[RFC PATCH 1/5] refcount, kref: add dec-and-test wrappers for rw_semaphores

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

Similar to the existing functions that take a mutex or spinlock if and
only if a reference count is decremented to zero, these new function
take an rwsem for writing just before the refcount reaches 0 (and
call a user-provided function in the case of kref_put_rwsem).

These will be used for statsfs_source data structures, which are
protected by an rw_semaphore to allow concurrent sysfs reads.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 include/linux/kref.h     | 11 +++++++++++
 include/linux/refcount.h |  2 ++
 lib/refcount.c           | 32 ++++++++++++++++++++++++++++++++
 3 files changed, 45 insertions(+)

diff --git a/include/linux/kref.h b/include/linux/kref.h
index d32e21a2538c..2dc935445f45 100644
--- a/include/linux/kref.h
+++ b/include/linux/kref.h
@@ -79,6 +79,17 @@ static inline int kref_put_mutex(struct kref *kref,
 	return 0;
 }
 
+static inline int kref_put_rwsem(struct kref *kref,
+				 void (*release)(struct kref *kref),
+				 struct rw_semaphore *rwsem)
+{
+	if (refcount_dec_and_down_write(&kref->refcount, rwsem)) {
+		release(kref);
+		return 1;
+	}
+	return 0;
+}
+
 static inline int kref_put_lock(struct kref *kref,
 				void (*release)(struct kref *kref),
 				spinlock_t *lock)
diff --git a/include/linux/refcount.h b/include/linux/refcount.h
index 0e3ee25eb156..a9d5038aec9a 100644
--- a/include/linux/refcount.h
+++ b/include/linux/refcount.h
@@ -99,6 +99,7 @@
 #include <linux/spinlock_types.h>
 
 struct mutex;
+struct rw_semaphore;
 
 /**
  * struct refcount_t - variant of atomic_t specialized for reference counts
@@ -313,6 +314,7 @@ static inline void refcount_dec(refcount_t *r)
 extern __must_check bool refcount_dec_if_one(refcount_t *r);
 extern __must_check bool refcount_dec_not_one(refcount_t *r);
 extern __must_check bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock);
+extern __must_check bool refcount_dec_and_down_write(refcount_t *r, struct rw_semaphore *rwsem);
 extern __must_check bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock);
 extern __must_check bool refcount_dec_and_lock_irqsave(refcount_t *r,
 						       spinlock_t *lock,
diff --git a/lib/refcount.c b/lib/refcount.c
index ebac8b7d15a7..03e113e1b43a 100644
--- a/lib/refcount.c
+++ b/lib/refcount.c
@@ -4,6 +4,7 @@
  */
 
 #include <linux/mutex.h>
+#include <linux/rwsem.h>
 #include <linux/refcount.h>
 #include <linux/spinlock.h>
 #include <linux/bug.h>
@@ -94,6 +95,37 @@ bool refcount_dec_not_one(refcount_t *r)
 }
 EXPORT_SYMBOL(refcount_dec_not_one);
 
+/**
+ * refcount_dec_and_down_write - return holding rwsem for writing if able to decrement
+ *                               refcount to 0
+ * @r: the refcount
+ * @lock: the mutex to be locked
+ *
+ * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
+ * to decrement when saturated at REFCOUNT_SATURATED.
+ *
+ * Provides release memory ordering, such that prior loads and stores are done
+ * before, and provides a control dependency such that free() must come after.
+ * See the comment on top.
+ *
+ * Return: true and hold rwsem for writing if able to decrement refcount to 0, false
+ *         otherwise
+ */
+bool refcount_dec_and_down_write(refcount_t *r, struct rw_semaphore *lock)
+{
+	if (refcount_dec_not_one(r))
+		return false;
+
+	down_write(lock);
+	if (!refcount_dec_and_test(r)) {
+		up_write(lock);
+		return false;
+	}
+
+	return true;
+}
+EXPORT_SYMBOL(refcount_dec_and_down_write);
+
 /**
  * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
  *                               refcount to 0
-- 
2.25.2