[PATCH net 1/3] kref: add kref_sub_return

["Michael S. Tsirkin" <mst@redhat.com>]

It is sometimes useful to get the value of the reference count after
decrement.
For example, vhost wants to execute some periodic cleanup operations
once number of references drops below a specific value, before it
reaches zero (for efficiency).

Add an API to do this atomically and efficiently using
atomic_sub_return.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

Greg, could you ack this API extension please?
I think it is cleanest to merge this through -net together
with the first user.

 include/linux/kref.h | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/linux/kref.h b/include/linux/kref.h
index 484604d..cb20550 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -61,7 +61,7 @@ static inline void kref_get(struct kref *kref)
  *
  * Subtract @count from the refcount, and if 0, call release().
  * Return 1 if the object was removed, otherwise return 0.  Beware, if this
- * function returns 0, you still can not count on the kref from remaining in
+ * function returns 0, you still can not count on the kref remaining in
  * memory.  Only use the return value if you want to see if the kref is now
  * gone, not present.
  */
@@ -78,6 +78,38 @@ static inline int kref_sub(struct kref *kref, unsigned int count,
 }
 
 /**
+ * kref_sub_return - subtract a number of refcounts for object.
+ * @kref: object.
+ * @count: Number of recounts to subtract.
+ * @release: pointer to the function that will clean up the object when the
+ *	     last reference to the object is released.
+ *	     This pointer is required, and it is not acceptable to pass kfree
+ *	     in as this function.  If the caller does pass kfree to this
+ *	     function, you will be publicly mocked mercilessly by the kref
+ *	     maintainer, and anyone else who happens to notice it.  You have
+ *	     been warned.
+ *
+ * Subtract @count from the refcount, and if 0, call release().
+ * Return the new refcount.  Beware, if this function returns > N, you still
+ * can not count on there being at least N other references, and in
+ * particular, on the kref remaining in memory.
+ * Only use the return value if you want to see if there are at most,
+ * not at least, N other references to kref,
+ */
+static inline int kref_sub_return(struct kref *kref, unsigned int count,
+				  void (*release)(struct kref *kref))
+{
+	int r;
+
+	WARN_ON(release == NULL);
+
+	r = atomic_sub_return((int) count, &kref->refcount);
+	if (!r)
+		release(kref);
+	return r;
+}
+
+/**
  * kref_put - decrement refcount for object.
  * @kref: object.
  * @release: pointer to the function that will clean up the object when the
-- 
MST