tracing: Have the reg function allow to fail

Some tracepoints have a registration function that gets enabled when the
tracepoint is enabled. There may be cases that the registraction function
must fail (for example, can't allocate enough memory). In this case, the
tracepoint should also fail to register, otherwise the user would not know
why the tracepoint is not working.

Cc: David Howells <dhowells@redhat.com>
Cc: Seiji Aguchi <seiji.aguchi@hds.com>
Cc: Anton Blanchard <anton@samba.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_benchmark.c

@@ -164,11 +164,12 @@ static int benchmark_event_kthread(void *arg)
  * When the benchmark tracepoint is enabled, it calls this
  * function and the thread that calls the tracepoint is created.
  */
-void trace_benchmark_reg(void)
+int trace_benchmark_reg(void)
 {
 	bm_event_thread = kthread_run(benchmark_event_kthread,
 				      NULL, "event_benchmark");
 	WARN_ON(!bm_event_thread);
+	return 0;
 }
 
 /*

kernel/trace/trace_benchmark.h

@@ -6,7 +6,7 @@
 
 #include <linux/tracepoint.h>
 
-extern void trace_benchmark_reg(void);
+extern int trace_benchmark_reg(void);
 extern void trace_benchmark_unreg(void);
 
 #define BENCHMARK_EVENT_STRLEN		128

kernel/tracepoint.c

@@ -194,9 +194,13 @@ static int tracepoint_add_func(struct tracepoint *tp,
 			       struct tracepoint_func *func, int prio)
 {
 	struct tracepoint_func *old, *tp_funcs;
+	int ret;
 
-	if (tp->regfunc && !static_key_enabled(&tp->key))
-		tp->regfunc();
+	if (tp->regfunc && !static_key_enabled(&tp->key)) {
+		ret = tp->regfunc();
+		if (ret < 0)
+			return ret;
+	}
 
 	tp_funcs = rcu_dereference_protected(tp->funcs,
 			lockdep_is_held(&tracepoints_mutex));
@@ -529,7 +533,7 @@ EXPORT_SYMBOL_GPL(for_each_kernel_tracepoint);
 /* NB: reg/unreg are called while guarded with the tracepoints_mutex */
 static int sys_tracepoint_refcount;
 
-void syscall_regfunc(void)
+int syscall_regfunc(void)
 {
 	struct task_struct *p, *t;
 
@@ -541,6 +545,8 @@ void syscall_regfunc(void)
 		read_unlock(&tasklist_lock);
 	}
 	sys_tracepoint_refcount++;
+
+	return 0;
 }
 
 void syscall_unregfunc(void)