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

At Sun, 17 Aug 2014 20:21:38 +0200,
Oleg Nesterov wrote:

On 08/17, Luis R. Rodriguez wrote:

quoted

In the last iteration that I have stress tested for corner cases I just
get_task_struct() on the init and then put_task_struct() at the exit, is that
fine too or are there reasons to prefer the module stuff?

I am fine either way.

I like the Takashi's idea because if sys_delete_module() is called before
initfn() completes it will return -EBUSY and not hang in TASK_UNINTERRUPTIBLE
state. But this is not necessarily good, so I leave this to you and Takashi.

Another merit of fiddling with module count is that the thread object
isn't referred in other than module_init.  That is, we'd need only
module_init() implementation like below (thanks to Oleg's advice):

#define module_long_probe_init(initfn)				\
	static int _long_probe_##initfn(void *arg)		\
	{							\
		module_put_and_exit(initfn());			\
		return 0;					\
	}							\
	static int __init __long_probe_##initfn(void)		\
	{							\
		struct task_struct *__init_thread =		\
			kthread_create(_long_probe_##initfn,	\
				       NULL, #initfn);		\
		if (IS_ERR(__init_thread))			\
			return PTR_ERR(__init_thread);		\
		__module_get(THIS_MODULE);			\
		wake_up_process(__init_thread);			\
		return 0;					\
	}							\
	module_init(__long_probe_##initfn)

... and module_exit() remains identical as the normal version.

But, it's really a small difference, and I don't mind much which way
to take, too.

quoted

+/*
+ * Linux device drivers must strive to handle driver initialization
+ * within less than 30 seconds,

Well, perhaps the comment should name the reason ;)

quoted

if device probing takes longer
+ * for whatever reason asynchronous probing of devices / loading
+ * firmware should be used. If a driver takes longer than 30 second
+ * on the initialization path

Or if the initialization code can't handle the errors properly (say,
mptsas can't handle the errors caused by SIGKILL).

quoted

+ * Drivers that use this helper should be considered broken and in need
+ * of some serious love.
+ */

Yes.

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_create(_long_probe_##initfn,\
+					       NULL,		\
+					       #initfn);	\
+		if (IS_ERR(__init_thread))			\
+			return PTR_ERR(__init_thread);		\
+		/*						\
+		 * callback won't check kthread_should_stop()	\
+		 * before bailing, so we need to protect it	\
+		 * before running it.				\
+		 */						\
+		get_task_struct(__init_thread); 		\
+		wake_up_process(__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)		\
+	{							\
+		int err;					\
+		/*						\
+		 * exitfn() will not be run if the driver's	\
+		 * real probe which is run on the kthread	\
+		 * failed for whatever reason, this will	\
+		 * wait for it to end.				\
+		 */						\
+		err = kthread_stop(__init_thread);		\
+		if (!err)					\
+			exitfn();				\
+		put_task_struct(__init_thread);	 		\
+	}							\
+	module_exit(__long_probe_##exitfn);

Both inline's look misleading, gcc will generate the code out-of-line
anyway. But this is cosmetic. And for cosmetic reasons, since the 1st
macro uses __init, the 2nd one should probably use __exit.

Yes, and it'd be better to mention not to mark initfn with __init
prefix.  (Meanwhile exitfn can be with __exit prefix.)


thanks,

Takashi