Re: [PATCH v4 2/3] v4l: async: do not hold list_lock when reprobing devices

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On 18/07/17 16:39, Niklas Söderlund wrote:
> Hi Hans,
> 
> Thanks for your feedback.
> 
> On 2017-07-18 16:22:14 +0200, Hans Verkuil wrote:
>> On 17/07/17 18:59, Niklas Söderlund wrote:
>>> There is no good reason to hold the list_lock when reprobing the devices
>>> and it prevents a clean implementation of subdevice notifiers. Move the
>>> actual release of the devices outside of the loop which requires the
>>> lock to be held.
>>>
>>> Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@xxxxxxxxxxxx>
>>> ---
>>>  drivers/media/v4l2-core/v4l2-async.c | 29 ++++++++++-------------------
>>>  1 file changed, 10 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/drivers/media/v4l2-core/v4l2-async.c b/drivers/media/v4l2-core/v4l2-async.c
>>> index 0acf288d7227ba97..8fc84f7962386ddd 100644
>>> --- a/drivers/media/v4l2-core/v4l2-async.c
>>> +++ b/drivers/media/v4l2-core/v4l2-async.c
>>> @@ -206,7 +206,7 @@ void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
>>>  	unsigned int notif_n_subdev = notifier->num_subdevs;
>>>  	unsigned int n_subdev = min(notif_n_subdev, V4L2_MAX_SUBDEVS);
>>>  	struct device **dev;
>>> -	int i = 0;
>>> +	int i, count = 0;
>>>  
>>>  	if (!notifier->v4l2_dev)
>>>  		return;
>>> @@ -222,37 +222,28 @@ void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
>>>  	list_del(&notifier->list);
>>>  
>>>  	list_for_each_entry_safe(sd, tmp, &notifier->done, async_list) {
>>> -		struct device *d;
>>> -
>>> -		d = get_device(sd->dev);
>>> +		if (dev)
>>> +			dev[count] = get_device(sd->dev);
>>> +		count++;
>>>  
>>>  		if (notifier->unbind)
>>>  			notifier->unbind(notifier, sd, sd->asd);
>>>  
>>>  		v4l2_async_cleanup(sd);
>>> +	}
>>>  
>>> -		/* If we handled USB devices, we'd have to lock the parent too */
>>> -		device_release_driver(d);
>>> +	mutex_unlock(&list_lock);
>>>  
>>> -		/*
>>> -		 * Store device at the device cache, in order to call
>>> -		 * put_device() on the final step
>>> -		 */
>>> +	for (i = 0; i < count; i++) {
>>> +		/* If we handled USB devices, we'd have to lock the parent too */
>>>  		if (dev)
>>> -			dev[i++] = d;
>>> -		else
>>> -			put_device(d);
>>> +			device_release_driver(dev[i]);
>>
>> This changes the behavior. If the alloc failed, then at least put_device was still called.
>> Now that no longer happens.
> 
> Yes, but also changes the behavior to also only call get_device() if the 
> allocation was successful. So the behavior is kept the same as far as I 
> understands it.

Ah, I missed that. Sorry about that.

But regardless of that the device_release_driver(d) isn't called anymore.
It's not clear at all to me whether that is a problem or not.

> 
>>
>> Frankly I don't understand this code, it is in desperate need of some comments explaining
>> this whole reprobing thing.
> 
> I agree that the code is in need of comments, but I feel a patch that 
> separates the v4l2-async work from the re-probing work is a step in the 
> right direction :-)

Would it help to simplify this function to:

        dev = kvmalloc_array(n_subdev, sizeof(*dev), GFP_KERNEL);
        if (!dev) {
                dev_err(notifier->v4l2_dev->dev,
                        "Failed to allocate device cache!\n");

	        mutex_lock(&list_lock);

	        list_del(&notifier->list);

		/* this assumes device_release_driver(d) isn't necessary */
        	list_for_each_entry_safe(sd, tmp, &notifier->done, async_list) {
	                if (notifier->unbind)
        	                notifier->unbind(notifier, sd, sd->asd);

               	        v4l2_async_cleanup(sd);
	        }

        	mutex_unlock(&list_lock);
		return;
	}

	...and here the code where dev is non-NULL...

Yes, there is some code duplication, but it is a lot easier to understand.

Regards,

	Hans

> 
>>
>> I have this strong feeling that this function needs to be reworked.
> 
> I also strongly agree with this.
> 
>>
>> Regards,
>>
>> 	Hans
>>
>>>  	}
>>>  
>>> -	mutex_unlock(&list_lock);
>>> -
>>>  	/*
>>>  	 * Call device_attach() to reprobe devices
>>> -	 *
>>> -	 * NOTE: If dev allocation fails, i is 0, and the whole loop won't be
>>> -	 * executed.
>>>  	 */
>>> -	while (i--) {
>>> +	for (i = 0; dev && i < count; i++) {
>>>  		struct device *d = dev[i];
>>>  
>>>  		if (d && device_attach(d) < 0) {
>>>
>>
> 




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