Merge tag 'tracefs-v7.1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace

Pull tracefs fixes from Steven Rostedt:

 - Use list_add_tail_rcu() for walking eventfs children

   The linked list of children is protected by SRCU and list walkers can
   walk the list with only using SRCU. Using just list_add_tail() on
   weakly ordered architectures can cause issues. Instead use
   list_add_tail_rcu().

 - Hold eventfs_mutex and SRCU for remount walk events

   The trace_apply_options() walks the tracefs_inodes where some are
   eventfs inodes and eventfs_remount() is called which in turn calls
   eventfs_set_attr(). This walk only holds normal RCU read locks, but
   the eventfs_mutex and SRCU should be held.

   Add a eventfs_remount_(un)lock() helpers to take the necessary locks
   before iterating the list.

* tag 'tracefs-v7.1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  eventfs: Hold eventfs_mutex and SRCU when remount walks events
  eventfs: Use list_add_tail_rcu() for SRCU-protected children list

Author: torvalds

fs/tracefs/event_inode.c

@@ -244,6 +244,8 @@ static void eventfs_set_attrs(struct eventfs_inode *ei, bool update_uid, kuid_t
 {
 	struct eventfs_inode *ei_child;
 
+	lockdep_assert_held(&eventfs_mutex);
+
 	/* Update events/<system>/<event> */
 	if (WARN_ON_ONCE(level > 3))
 		return;
@@ -706,7 +708,7 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
 
 	scoped_guard(mutex, &eventfs_mutex) {
 		if (!parent->is_freed)
-			list_add_tail(&ei->list, &parent->children);
+			list_add_tail_rcu(&ei->list, &parent->children);
 	}
 	/* Was the parent freed? */
 	if (list_empty(&ei->list)) {
@@ -886,3 +888,15 @@ void eventfs_remove_events_dir(struct eventfs_inode *ei)
 	d_invalidate(dentry);
 	d_make_discardable(dentry);
 }
+
+int eventfs_remount_lock(void)
+{
+	mutex_lock(&eventfs_mutex);
+	return srcu_read_lock(&eventfs_srcu);
+}
+
+void eventfs_remount_unlock(int srcu_idx)
+{
+	srcu_read_unlock(&eventfs_srcu, srcu_idx);
+	mutex_unlock(&eventfs_mutex);
+}

fs/tracefs/inode.c

@@ -313,6 +313,7 @@ static int tracefs_apply_options(struct super_block *sb, bool remount)
 	struct inode *inode = d_inode(sb->s_root);
 	struct tracefs_inode *ti;
 	bool update_uid, update_gid;
+	int srcu_idx;
 	umode_t tmp_mode;
 
 	/*
@@ -337,6 +338,7 @@ static int tracefs_apply_options(struct super_block *sb, bool remount)
 		update_uid = fsi->opts & BIT(Opt_uid);
 		update_gid = fsi->opts & BIT(Opt_gid);
 
+		srcu_idx = eventfs_remount_lock();
 		rcu_read_lock();
 		list_for_each_entry_rcu(ti, &tracefs_inodes, list) {
 			if (update_uid) {
@@ -358,6 +360,7 @@ static int tracefs_apply_options(struct super_block *sb, bool remount)
 				eventfs_remount(ti, update_uid, update_gid);
 		}
 		rcu_read_unlock();
+		eventfs_remount_unlock(srcu_idx);
 	}
 
 	return 0;
@@ -403,7 +406,7 @@ static int tracefs_drop_inode(struct inode *inode)
 	 * This inode is being freed and cannot be used for
 	 * eventfs. Clear the flag so that it doesn't call into
 	 * eventfs during the remount flag updates. The eventfs_inode
-	 * gets freed after an RCU cycle, so the content will still
+	 * gets freed after an SRCU cycle, so the content will still
 	 * be safe if the iteration is going on now.
 	 */
 	ti->flags &= ~TRACEFS_EVENT_INODE;

fs/tracefs/internal.h

@@ -76,4 +76,7 @@ struct inode *tracefs_get_inode(struct super_block *sb);
 void eventfs_remount(struct tracefs_inode *ti, bool update_uid, bool update_gid);
 void eventfs_d_release(struct dentry *dentry);
 
+int eventfs_remount_lock(void);
+void eventfs_remount_unlock(int srcu_idx);
+
 #endif /* _TRACEFS_INTERNAL_H */