[PATCH -mm 1/2] ptrace_stop: fix the race with ptrace detach+attach

[Oleg Nesterov <oleg@tv-sign.ru>]

If the tracer went away (may_ptrace_stop() failed), ptrace_stop() drops tasklist
and then changes the ->state from TASK_TRACED to TASK_RUNNING.

This can fool another tracer which attaches to us in between. Change the ->state
under tasklist_lock to ensure that ptrace_check_attach() can't wrongly succeed.
Also, remove the unnecessary mb().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- PT/kernel/signal.c~1_ptrace_stop	2007-11-21 21:41:02.000000000 +0300
+++ PT/kernel/signal.c	2007-11-22 16:59:35.000000000 +0300
@@ -1628,11 +1628,11 @@ static void ptrace_stop(int exit_code, i
 	} else {
 		/*
 		 * By the time we got the lock, our tracer went away.
-		 * Don't stop here.
+		 * Don't drop the lock yet, another tracer may come.
 		 */
-		read_unlock(&tasklist_lock);
-		set_current_state(TASK_RUNNING);
+		__set_current_state(TASK_RUNNING);
 		current->exit_code = nostop_code;
+		read_unlock(&tasklist_lock);
 	}
 
 	/*