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

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On 2017-07-18 16:50:15 +0200, Hans Verkuil wrote:
> 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.

You are right I missed that, thanks for pointing it out, please see 
bellow.

> 
> > 
> >>
> >> 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.

I be fine with this, or simply aborting with -ENOMEM if the allocation 
fails. If the allocation fails I say we are in a lot of trouble anyhow, 
as Geert pointed out the kernel would already printed a warning and 
invoked the OOM-killer.

If you are OK with it I will rework the next version of this series to 
introduce this behavior. Let me know what you think.

> 
> 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) {
> >>>
> >>
> > 
> 

-- 
Regards,
Niklas Söderlund



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