Re: [PATCH net-next 2/3] bpf: permit multiple bpf attachments for a single perf event

On Mon, Oct 23, 2017 at 10:58:04AM -0700, Yonghong Song wrote:

This patch enables multiple bpf attachments for a
kprobe/uprobe/tracepoint single trace event.

This forgets to explain _why_ this is a good thing to do.

+static DEFINE_MUTEX(bpf_event_mutex);
+
+int perf_event_attach_bpf_prog(struct perf_event *event,
+			   struct bpf_prog *prog)
+{
+	struct bpf_prog_array __rcu *old_array;
+	struct bpf_prog_array *new_array;
+	int ret;
+
+	mutex_lock(&bpf_event_mutex);
+
+	if (event->prog)
+		return -EEXIST;
+
+	old_array = rcu_dereference_protected(event->tp_event->prog_array,
+					      lockdep_is_held(&bpf_event_mutex));

Since all modifications to prog_array are serialized by this one mutex;
you don't need rcu_dereference() here, there are no possible ordering
problems.

+	ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
+	if (ret < 0)
+		goto out;
+
+	/* 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);
+
+	if (old_array)
+		bpf_prog_array_free(old_array);
+
+out:

Its customary to call that unlock:

+	mutex_unlock(&bpf_event_mutex);
+	return ret;
+}