2 files changed, 66 insertions, 7 deletions

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 6d843d60690d..dd2a08600453 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -63,13 +63,30 @@ struct percpu_ref {
 	 */
 	unsigned __percpu	*pcpu_count;
 	percpu_ref_func_t	*release;
+	percpu_ref_func_t	*confirm_kill;
 	struct rcu_head		rcu;
 };
 
 int __must_check percpu_ref_init(struct percpu_ref *ref,
 				 percpu_ref_func_t *release);
 void percpu_ref_cancel_init(struct percpu_ref *ref);
-void percpu_ref_kill(struct percpu_ref *ref);
+void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
+				 percpu_ref_func_t *confirm_kill);
+
+/**
+ * percpu_ref_kill - drop the initial ref
+ * @ref: percpu_ref to kill
+ *
+ * Must be used to drop the initial ref on a percpu refcount; must be called
+ * precisely once before shutdown.
+ *
+ * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
+ * percpu counters and dropping the initial ref.
+ */
+static inline void percpu_ref_kill(struct percpu_ref *ref)
+{
+	return percpu_ref_kill_and_confirm(ref, NULL);
+}
 
 #define PCPU_STATUS_BITS	2
 #define PCPU_STATUS_MASK	((1 << PCPU_STATUS_BITS) - 1)
@@ -101,6 +118,37 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
 }
 
 /**
+ * percpu_ref_tryget - try to increment a percpu refcount
+ * @ref: percpu_ref to try-get
+ *
+ * Increment a percpu refcount unless it has already been killed.  Returns
+ * %true on success; %false on failure.
+ *
+ * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget
+ * 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().
+ */
+static inline bool percpu_ref_tryget(struct percpu_ref *ref)
+{
+	unsigned __percpu *pcpu_count;
+	int ret = false;
+
+	rcu_read_lock();
+
+	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
+
+	if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) {
+		__this_cpu_inc(*pcpu_count);
+		ret = true;
+	}
+
+	rcu_read_unlock();
+
+	return ret;
+}
+
+/**
  * percpu_ref_put - decrement a percpu refcount
  * @ref: percpu_ref to put
  *
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index ebeaac274cb9..8bf9e719cca0 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -118,6 +118,10 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu)
 
 	atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
 
+	/* @ref is viewed as dead on all CPUs, send out kill confirmation */
+	if (ref->confirm_kill)
+		ref->confirm_kill(ref);
+
 	/*
 	 * Now we're in single atomic_t mode with a consistent refcount, so it's
 	 * safe to drop our initial ref:
@@ -126,22 +130,29 @@ static void percpu_ref_kill_rcu(struct rcu_head *rcu)
 }
 
 /**
- * percpu_ref_kill - safely drop initial ref
+ * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
  * @ref: percpu_ref to kill
+ * @confirm_kill: optional confirmation callback
  *
- * Must be used to drop the initial ref on a percpu refcount; must be called
- * precisely once before shutdown.
+ * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
+ * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
+ * called after @ref is seen as dead from all CPUs - all further
+ * invocations of percpu_ref_tryget() will fail.  See percpu_ref_tryget()
+ * for more details.
  *
- * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
- * percpu counters and dropping the initial ref.
+ * Due to the way percpu_ref is implemented, @confirm_kill will be called
+ * after at least one full RCU grace period has passed but this is an
+ * implementation detail and callers must not depend on it.
  */
-void percpu_ref_kill(struct percpu_ref *ref)
+void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
+				 percpu_ref_func_t *confirm_kill)
 {
 	WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
 		  "percpu_ref_kill() called more than once!\n");
 
 	ref->pcpu_count = (unsigned __percpu *)
 		(((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
+	ref->confirm_kill = confirm_kill;
 
 	call_rcu(&ref->rcu, percpu_ref_kill_rcu);
 }