tracing: Quiet gcc warning about maybe unused link variable

From: "Steven Rostedt (VMware)" <[email protected]>

Commit 57ea2a34adf4 ("tracing/kprobes: Fix trace_probe flags on enable_trace_kprobe() failure") added an if statement that depends on another if statement that gcc doesn't see will initialize the "link" variable and gives the warning:

"warning: 'link' may be used uninitialized in this function"

It is really a false positive, but to quiet the warning, and also to make sure that it never actually is used uninitialized, initialize the "link" variable to NULL and add an if (!WARN_ON_ONCE(!link)) where the compiler thinks it could be used uninitialized.

---
 kernel/trace/trace_kprobe.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index 27ace4513c43..6b71860f3998 100644
@@ -400,7 +400,7 @@ static struct trace_kprobe *find_trace_kprobe(const char *event,
 static int
 enable_trace_kprobe(struct trace_kprobe *tk, struct trace_event_file *file)
 {
-	struct event_file_link *link;
+	struct event_file_link *link = NULL;
 	int ret = 0;
 
 	if (file) {

@@ -426,7 +426,9 @@ enable_trace_kprobe(struct trace_kprobe *tk, struct trace_event_file *file)
 
 	if (ret) {
 		if (file) {
-			list_del_rcu(&link->list);
+			/* Notice the if is true on not WARN() */
+			if (!WARN_ON_ONCE(!link))
+				list_del_rcu(&link->list);
 			kfree(link);
 			tk->tp.flags &= ~TP_FLAG_TRACE;
 		} else {

2.17.1