Re: [PATCH][RFC] ASoC: soc-pcm: fixup try_module_get() calling timing

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

On 5/18/19 12:54 PM, Ranjani Sridharan wrote:
> On Fri, 2019-05-17 at 08:22 -0500, Pierre-Louis Bossart wrote:
>>
>> On 5/17/19 1:08 AM, Kuninori Morimoto wrote:
>>>
>>> From: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
>>>
>>> soc_pcm_components_open() try to call try_module_get()
>>> based on component->driver->module_get_upon_open.
>>> But, it should be always called, not relatead to .open callback.
>>> It should be called at (A) istead of (B)
>>>
>>> => (A)
>>> 	if (!component->driver->ops ||
>>> 	    !component->driver->ops->open)
>>> 		continue;
>>> => (B)
>>> 	if (component->driver->module_get_upon_open &&
>>> 	    !try_module_get(component->dev->driver->owner)) {
>>> 		...
>>> 	}
>>>
>>> 	ret = component->driver->ops->open(substream);
>>>
>>> Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
>>> ---
>>> Mark, Pierre-Louis, Vinod, Liam
>>>
>>> I think this patch is correct, but I'm not sure.
>>> I'm happy if someone can confirm it.
>>
>> The try_module_get()/module_put() mechanism is based on the
>> assumption
>> that the .open and .close callbacks are both mandatory.
> Hi Pierre,
> But is this enforced? We could end up doing a try_module_get() without
> checking if there is a close callback in which case we'd never do the
> module_put(), isnt it?


My initial feedback was that changing the open case only wouldn't work.

We need to enforce that both the open/close callbacks are required and 
leave the code as is, or we apply both of Morimoto-san's patches (which 
unfortunately have the same subject to cover the two cases) and both 
open and close are optional - though I am having a hard time figuring 
out case where we we'd use one and the other.

> 
> Thanks,
> Ranjani
>>
>> open flow:
>> 		if (!component->driver->ops ||
>> 		    !component->driver->ops->open)
>> 			continue;
>>
>> 		if (component->driver->module_get_upon_open &&
>> 		    !try_module_get(component->dev->driver->owner)) {
>> 			ret = -ENODEV;
>> 			goto module_err;
>> 		}
>>
>> 		ret = component->driver->ops->open(substream);
>>
>> close flow:
>> 		if (!component->driver->ops ||
>> 		    !component->driver->ops->close)
>> 			continue;
>>
>> 		component->driver->ops->close(substream);
>>
>> 		if (component->driver->module_get_upon_open)
>> 			module_put(component->dev->driver->owner);
>>
>> it'd be odd to allow the refcount to be increased when there is no
>> .open, since if there is no .close either then the refcount never
>> decreases.
>>
>>>
>>>
>>>    sound/soc/soc-pcm.c | 8 ++++----
>>>    1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/sound/soc/soc-pcm.c b/sound/soc/soc-pcm.c
>>> index 7737e00..7b4cda6 100644
>>> --- a/sound/soc/soc-pcm.c
>>> +++ b/sound/soc/soc-pcm.c
>>> @@ -440,10 +440,6 @@ static int soc_pcm_components_open(struct
>>> snd_pcm_substream *substream,
>>>    		component = rtdcom->component;
>>>    		*last = component;
>>>    
>>> -		if (!component->driver->ops ||
>>> -		    !component->driver->ops->open)
>>> -			continue;
>>> -
>>>    		if (component->driver->module_get_upon_open &&
>>>    		    !try_module_get(component->dev->driver->owner)) {
>>>    			dev_err(component->dev,
>>> @@ -452,6 +448,10 @@ static int soc_pcm_components_open(struct
>>> snd_pcm_substream *substream,
>>>    			return -ENODEV;
>>>    		}
>>>    
>>> +		if (!component->driver->ops ||
>>> +		    !component->driver->ops->open)
>>> +			continue;
>>> +
>>>    		ret = component->driver->ops->open(substream);
>>>    		if (ret < 0) {
>>>    			dev_err(component->dev,
>>>
>>
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