bpf: avoid rcu_dereference inside bpf_event_mutex lock region

During perf event attaching/detaching bpf programs,
the tp_event->prog_array change is protected by the
bpf_event_mutex lock in both attaching and deteching
functions. Although tp_event->prog_array is a rcu
pointer, rcu_derefrence is not needed to access it
since mutex lock will guarantee ordering.

Verified through "make C=2" that sparse
locking check still happy with the new change.

Also change the label name in perf_event_{attach,detach}_bpf_prog
from "out" to "unlock" to reflect the code action after the label.
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/trace/bpf_trace.c

@@ -769,20 +769,19 @@ int perf_event_attach_bpf_prog(struct perf_event *event,
 	mutex_lock(&bpf_event_mutex);
 
 	if (event->prog)
-		goto out;
+		goto unlock;
 
-	old_array = rcu_dereference_protected(event->tp_event->prog_array,
-					      lockdep_is_held(&bpf_event_mutex));
+	old_array = event->tp_event->prog_array;
 	ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
 	if (ret < 0)
-		goto out;
+		goto unlock;
 
 	/* set the new array to event->tp_event and set event->prog */
 	event->prog = prog;
 	rcu_assign_pointer(event->tp_event->prog_array, new_array);
 	bpf_prog_array_free(old_array);
 
-out:
+unlock:
 	mutex_unlock(&bpf_event_mutex);
 	return ret;
 }
@@ -796,11 +795,9 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
 	mutex_lock(&bpf_event_mutex);
 
 	if (!event->prog)
-		goto out;
-
-	old_array = rcu_dereference_protected(event->tp_event->prog_array,
-					      lockdep_is_held(&bpf_event_mutex));
+		goto unlock;
 
+	old_array = event->tp_event->prog_array;
 	ret = bpf_prog_array_copy(old_array, event->prog, NULL, &new_array);
 	if (ret < 0) {
 		bpf_prog_array_delete_safe(old_array, event->prog);
@@ -812,6 +809,6 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
 	bpf_prog_put(event->prog);
 	event->prog = NULL;
 
-out:
+unlock:
 	mutex_unlock(&bpf_event_mutex);
 }