tracing: Use hashtable.h for event_hash

Convert the event_hash array in trace_output.c to use the generic
hashtable implementation from hashtable.h instead of the manually
implemented hash table.

This simplifies the code and makes it more maintainable by using the
standard hashtable API defined in hashtable.h.

Rename EVENT_HASHSIZE to EVENT_HASH_BITS to properly reflect its new
meaning as the number of bits for the hashtable size.

Link: https://lore.kernel.org/20250323132800.3010783-1-sashal@kernel.org
Link: https://lore.kernel.org/20250319190545.3058319-1-sashal@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: Sasha Levin

kernel/trace/trace_output.c

@@ -14,16 +14,17 @@
 #include <linux/idr.h>
 #include <linux/btf.h>
 #include <linux/bpf.h>
+#include <linux/hashtable.h>
 
 #include "trace_output.h"
 #include "trace_btf.h"
 
-/* must be a power of 2 */
-#define EVENT_HASHSIZE	128
+/* 2^7 = 128 */
+#define EVENT_HASH_BITS 7
 
 DECLARE_RWSEM(trace_event_sem);
 
-static struct hlist_head event_hash[EVENT_HASHSIZE] __read_mostly;
+static DEFINE_HASHTABLE(event_hash, EVENT_HASH_BITS);
 
 enum print_line_t trace_print_bputs_msg_only(struct trace_iterator *iter)
 {
@@ -779,11 +780,8 @@ void print_function_args(struct trace_seq *s, unsigned long *args,
 struct trace_event *ftrace_find_event(int type)
 {
 	struct trace_event *event;
-	unsigned key;
 
-	key = type & (EVENT_HASHSIZE - 1);
-
-	hlist_for_each_entry(event, &event_hash[key], node) {
+	hash_for_each_possible(event_hash, event, node, type) {
 		if (event->type == type)
 			return event;
 	}
@@ -838,7 +836,6 @@ void trace_event_read_unlock(void)
  */
 int register_trace_event(struct trace_event *event)
 {
-	unsigned key;
 	int ret = 0;
 
 	down_write(&trace_event_sem);
@@ -871,9 +868,7 @@ int register_trace_event(struct trace_event *event)
 	if (event->funcs->binary == NULL)
 		event->funcs->binary = trace_nop_print;
 
-	key = event->type & (EVENT_HASHSIZE - 1);
-
-	hlist_add_head(&event->node, &event_hash[key]);
+	hash_add(event_hash, &event->node, event->type);
 
 	ret = event->type;
  out:
@@ -888,7 +883,7 @@ EXPORT_SYMBOL_GPL(register_trace_event);
  */
 int __unregister_trace_event(struct trace_event *event)
 {
-	hlist_del(&event->node);
+	hash_del(&event->node);
 	free_trace_event_type(event->type);
 	return 0;
 }