Yang Xiaopeng writes:

>I'm sure that no user process use old root now,  but when run "umount
>/old_root", it says:
>   umount: /old_root: Device or resource busy
>
>I have tried to remount /proc within the new root *after* chroot, but
>get the same result.
>
>
I found the problem, I said that no user process use old root when run
my scripts, but
I'm wrong, actually there is a '3' fd open the file
"/old_root/dev/console". By adding
debug message in init/init.c, I found the problem: when init restart(in
exec_signal()),
before open the new terminal device, there is still a file opened(I
don't know which file it is), so the
terminal device(stdin) get fd '1', and the first dup(0)(stdout) return
'2', the second(stderr) return '3'.

I attach a simple patch to solve this problem.

init/init.c

@@ -735,6 +735,11 @@ static void exec_signal(int sig)
 			sigaddset(&unblock_signals, SIGTSTP);
 			sigprocmask(SIG_UNBLOCK, &unblock_signals, NULL);
 
+			/* Close whatever files are open. */
+			close(0);
+			close(1);
+			close(2);
+
 			/* Open the new terminal device */
 			if ((device_open(a->terminal, O_RDWR)) < 0) {
 				if (stat(a->terminal, &sb) != 0) {