notifiers: add tracepoints to the notifiers infrastructure

Currently there is no way to show the callback names for registered,
unregistered or executed notifiers. This is very useful for debug
purposes, hence add this functionality here in the form of notifiers'
tracepoints, one per operation.

[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/20230314200058.1326909-1-gpiccoli@igalia.comSigned-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Michael Kelley <mikelley@microsoft.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Xiaoming Ni <nixiaoming@huawei.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: Guilherme G. Piccoli <gpiccoli@igalia.com>
Cc: Guilherme G. Piccoli <kernel@gpiccoli.net>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/trace/events/notifier.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM notifier
+
+#if !defined(_TRACE_NOTIFIERS_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_NOTIFIERS_H
+
+#include <linux/tracepoint.h>
+
+DECLARE_EVENT_CLASS(notifier_info,
+
+	TP_PROTO(void *cb),
+
+	TP_ARGS(cb),
+
+	TP_STRUCT__entry(
+		__field(void *, cb)
+	),
+
+	TP_fast_assign(
+		__entry->cb = cb;
+	),
+
+	TP_printk("%ps", __entry->cb)
+);
+
+/*
+ * notifier_register - called upon notifier callback registration
+ *
+ * @cb:		callback pointer
+ *
+ */
+DEFINE_EVENT(notifier_info, notifier_register,
+
+	TP_PROTO(void *cb),
+
+	TP_ARGS(cb)
+);
+
+/*
+ * notifier_unregister - called upon notifier callback unregistration
+ *
+ * @cb:		callback pointer
+ *
+ */
+DEFINE_EVENT(notifier_info, notifier_unregister,
+
+	TP_PROTO(void *cb),
+
+	TP_ARGS(cb)
+);
+
+/*
+ * notifier_run - called upon notifier callback execution
+ *
+ * @cb:		callback pointer
+ *
+ */
+DEFINE_EVENT(notifier_info, notifier_run,
+
+	TP_PROTO(void *cb),
+
+	TP_ARGS(cb)
+);
+
+#endif /* _TRACE_NOTIFIERS_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

kernel/notifier.c

@@ -7,6 +7,9 @@
 #include <linux/vmalloc.h>
 #include <linux/reboot.h>
 
+#define CREATE_TRACE_POINTS
+#include <trace/events/notifier.h>
+
 /*
  *	Notifier list for kernel code which wants to be called
  *	at shutdown. This is used to stop any idling DMA operations
@@ -37,6 +40,7 @@ static int notifier_chain_register(struct notifier_block **nl,
 	}
 	n->next = *nl;
 	rcu_assign_pointer(*nl, n);
+	trace_notifier_register((void *)n->notifier_call);
 	return 0;
 }
 
@@ -46,6 +50,7 @@ static int notifier_chain_unregister(struct notifier_block **nl,
 	while ((*nl) != NULL) {
 		if ((*nl) == n) {
 			rcu_assign_pointer(*nl, n->next);
+			trace_notifier_unregister((void *)n->notifier_call);
 			return 0;
 		}
 		nl = &((*nl)->next);
@@ -84,6 +89,7 @@ static int notifier_call_chain(struct notifier_block **nl,
 			continue;
 		}
 #endif
+		trace_notifier_run((void *)nb->notifier_call);
 		ret = nb->notifier_call(nb, val, v);
 
 		if (nr_calls)