percpu_ref: remove unnecessary RCU grace period for staggered atomic switching confirmation

At the beginning, percpu_ref guaranteed a RCU grace period between a
call to percpu_ref_kill_and_confirm() and the invocation of the
confirmation callback.  This guarantee exposed internal implementation
details and got rescinded while switching over to sched RCU; however,
__percpu_ref_switch_to_atomic() still inserts a full sched RCU grace
period even when it can simply wait for the previous attempt.

Remove the unnecessary grace period and perform the confirmation
synchronously for staggered atomic switching attempts.  Update
comments accordingly.
Signed-off-by: Tejun Heo <tj@kernel.org>

lib/percpu-refcount.c

@@ -177,17 +177,11 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 		call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
 	} else if (confirm_switch) {
 		/*
-		 * Somebody already set ATOMIC.  Switching may still be in
-		 * progress.  @confirm_switch must be invoked after the
-		 * switching is complete and a full sched RCU grace period
-		 * has passed.  Wait synchronously for the previous
-		 * switching and schedule @confirm_switch invocation.
+		 * Somebody else already set ATOMIC.  Wait for its
+		 * completion and invoke @confirm_switch() directly.
 		 */
 		wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
-		ref->confirm_switch = confirm_switch;
-
-		percpu_ref_get(ref);	/* put after confirmation */
-		call_rcu_sched(&ref->rcu, percpu_ref_call_confirm_rcu);
+		confirm_switch(ref);
 	}
 }
 
@@ -211,10 +205,6 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
  * but it may block if @confirm_kill is specified and @ref is already in
  * the process of switching to atomic mode.  In such cases, @confirm_switch
  * will be invoked after the switching is complete.
- *
- * Due to the way percpu_ref is implemented, @confirm_switch will be called
- * after at least one full sched RCU grace period has passed but this is an
- * implementation detail and must not be depended upon.
  */
 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_switch)
@@ -290,11 +280,7 @@ void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
  *
  * This function normally doesn't block and can be called from any context
  * but it may block if @confirm_kill is specified and @ref is in the
- * process of switching to atomic mode by percpu_ref_switch_atomic().
- *
- * Due to the way percpu_ref is implemented, @confirm_switch will be called
- * after at least one full sched RCU grace period has passed but this is an
- * implementation detail and must not be depended upon.
+ * process of switching to atomic mode by percpu_ref_switch_to_atomic().
  */
 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 				 percpu_ref_func_t *confirm_kill)