ptrace_stop: fix the race with ptrace detach+attach

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>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -1638,11 +1638,11 @@ static void ptrace_stop(int exit_code, int nostop_code, siginfo_t *info)
 	} 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);
 	}
 
 	/*