[PATCH 08/14] namespace: Fix invalid wakeup in zap_pid_ns_processes

[Libin]

If thread is preempted before calling set_current_state(TASK_UNINTERRUPTIBLE),
and the other thread set the condition followed with wake_up_process. After
that when this thread is re-scheduled, calling set_current_state to set itself
as state TASK_UNINTERRUPTIBLE, if it is preempted again after that and before
__set_current_state(TASK_RUNNING), it triggers the invalid wakeup problem.
----------------------
zap_pid_ns_processes()
----------------------
...
for (;;) {
	set_current_state(TASK_UNINTERRUPTIBLE);
	if (pid_ns->nr_hashed == init_pids)
		break;
	schedule();
}
__set_current_state(TASK_RUNNING);
...

To solve this problem, using preempt_disable() to bound the operaion that
setting the task state and the conditions(set by the wake thread) validation.

Signed-off-by: Libin <huawei.libin@xxxxxxxxxx>
---
 kernel/pid_namespace.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 6917e8e..e3696a1 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -227,13 +227,17 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	 * sys_wait4() above can't reap the TASK_DEAD children.
 	 * Make sure they all go away, see free_pid().
 	 */
+	preempt_disable();
 	for (;;) {
 		set_current_state(TASK_UNINTERRUPTIBLE);
 		if (pid_ns->nr_hashed == init_pids)
 			break;
+		preempt_enable();
 		schedule();
+		preempt_disable();
 	}
 	__set_current_state(TASK_RUNNING);
+	preempt_enable();
 
 	if (pid_ns->reboot)
 		current->signal->group_exit_code = pid_ns->reboot;
-- 
1.8.2.1


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