percpu-refcount: init ->confirm_switch member properly

This patch targets two things which are related to ->confirm_switch:

 1. Init ->confirm_switch pointer with NULL on percpu_ref_init() or
    kernel frightfully complains with WARN_ON_ONCE(ref->confirm_switch)
    at __percpu_ref_switch_to_atomic if memory chunk was not properly
    zeroed.

 2. Warn if RCU callback is still in progress on percpu_ref_exit().
    The race still exists, because percpu_ref_call_confirm_rcu()
    drops ->confirm_switch to NULL early, but that is only a warning
    and still the caller is responsible that ref is no longer in
    active use.  Hopefully that can help to catch incorrect usage
    of percpu-refcount.
Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Tejun Heo <tj@kernel.org>

lib/percpu-refcount.c

@@ -83,6 +83,7 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
 	atomic_long_set(&ref->count, start_count);
 
 	ref->release = release;
+	ref->confirm_switch = NULL;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(percpu_ref_init);
@@ -102,6 +103,8 @@ void percpu_ref_exit(struct percpu_ref *ref)
 	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
 
 	if (percpu_count) {
+		/* non-NULL confirm_switch indicates switching in progress */
+		WARN_ON_ONCE(ref->confirm_switch);
 		free_percpu(percpu_count);
 		ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
 	}