */
-#if defined(CONFIG_PREEMPT) && defined(CONFIG_SMP)
-# define need_lockbreak(lock) ((lock)->break_lock)
-#else
-# define need_lockbreak(lock) 0
-#endif
-
-/*
- * Does a critical section need to be broken due to another
- * task waiting or preemption being signalled:
- */
-static inline int lock_need_resched(spinlock_t *lock)
+static inline int spin_needbreak(spinlock_t *lock)
 {
-	if (need_lockbreak(lock) || need_resched())
-		return 1;
+#ifdef CONFIG_PREEMPT
+	return spin_is_contended(lock);
+#else
 	return 0;
+#endif
 }
 
 /*
diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index c376f3b36c8..124449733c5 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -120,6 +120,12 @@ do {								\
 
 #define spin_is_locked(lock)	__raw_spin_is_locked(&(lock)->raw_lock)
 
+#ifdef CONFIG_GENERIC_LOCKBREAK
+#define spin_is_contended(lock) ((lock)->break_lock)
+#else
+#define spin_is_contended(lock)	__raw_spin_is_contended(&(lock)->raw_lock)
+#endif
+
 /**
  * spin_unlock_wait - wait until the spinlock gets unlocked
  * @lock: the spinlock in question.
diff --git a/include/linux/spinlock_types.h b/include/linux/spinlock_types.h
index f6a3a951b79..68d88f71f1a 100644
--- a/include/linux/spinlock_types.h
+++ b/include/linux/spinlock_types.h
@@ -19,7 +19,7 @@
 
 typedef struct {
 	raw_spinlock_t raw_lock;
-#if defined(CONFIG_PREEMPT) && defined(CONFIG_SMP)
+#ifdef CONFIG_GENERIC_LOCKBREAK
 	unsigned int break_lock;
 #endif
 #ifdef CONFIG_DEBUG_SPINLOCK
@@ -35,7 +35,7 @@ typedef struct {
 
 typedef struct {
 	raw_rwlock_t raw_lock;
-#if defined(CONFIG_PREEMPT) && defined(CONFIG_SMP)
+#ifdef CONFIG_GENERIC_LOCKBREAK
 	unsigned int break_lock;
 #endif
 #ifdef CONFIG_DEBUG_SPINLOCK
diff --git a/include/linux/spinlock_up.h b/include/linux/spinlock_up.h
index ea54c4c9a4e..938234c4a99 100644
--- a/include/linux/spinlock_up.h
+++ b/include/linux/spinlock_up.h
@@ -64,6 +64,8 @@ static inline void __raw_spin_unlock(raw_spinlock_t *lock)
 # define __raw_spin_trylock(lock)	({ (void)(lock); 1; })
 #endif /* DEBUG_SPINLOCK */
 
+#define __raw_spin_is_contended(lock)	(((void)(lock), 0))
+
 #define __raw_read_can_lock(lock)	(((void)(lock), 1))
 #define __raw_write_can_lock(lock)	(((void)(lock), 1))