kill do_each_thread()

Eric has pointed out that we still have 3 users of do_each_thread().
Change them to use for_each_process_thread() and kill this helper.

There is a subtle change, after do_each_thread/while_each_thread g == t ==
&init_task, while after for_each_process_thread() they both point to
nowhere, but this doesn't matter.

> Why is for_each_process_thread() better than do_each_thread()?

Say, for_each_process_thread() is rcu safe, do_each_thread() is not.

And certainly

	for_each_process_thread(p, t) {
		do_something(p, t);
	}

looks better than

	do_each_thread(p, t) {
		do_something(p, t);
	} while_each_thread(p, t);

And again, there are only 3 users of this awkward helper left.  It should
have been killed years ago and in fact I thought it had already been
killed.  It uses while_each_thread() which needs some changes.

Link: https://lkml.kernel.org/r/20230817163708.GA8248@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: "Christian Brauner (Microsoft)" <brauner@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Jiri Slaby <jirislaby@kernel.org> # tty/serial
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: oleg-nesterov

arch/ia64/kernel/mca.c

@@ -1630,10 +1630,10 @@ default_monarch_init_process(struct notifier_block *self, unsigned long val, voi
 	}
 	printk("\n\n");
 	if (read_trylock(&tasklist_lock)) {
-		do_each_thread (g, t) {
+		for_each_process_thread(g, t) {
 			printk("\nBacktrace of pid %d (%s)\n", t->pid, t->comm);
 			show_stack(t, NULL, KERN_DEFAULT);
-		} while_each_thread (g, t);
+		}
 		read_unlock(&tasklist_lock);
 	}
 	/* FIXME: This will not restore zapped printk locks. */

drivers/tty/tty_io.c

@@ -3031,7 +3031,7 @@ void __do_SAK(struct tty_struct *tty)
 	} while_each_pid_task(session, PIDTYPE_SID, p);
 
 	/* Now kill any processes that happen to have the tty open */
-	do_each_thread(g, p) {
+	for_each_process_thread(g, p) {
 		if (p->signal->tty == tty) {
 			tty_notice(tty, "SAK: killed process %d (%s): by controlling tty\n",
 				   task_pid_nr(p), p->comm);
@@ -3048,7 +3048,7 @@ void __do_SAK(struct tty_struct *tty)
 					PIDTYPE_SID);
 		}
 		task_unlock(p);
-	} while_each_thread(g, p);
+	}
 	read_unlock(&tasklist_lock);
 	put_pid(session);
 }

fs/fs_struct.c

@@ -62,7 +62,7 @@ void chroot_fs_refs(const struct path *old_root, const struct path *new_root)
 	int count = 0;
 
 	read_lock(&tasklist_lock);
-	do_each_thread(g, p) {
+	for_each_process_thread(g, p) {
 		task_lock(p);
 		fs = p->fs;
 		if (fs) {
@@ -79,7 +79,7 @@ void chroot_fs_refs(const struct path *old_root, const struct path *new_root)
 			spin_unlock(&fs->lock);
 		}
 		task_unlock(p);
-	} while_each_thread(g, p);
+	}
 	read_unlock(&tasklist_lock);
 	while (count--)
 		path_put(old_root);

include/linux/sched/signal.h

@@ -648,13 +648,6 @@ extern void flush_itimer_signals(void);
 
 extern bool current_is_single_threaded(void);
 
-/*
- * Careful: do_each_thread/while_each_thread is a double loop so
- *          'break' will not work as expected - use goto instead.
- */
-#define do_each_thread(g, t) \
-	for (g = t = &init_task ; (g = t = next_task(g)) != &init_task ; ) do
-
 #define while_each_thread(g, t) \
 	while ((t = next_thread(t)) != g)