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 14:55:05 +0200

Damn, sorry for noise ;)

I was going to suggest to introduce module_put_and_exit() to simplify
this and potentially other users, but it already exists. So this code
can use it too without additional complications.

On 08/17, Oleg Nesterov wrote:
> On 08/17, Oleg Nesterov wrote:
> >
> > On 08/17, Takashi Iwai wrote:
> > >
> > > How about just increasing/decreasing the module count for blocking the
> > > exit call?  For example:
> > >
> > > #define module_long_probe_init(initfn)				\
> > > 	static int _long_probe_##initfn(void *arg)		\
> > > 	{							\
> > > 		int ret = initfn();				\
> > > 		module_put(THIS_MODULE);			\
> >
> > WINDOW, please see below.
> >
> > > 		return ret;					\
> > > 	}							\
> > > 	static inline __init int __long_probe_##initfn(void)	\
> > > 	{							\
> > > 		struct task_struct *__init_thread;		\
> > > 		__module_get(THIS_MODULE);			\
> > > 		__init_thread = kthread_run(_long_probe_##initfn,\
> > > 					    NULL,		\
> > > 					    #initfn);		\
> > > 		if (IS_ERR(__init_thread)) {			\
> > > 			module_put(THIS_MODULE);		\
> > > 			return PTR_ERR(__init_thread);		\
> > > 		}						\
> > > 		return 0;					\
> > > 	}							\
> >
> > I leave this to you and Luis, but personally I think this is very
> > nice idea, I like it. Because sys_delete_module() won't hang in D
> > state waiting for initfn().
> >
> > There is a small problem. This module can be unloaded right after
> > module_put() above. In this case its memory can be unmapped and
> > the exiting thread can crash.
> >
> > This is very unlikely, this thread needs to execute just a few insn
> > and escape from this module's memory. Given that only the buggy
> > modules should use this hack, perhaps we can even ignore this race.
> >
> > But perhaps it makes sense to close this race anyway, and we already
> > have complete_and_exit() which can be used instead of "return ret"
> > above. Just we need the additional "static struct completion" and
> > module_exit() should call wait_for_completion.
> 
> Forgot to mention... and __long_probe_##initfn() could be simpler
> without kthread_run,
> 
> 	__init_thread = kthread_create(...);
> 	if (IS_ERR(__init_thread))
> 		return PTR_ERR();
> 
> 	module_get(THIS_MODULE);
> 	wake_up_process(__init_thread);
> 	return 0;
> 
> but this is subjective, up to you.
> 
> Oleg.