Re: module: fix module_refcount() return when running in a module exit routine

[Masami Hiramatsu]

(2015/01/20 9:45), Rusty Russell wrote:
> James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> writes:
>> On Mon, 2015-01-19 at 16:21 +1030, Rusty Russell wrote:
>>> Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> writes:
>>>> (2015/01/19 1:55), James Bottomley wrote:
>>>>> From: James Bottomley <JBottomley@xxxxxxxxxxxxx>
>>>>>
>>>>> After e513cc1 module: Remove stop_machine from module unloading,
>>>>> module_refcount() is returning (unsigned long)-1 when called from within
>>>>> a routine that runs in module_exit.  This is confusing the scsi device
>>>>> put code which is coded to detect a module_refcount() of zero for
>>>>> running within a module exit routine and not try to do another
>>>>> module_put.  The fix is to restore the original behaviour of
>>>>> module_refcount() and return zero if we're running inside an exit
>>>>> routine.
>>>>>
>>>>> Fixes: e513cc1c07e2ab93a4514eec9833e031df3e30bb
>>>>> Reported-by: Bart Van Assche <bvanassche@xxxxxxx>
>>>>> Signed-off-by: James Bottomley <JBottomley@xxxxxxxxxxxxx>
>>>>>
>>>>
>>>> Yes, this should be fixed as you said, since it must return "unsigned long" value.
>>>
>>> But there are only three non-module callers:
>>>
>>> drivers/scsi/scsi.c:1012:	if (module && module_refcount(module) != 0)
>>> drivers/staging/lustre/lustre/obdclass/lu_object.c:1359:			LINVRNT(module_refcount(key->lct_owner) > 0);
>>> include/linux/module.h:447:unsigned long module_refcount(struct module *mod);
>>> kernel/debug/kdb/kdb_main.c:2026:		kdb_printf("%4ld ", module_refcount(mod));
>>> kernel/module.c:775:unsigned long module_refcount(struct module *mod)
>>> kernel/module.c:779:EXPORT_SYMBOL(module_refcount);
>>> kernel/module.c:859:	seq_printf(m, " %lu ", module_refcount(mod));
>>> kernel/module.c:911:	return sprintf(buffer, "%lu\n", module_refcount(mk->mod));
>>>
>>> The first one I think should be eliminated, and the second one is simply
>>> an assertion before calling module_put() (which should probably be
>>> eliminated).  The others are just printing information.
>>
>> If you really want to insist on module_reference() returning -1 when the
>> module is in it's exit phase, OK, but in that case, I think it should
>> return a signed value, not an unsigned one.
> 
> Sure; I just didn't want to paper over the problem here.  And I'm not
> sure we want to lose information, eg. in kgdb we're presumably looking
> at it because something went wrong...
> 
> Thanks,
> Rusty.
> 
> Subject: module: make module_refcount() a signed integer.
> 
> James Bottomley points out that it will be -1 during unload.  It's
> only used for diagnostics, so let's not hide that as it could be a
> clue as to what's gone wrong.

Thanks!
This looks good to me too :)

Reviewed-by: Masami Hiramatsu <maasami.hiramatsu.pt@xxxxxxxxxxx>

> 
> Cc: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
> Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index ebfb0e153c6a..b653d7c0a05a 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -444,7 +444,7 @@ extern void __module_put_and_exit(struct module *mod, long code)
>  #define module_put_and_exit(code) __module_put_and_exit(THIS_MODULE, code)
>  
>  #ifdef CONFIG_MODULE_UNLOAD
> -unsigned long module_refcount(struct module *mod);
> +int module_refcount(struct module *mod);
>  void __symbol_put(const char *symbol);
>  #define symbol_put(x) __symbol_put(VMLINUX_SYMBOL_STR(x))
>  void symbol_put_addr(void *addr);
> diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
> index f191bddf64b8..7b40c5f07dce 100644
> --- a/kernel/debug/kdb/kdb_main.c
> +++ b/kernel/debug/kdb/kdb_main.c
> @@ -2023,7 +2023,7 @@ static int kdb_lsmod(int argc, const char **argv)
>  		kdb_printf("%-20s%8u  0x%p ", mod->name,
>  			   mod->core_size, (void *)mod);
>  #ifdef CONFIG_MODULE_UNLOAD
> -		kdb_printf("%4ld ", module_refcount(mod));
> +		kdb_printf("%4d ", module_refcount(mod));
>  #endif
>  		if (mod->state == MODULE_STATE_GOING)
>  			kdb_printf(" (Unloading)");
> diff --git a/kernel/module.c b/kernel/module.c
> index 3965511ae133..2387c98347c1 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -772,9 +772,9 @@ static int try_stop_module(struct module *mod, int flags, int *forced)
>  	return 0;
>  }
>  
> -unsigned long module_refcount(struct module *mod)
> +int module_refcount(struct module *mod)
>  {
> -	return (unsigned long)atomic_read(&mod->refcnt) - MODULE_REF_BASE;
> +	return atomic_read(&mod->refcnt) - MODULE_REF_BASE;
>  }
>  EXPORT_SYMBOL(module_refcount);
>  
> @@ -856,7 +856,7 @@ static inline void print_unload_info(struct seq_file *m, struct module *mod)
>  	struct module_use *use;
>  	int printed_something = 0;
>  
> -	seq_printf(m, " %lu ", module_refcount(mod));
> +	seq_printf(m, " %i ", module_refcount(mod));
>  
>  	/*
>  	 * Always include a trailing , so userspace can differentiate
> @@ -908,7 +908,7 @@ EXPORT_SYMBOL_GPL(symbol_put_addr);
>  static ssize_t show_refcnt(struct module_attribute *mattr,
>  			   struct module_kobject *mk, char *buffer)
>  {
> -	return sprintf(buffer, "%lu\n", module_refcount(mk->mod));
> +	return sprintf(buffer, "%i\n", module_refcount(mk->mod));
>  }
>  
>  static struct module_attribute modinfo_refcnt =
> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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