linux-kernel - Re: [PATCH v5 04/13] s390: vfio-ap: base implementation of VFIO AP device driver

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Date:   Wed, 13 Jun 2018 14:12:21 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Pierre Morel <pmorel@...ux.ibm.com>
Cc:     Tony Krowiak <akrowiak@...ux.vnet.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        kwankhede@...dia.com, bjsdjshi@...ux.vnet.ibm.com,
        pbonzini@...hat.com, alex.williamson@...hat.com,
        pmorel@...ux.vnet.ibm.com, alifm@...ux.vnet.ibm.com,
        mjrosato@...ux.vnet.ibm.com, jjherne@...ux.vnet.ibm.com,
        thuth@...hat.com, pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com
Subject: Re: [PATCH v5 04/13] s390: vfio-ap: base implementation of VFIO AP
 device driver

On Wed, 13 Jun 2018 14:01:54 +0200
Pierre Morel <pmorel@...ux.ibm.com> wrote:

> On 13/06/2018 13:14, Cornelia Huck wrote:
> > On Wed, 13 Jun 2018 12:54:40 +0200
> > Pierre Morel <pmorel@...ux.ibm.com> wrote:
> >  
> >> On 13/06/2018 09:48, Cornelia Huck wrote:  
> >>> On Wed, 13 Jun 2018 09:41:16 +0200
> >>> Pierre Morel <pmorel@...ux.ibm.com> wrote:
> >>>     
> >>>> On 07/05/2018 17:11, Tony Krowiak wrote:  
> >>>>> Introduces a new AP device driver. This device driver
> >>>>> is built on the VFIO mediated device framework. The framework
> >>>>> provides sysfs interfaces that facilitate passthrough
> >>>>> access by guests to devices installed on the linux host.  
> >>>> ...snip...
> >>>>     
> >>>>> +static int vfio_ap_matrix_dev_create(void)
> >>>>> +{
> >>>>> +	int ret;
> >>>>> +
> >>>>> +	vfio_ap_root_device = root_device_register(VFIO_AP_ROOT_NAME);
> >>>>> +
> >>>>> +	if (IS_ERR(vfio_ap_root_device)) {
> >>>>> +		ret = PTR_ERR(vfio_ap_root_device);
> >>>>> +		goto done;
> >>>>> +	}
> >>>>> +
> >>>>> +	ap_matrix = kzalloc(sizeof(*ap_matrix), GFP_KERNEL);
> >>>>> +	if (!ap_matrix) {
> >>>>> +		ret = -ENOMEM;
> >>>>> +		goto matrix_alloc_err;
> >>>>> +	}
> >>>>> +
> >>>>> +	ap_matrix->device.type = &vfio_ap_dev_type;
> >>>>> +	dev_set_name(&ap_matrix->device, "%s", VFIO_AP_DEV_NAME);
> >>>>> +	ap_matrix->device.parent = vfio_ap_root_device;
> >>>>> +	ap_matrix->device.release = vfio_ap_matrix_dev_release;
> >>>>> +	ap_matrix->device.driver = &vfio_ap_drv.driver;
> >>>>> +
> >>>>> +	ret = device_register(&ap_matrix->device);
> >>>>> +	if (ret)
> >>>>> +		goto matrix_reg_err;
> >>>>> +
> >>>>> +	goto done;
> >>>>> +
> >>>>> +matrix_reg_err:
> >>>>> +	put_device(&ap_matrix->device);  
> >>>> Did not see this before but here you certainly want to
> >>>> do a kfree and not a put_device.  
> >>> No, this must not be a kfree. Once you've tried to register something
> >>> embedding a struct device with the driver core, you need to use
> >>> put_device, as another path may have acquired a reference, even if
> >>> registering ultimately failed. See the comment for device_register().
> >>> IOW, the code is correct.  
> >> learned something again :) ,
> >> but still, a kfree is needed for the kzalloc.
> >> Does'nt it?  
> > No, the put callback for the embedding structure needs to take care of
> > freeing things. Otherwise it is buggy.  
> 
> Seems buggy to me.
> How does the put_device knows if it is embedded and then in what it is 
> embedded ?

It does not need to know; the code registering the structure needs to
set up device->release correctly.

> 
> >  
> >>>     
> >>>>     
> >>>>> +
> >>>>> +matrix_alloc_err:
> >>>>> +	root_device_unregister(vfio_ap_root_device);
> >>>>> +
> >>>>> +done:
> >>>>> +	return ret;
> >>>>> +}
> >>>>> +
> >>>>> +static void vfio_ap_matrix_dev_destroy(struct ap_matrix *ap_matrix)
> >>>>> +{
> >>>>> +	device_unregister(&ap_matrix->device);
> >>>>> +	root_device_unregister(vfio_ap_root_device);  
> >>>> Also here you need a kfree(ap_matrix) too .  
> >>> Same here.
> >>>     
> 
>