[PATCH percpu/for-3.16 2/2] percpu-refcount: implement percpu_ref_tryget()

[Tejun Heo]

Implement percpu_ref_tryget() which fails if the refcnt already
reached zero.  Note that this is different from the recently renamed
percpu_ref_tryget_live() which fails if the refcnt has been killed and
is draining the remaining references.  percpu_ref_tryget() succeeds on
a killed refcnt as long as its current refcnt is above zero.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Kent Overstreet <kmo@xxxxxxxxxxxxx>
---
 include/linux/percpu-refcount.h |   32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -118,6 +118,36 @@ static inline void percpu_ref_get(struct
 }
 
 /**
+ * percpu_ref_tryget - try to increment a percpu refcount
+ * @ref: percpu_ref to try-get
+ *
+ * Increment a percpu refcount unless its count already reached zero.
+ * Returns %true on success; %false on failure.
+ *
+ * The caller is responsible for ensuring that @ref stays accessible.
+ */
+static inline bool percpu_ref_tryget(struct percpu_ref *ref)
+{
+	unsigned __percpu *pcpu_count;
+	int ret = false;
+
+	rcu_read_lock_sched();
+
+	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
+
+	if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) {
+		__this_cpu_inc(*pcpu_count);
+		ret = true;
+	} else {
+		ret = atomic_inc_not_zero(&ref->count);
+	}
+
+	rcu_read_unlock_sched();
+
+	return ret;
+}
+
+/**
  * percpu_ref_tryget_live - try to increment a live percpu refcount
  * @ref: percpu_ref to try-get
  *
@@ -128,6 +158,8 @@ static inline void percpu_ref_get(struct
  * will fail.  For such guarantee, percpu_ref_kill_and_confirm() should be
  * used.  After the confirm_kill callback is invoked, it's guaranteed that
  * no new reference will be given out by percpu_ref_tryget().
+ *
+ * The caller is responsible for ensuring that @ref stays accessible.
  */
 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 {
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