do_wait: fix the theoretical race with stop/trace/cont
Oleg Nesterov [Wed, 17 Jun 2009 23:27:42 +0000 (16:27 -0700)]
do_wait:

current->state = TASK_INTERRUPTIBLE;

read_lock(&tasklist_lock);
... search for the task to reap ...

In theory, the ->state changing can leak into the critical section.  Since
the child can change its status under read_lock(tasklist) in parallel
(finish_stop/ptrace_stop), we can miss the wakeup if __wake_up_parent()
sees us in TASK_RUNNING state.  Add the barrier.

Also, use __set_current_state() to set TASK_RUNNING.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
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/exit.c

index dd83c84..7ef355d 100644 (file)
@@ -1581,7 +1581,7 @@ repeat:
           (!wo->wo_pid || hlist_empty(&wo->wo_pid->tasks[wo->wo_type])))
                goto notask;
 
-       current->state = TASK_INTERRUPTIBLE;
+       set_current_state(TASK_INTERRUPTIBLE);
        read_lock(&tasklist_lock);
        tsk = current;
        do {
@@ -1608,7 +1608,7 @@ notask:
                }
        }
 end:
-       current->state = TASK_RUNNING;
+       __set_current_state(TASK_RUNNING);
        remove_wait_queue(&current->signal->wait_chldexit,&wait);
        if (wo->wo_info) {
                struct siginfo __user *infop = wo->wo_info;