[PATCH 5/5] do_wait: fix the theoretical race with stop/trace/cont

Oleg Nesterov
2009-05-07T06:52:49Z

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>
---

kernel/exit.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- PTRACE/kernel/exit.c~5_MB 2009-05-07 05:34:40.000000000 +0200
+++ PTRACE/kernel/exit.c 2009-05-07 05:43:06.000000000 +0200
@@ -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;