Re: [PATCH v3 1/3] init / kthread: add module_long_probe_init() and module_long_probe_exit()

On 08/15, Luis R. Rodriguez wrote:

On Wed, Aug 13, 2014 at 07:51:01PM +0200, Oleg Nesterov wrote:

quoted

On 08/12, Luis R. Rodriguez wrote:

quoted

+/* To be used by modules which can take over 30 seconds at probe */

Probably the comment should explain that this hack should only be
used if the driver is buggy and is wating for "real fix".

quoted

+#define module_long_probe_init(initfn)				\
+	static struct task_struct *__init_thread;		\
+	static int _long_probe_##initfn(void *arg)		\
+	{							\
+		return initfn();				\
+	}							\
+	static inline __init int __long_probe_##initfn(void)	\
+	{							\
+		__init_thread = kthread_run(_long_probe_##initfn,\
+					    NULL,		\
+					    #initfn);		\
+		if (IS_ERR(__init_thread))			\
+			return PTR_ERR(__init_thread);		\
+		return 0;					\
+	}							\
+	module_init(__long_probe_##initfn);
+/* To be used by modules that require module_long_probe_init() */
+#define module_long_probe_exit(exitfn)				\
+	static inline void __long_probe_##exitfn(void)		\
+	{							\
+		exitfn();					\
+		if (__init_thread)				\
+			kthread_stop(__init_thread);		\
+	}							\

exitfn() should be called after kthread_stop(), and only if initfn()
returns 0. So it should probably do

	int err = kthread_stop(__init_thread);
	if (!err)
		exitfn();

Thanks! With the check for __init_thread as well as it can be
ERR_PTR(-ENOMEM), ERR_PTR(-EINTR), or NULL (for whatever other
reason).

Do you mean __long_probe_##exitfn() should also check ERR_PTR(__init_thread)?
I don't think so. If kthread_run() above fails, module_init() should return
the error (it does), so module_exit() won't be called.

quoted

But there is an additional complication, you can't use __init_thread
without get_task_struct(),

Can you elaborate why ? kthread_stop() uses get_task_struct(),

This is too late. This task_struct can be already freed/reused. See below.

wake_up_process() and finally put_task_struct(), and we're the
only user of this thread. Also kthread_run() ensures wake_up_process()
gets called on startup, so not sure where the race would be provided
all users here and with the respective helpers on buggy drivers.

quoted

so  __long_probe_##initfn() can't use
kthread_run(). It needs kthread_create() + get_task_struct() + wakeup.

I fail to see why we'd need to add get_task_struct() on
module_long_probe_init(), can you clarify?

kthread_stop(kthread_run(callback)) is only safe if callback() can not exit
on its own, without checking kthread_should_stop(). And btw that is why
kthread_stop() does get_task_struct()).

If callback() can exit (if it calls do_exit() or simply returns), then nothing
protects this task_struct, it will be freed.

Oleg.