percpu: READ_ONCE() now implies smp_read_barrier_depends()

Because READ_ONCE() now implies smp_read_barrier_depends(), this commit
removes the now-redundant smp_read_barrier_depends() following the
READ_ONCE() in __ref_is_percpu().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NTejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>

include/linux/percpu-refcount.h

@@ -139,12 +139,12 @@ static inline bool __ref_is_percpu(struct percpu_ref *ref,
 	 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
 	 * between contaminating the pointer value, meaning that
 	 * READ_ONCE() is required when fetching it.
+	 *
+	 * The smp_read_barrier_depends() implied by READ_ONCE() pairs
+	 * with smp_store_release() in __percpu_ref_switch_to_percpu().
 	 */
 	percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
 
-	/* paired with smp_store_release() in __percpu_ref_switch_to_percpu() */
-	smp_read_barrier_depends();
-
 	/*
 	 * Theoretically, the following could test just ATOMIC; however,
 	 * then we'd have to mask off DEAD separately as DEAD may be

lib/percpu-refcount.c

@@ -197,10 +197,10 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
 	atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
 
 	/*
-	 * Restore per-cpu operation.  smp_store_release() is paired with
-	 * smp_read_barrier_depends() in __ref_is_percpu() and guarantees
-	 * that the zeroing is visible to all percpu accesses which can see
-	 * the following __PERCPU_REF_ATOMIC clearing.
+	 * Restore per-cpu operation.  smp_store_release() is paired
+	 * with READ_ONCE() in __ref_is_percpu() and guarantees that the
+	 * zeroing is visible to all percpu accesses which can see the
+	 * following __PERCPU_REF_ATOMIC clearing.
 	 */
 	for_each_possible_cpu(cpu)
 		*per_cpu_ptr(percpu_count, cpu) = 0;