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

From: Oleg Nesterov <[email-masked]>
Date: Sun, 17 Aug 2014 20:21:38 +0200

On 08/17, Luis R. Rodriguez wrote:
>
> 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.

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

Well, perhaps the comment should name the reason ;)

> 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).

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

Yes.

> +#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.

I believe this version is correct.

Oleg.