Introduce down_try() so we can move away from down_trylock()

I planned on removing the much-disliked down_trylock() (with its
backwards return codes) in 2.6.27, but it's creating something of a
logjam with other patches in -mm and linux-next.

Andrew suggested introducing "down_try" as a wrapper now, to make
the transition easier.  Linus, please apply.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
---
 include/linux/semaphore.h |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff -r 92664ae4130b include/linux/semaphore.h
--- a/include/linux/semaphore.h	Wed May 21 14:54:40 2008 +1000
+++ b/include/linux/semaphore.h	Wed May 21 15:07:31 2008 +1000
@@ -48,4 +48,18 @@ extern int __must_check down_timeout(str
 extern int __must_check down_timeout(struct semaphore *sem, long jiffies);
 extern void up(struct semaphore *sem);
 
+/**
+ * down_try - try to down a semaphore, but don't block
+ * @sem: the semaphore
+ *
+ * This is equivalent to down_trylock(), but has the same return codes as
+ * spin_trylock and mutex_trylock: 1 if semaphore acquired, 0 if not.
+ *
+ * down_trylock() with its confusing return codes will be deprecated
+ * soon.  It will not be missed.
+ */
+static inline int __must_check down_try(struct semaphore *sem)
+{
+        return !down_trylock(sem);
+}
 #endif /* __LINUX_SEMAPHORE_H */
--
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