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Subject[tip:perfcounters/core] mutex: add atomic_dec_and_mutex_lock()
Commit-ID:  3a2d367d9aabac486ac4444c6c7ec7a1dab16267
Gitweb: http://git.kernel.org/tip/3a2d367d9aabac486ac4444c6c7ec7a1dab16267
Author: Eric Paris <eparis@redhat.com>
AuthorDate: Mon, 23 Mar 2009 18:22:09 +0100
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Mon, 23 Mar 2009 21:45:10 +0100
mutex: add atomic_dec_and_mutex_lock()

Much like the atomic_dec_and_lock() function in which we take an hold a
spin_lock if we drop the atomic to 0 this function takes and holds the
mutex if we dec the atomic to 0.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20090323172417.410913479@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
include/linux/mutex.h | 23 +++++++++++++++++++++++
1 files changed, 23 insertions(+), 0 deletions(-)
diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 3069ec7..93054fc 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -151,4 +151,27 @@ extern int __must_check mutex_lock_killable(struct mutex *lock);
extern int mutex_trylock(struct mutex *lock);
extern void mutex_unlock(struct mutex *lock);

+/**
+ * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
+ * @cnt: the atomic which we are to dec
+ * @lock: the mutex to return holding if we dec to 0
+ *
+ * return true and hold lock if we dec to 0, return false otherwise
+ */
+static inline int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
+{
+ /* dec if we can't possibly hit 0 */
+ if (atomic_add_unless(cnt, -1, 1))
+ return 0;
+ /* we might hit 0, so take the lock */
+ mutex_lock(lock);
+ if (!atomic_dec_and_test(cnt)) {
+ /* when we actually did the dec, we didn't hit 0 */
+ mutex_unlock(lock);
+ return 0;
+ }
+ /* we hit 0, and we hold the lock */
+ return 1;
+}
+
#endif

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