tracing/user_events: Remove RCU lock while pinning pages

pin_user_pages_remote() can reschedule which means we cannot hold any
RCU lock while using it. Now that enablers are not exposed out to the
tracing register callbacks during fork(), there is clearly no need to
require the RCU lock as event_mutex is enough to protect changes.

Remove unneeded RCU usages when pinning pages and walking enablers with
event_mutex held. Cleanup a misleading "safe" list walk that is not
needed. During fork() duplication, remove unneeded RCU list add, since
the list is not exposed yet.

Link: https://lkml.kernel.org/r/20230519230741.669-3-beaub@linux.microsoft.com
Link: https://lore.kernel.org/linux-trace-kernel/CAHk-=wiiBfT4zNS29jA0XEsy8EmbqTH1hAPdRJCDAJMD8Gxt5A@mail.gmail.com/

Fixes: 72357590 ("tracing/user_events: Use remote writes for event enablement")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[ change log written by Beau Belgrave ]
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events_user.c

@@ -439,9 +439,8 @@ static bool user_event_enabler_exists(struct user_event_mm *mm,
 				      unsigned long uaddr, unsigned char bit)
 {
 	struct user_event_enabler *enabler;
-	struct user_event_enabler *next;
 
-	list_for_each_entry_safe(enabler, next, &mm->enablers, link) {
+	list_for_each_entry(enabler, &mm->enablers, link) {
 		if (enabler->addr == uaddr && ENABLE_BIT(enabler) == bit)
 			return true;
 	}
@@ -456,19 +455,19 @@ static void user_event_enabler_update(struct user_event *user)
 	struct user_event_mm *next;
 	int attempt;
 
+	lockdep_assert_held(&event_mutex);
+
 	while (mm) {
 		next = mm->next;
 		mmap_read_lock(mm->mm);
-		rcu_read_lock();
 
-		list_for_each_entry_rcu(enabler, &mm->enablers, link) {
+		list_for_each_entry(enabler, &mm->enablers, link) {
 			if (enabler->event == user) {
 				attempt = 0;
 				user_event_enabler_write(mm, enabler, true, &attempt);
 			}
 		}
 
-		rcu_read_unlock();
 		mmap_read_unlock(mm->mm);
 		user_event_mm_put(mm);
 		mm = next;
@@ -496,7 +495,9 @@ static bool user_event_enabler_dup(struct user_event_enabler *orig,
 	enabler->values = orig->values & ENABLE_VAL_DUP_MASK;
 
 	refcount_inc(&enabler->event->refcnt);
-	list_add_rcu(&enabler->link, &mm->enablers);
+
+	/* Enablers not exposed yet, RCU not required */
+	list_add(&enabler->link, &mm->enablers);
 
 	return true;
 }