Re: [PATCH 1/2] PM / devfreq: Fix governor module load failure

[Chanwoo Choi <cw00.choi@samsung.com>]

On 19. 6. 20. 오후 4:59, Chanwoo Choi wrote:
> Hi,
> 
> On 19. 6. 6. 오전 4:00, Ezequiel Garcia wrote:
>> A bit unexpectedly (but still documented), request_module may
>> return a positive value, in case of a modprobe error.
>> This is currently causing issues in the devfreq framework.
>>
>> When a request_module exits with a positive value, we currently
>> return that via ERR_PTR. However, because the value is positive,
>> it's not a ERR_VALUE proper, and is therefore treated as a
>> valid struct devfreq_governor pointer, leading to a kernel oops.
>>
>> The right way to fix this is hinted in __request_module documentation:
>>
>> """
>> [snip] The function returns
>> zero on success or a negative errno code or positive exit code from
>> "modprobe" on failure. Note that a successful module load does not mean
>> the module did not then unload and exit on an error of its own. Callers
>> must check that the service they requested is now available not blindly
>> invoke it.
>> """
>>
>> Therefore, drop the return value check, which is not useful, and instead
>> just re-try to find the (hopefully now loaded) governor.
>>
>> Fixes: 23c7b54ca1cd1 ("PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.")
>> Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
>> ---
>>  drivers/devfreq/devfreq.c | 8 ++------
>>  1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>> index 6b6991f0e873..8868ad9472d2 100644
>> --- a/drivers/devfreq/devfreq.c
>> +++ b/drivers/devfreq/devfreq.c
>> @@ -236,7 +236,6 @@ static struct devfreq_governor *find_devfreq_governor(const char *name)
>>  static struct devfreq_governor *try_then_request_governor(const char *name)
>>  {
>>  	struct devfreq_governor *governor;
>> -	int err = 0;
>>  
>>  	if (IS_ERR_OR_NULL(name)) {
>>  		pr_err("DEVFREQ: %s: Invalid parameters\n", __func__);
>> @@ -251,13 +250,10 @@ static struct devfreq_governor *try_then_request_governor(const char *name)
>>  
>>  		if (!strncmp(name, DEVFREQ_GOV_SIMPLE_ONDEMAND,
>>  			     DEVFREQ_NAME_LEN))
>> -			err = request_module("governor_%s", "simpleondemand");
>> +			request_module("governor_%s", "simpleondemand");
> 
> I don't agree to remove the exception handling. Even if request_module()
> returns positive value, 

Sorry, I wrote the wrong comment. It have to handle the positive return value
for exception handling.

> 
>>  		else
>> -			err = request_module("governor_%s", name);
>> -		/* Restore previous state before return */
>> +			request_module("governor_%s", name);
> 
> ditto.
> 
>>  		mutex_lock(&devfreq_list_lock);
>> -		if (err)> -			return ERR_PTR(err);
> 
> You better to modify it as following:
> 
> if (err < 0)
> 	return ERR_PTR(err);
> else if (err > 0)
> 	return ERR_PTR(-EINVAL);
> 
> 
>>  
>>  		governor = find_devfreq_governor(name);
>>  	}
>>
> 
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics