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Subject[PATCH percpu/for-3.16 2/2] percpu-refcount: implement percpu_ref_tryget()
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@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
---
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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