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

[Luis R. Rodriguez]

On Wed, Aug 13, 2014 at 07:51:01PM +0200, Oleg Nesterov wrote:
> On 08/12, Luis R. Rodriguez wrote:
> >
> > +/* 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".
> 
> > +#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).

> 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(), 
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.

> 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?

  Luis
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