[PATCH] x86 rwlock *_can_lock() primitives

This introduces the following 3 new locking primitives:

  spin_can_lock(lock)
  read_can_lock(lock)
  write_can_lock(lock)

which are a nonintrusive test to check whether the real (intrusive)
trylock op would succeed or not.  Semantics and naming is completely
symmetric to the trylock counterpart.

Architectures that want to support PREEMPT will need to add these
definitions.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/asm-i386/spinlock.h

@@ -186,6 +186,18 @@ typedef struct {
 
 #define rwlock_init(x)	do { *(x) = RW_LOCK_UNLOCKED; } while(0)
 
+/**
+ * read_can_lock - would read_trylock() succeed?
+ * @lock: the rwlock in question.
+ */
+#define read_can_lock(x) ((int)(x)->lock > 0)
+
+/**
+ * write_can_lock - would write_trylock() succeed?
+ * @lock: the rwlock in question.
+ */
+#define write_can_lock(x) ((x)->lock == RW_LOCK_BIAS)
+
 /*
  * On x86, we implement read-write locks as a 32-bit counter
  * with the high bit (sign) being the "contended" bit.

include/linux/spinlock.h

@@ -584,4 +584,10 @@ static inline int bit_spin_is_locked(int bitnum, unsigned long *addr)
 #define DEFINE_SPINLOCK(x) spinlock_t x = SPIN_LOCK_UNLOCKED
 #define DEFINE_RWLOCK(x) rwlock_t x = RW_LOCK_UNLOCKED
 
+/**
+ * spin_can_lock - would spin_trylock() succeed?
+ * @lock: the spinlock in question.
+ */
+#define spin_can_lock(lock)		(!spin_is_locked(lock))
+
 #endif /* __LINUX_SPINLOCK_H */