percpu-refcount: consistently use plain (non-sched) RCU

percpu_ref_get/put() are using preempt_disable/enable() while
percpu_ref_kill() is using plain call_rcu() instead of
call_rcu_sched().  This is buggy as grace periods of the two may not
match.  Fix it by using plain RCU in percpu_ref_get/put().

(I suggested using sched RCU in the first place but there's no actual
 benefit in doing so unless we're gonna introduce different variants
 of get/put to be called while preemption is alredy disabled, which we
 definitely shouldn't.)

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Kent Overstreet <koverstreet@google.com>

include/linux/percpu-refcount.h

@@ -85,7 +85,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
 {
 	unsigned __percpu *pcpu_count;
 
-	preempt_disable();
+	rcu_read_lock();
 
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 
@@ -94,7 +94,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
 	else
 		atomic_inc(&ref->count);
 
-	preempt_enable();
+	rcu_read_unlock();
 }
 
 /**
@@ -107,7 +107,7 @@ static inline void percpu_ref_put(struct percpu_ref *ref)
 {
 	unsigned __percpu *pcpu_count;
 
-	preempt_disable();
+	rcu_read_lock();
 
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 
@@ -116,7 +116,7 @@ static inline void percpu_ref_put(struct percpu_ref *ref)
 	else if (unlikely(atomic_dec_and_test(&ref->count)))
 		ref->release(ref);
 
-	preempt_enable();
+	rcu_read_unlock();
 }
 
 #endif