percpu-refcount: Introduce percpu_ref_resurrect()

This function will be used in a later patch to switch the struct
request_queue q_usage_counter from killed back to live. In contrast
to percpu_ref_reinit(), this new function does not require that the
refcount is zero.

Signed-off-by: Bart Van Assche <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Jianchao Wang <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

include/linux/percpu-refcount.h

@@ -108,6 +108,7 @@ void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
 void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_kill);
+void percpu_ref_resurrect(struct percpu_ref *ref);
 void percpu_ref_reinit(struct percpu_ref *ref);
 
 /**

lib/percpu-refcount.c

@@ -356,16 +356,40 @@ EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
  */
 void percpu_ref_reinit(struct percpu_ref *ref)
 {
+	WARN_ON_ONCE(!percpu_ref_is_zero(ref));
+
+	percpu_ref_resurrect(ref);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_reinit);
+
+/**
+ * percpu_ref_resurrect - modify a percpu refcount from dead to live
+ * @ref: perpcu_ref to resurrect
+ *
+ * Modify @ref so that it's in the same state as before percpu_ref_kill() was
+ * called. @ref must be dead but must not yet have exited.
+ *
+ * If @ref->release() frees @ref then the caller is responsible for
+ * guaranteeing that @ref->release() does not get called while this
+ * function is in progress.
+ *
+ * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
+ * this function is in progress.
+ */
+void percpu_ref_resurrect(struct percpu_ref *ref)
+{
+	unsigned long __percpu *percpu_count;
 	unsigned long flags;
 
 	spin_lock_irqsave(&percpu_ref_switch_lock, flags);
 
-	WARN_ON_ONCE(!percpu_ref_is_zero(ref));
+	WARN_ON_ONCE(!(ref->percpu_count_ptr & __PERCPU_REF_DEAD));
+	WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
 
 	ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
 	percpu_ref_get(ref);
 	__percpu_ref_switch_mode(ref, NULL);
 
 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
 }
-EXPORT_SYMBOL_GPL(percpu_ref_reinit);
+EXPORT_SYMBOL_GPL(percpu_ref_resurrect);