linux-kernel - Re: [PATCH 4/5] virtio: introduce a vDPA based transport

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Date:   Mon, 20 Jan 2020 15:52:12 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     "mst@...hat.com" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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        "cunming.liang@...el.com" <cunming.liang@...el.com>,
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        Parav Pandit <parav@...lanox.com>,
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        Shahaf Shuler <shahafs@...lanox.com>,
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        "mhabets@...arflare.com" <mhabets@...arflare.com>
Subject: Re: [PATCH 4/5] virtio: introduce a vDPA based transport


On 2020/1/17 下午10:00, Jason Gunthorpe wrote:
> On Fri, Jan 17, 2020 at 05:32:35PM +0800, Jason Wang wrote:
>>>> +	const struct vdpa_config_ops *ops = vdpa->config;
>>>> +	struct virtio_vdpa_device *vd_dev;
>>>> +	int rc;
>>>> +
>>>> +	vd_dev = devm_kzalloc(dev, sizeof(*vd_dev), GFP_KERNEL);
>>>> +	if (!vd_dev)
>>>> +		return -ENOMEM;
>>> This is not right, the struct device lifetime is controled by a kref,
>>> not via devm. If you want to use a devm unwind then the unwind is
>>> put_device, not devm_kfree.
>> I'm not sure I get the point here. The lifetime is bound to underlying vDPA
>> device and devres allow to be freed before the vpda device is released. But
>> I agree using devres of underlying vdpa device looks wired.
> Once device_initialize is called the only way to free a struct device
> is via put_device, while here you have a devm trigger that will
> unconditionally do kfree on a struct device without respecting the
> reference count.
>
> reference counted memory must never be allocated with devm.


Right, fixed.


>
>>>> +	vd_dev->vdev.dev.release = virtio_vdpa_release_dev;
>>>> +	vd_dev->vdev.config = &virtio_vdpa_config_ops;
>>>> +	vd_dev->vdpa = vdpa;
>>>> +	INIT_LIST_HEAD(&vd_dev->virtqueues);
>>>> +	spin_lock_init(&vd_dev->lock);
>>>> +
>>>> +	vd_dev->vdev.id.device = ops->get_device_id(vdpa);
>>>> +	if (vd_dev->vdev.id.device == 0)
>>>> +		return -ENODEV;
>>>> +
>>>> +	vd_dev->vdev.id.vendor = ops->get_vendor_id(vdpa);
>>>> +	rc = register_virtio_device(&vd_dev->vdev);
>>>> +	if (rc)
>>>> +		put_device(dev);
>>> And a ugly unwind like this is why you want to have device_initialize()
>>> exposed to the driver,
>> In this context, which "driver" did you mean here? (Note, virtio-vdpa is the
>> driver for vDPA bus here).
> 'driver' is the thing using the 'core' library calls to implement a
> device, so here the 'vd_dev' is the driver and
> 'register_virtio_device' is the core


Ok.


>
>>> Where is the various THIS_MODULE's I expect to see in a scheme like
>>> this?
>>>
>>> All function pointers must be protected by a held module reference
>>> count, ie the above probe/remove and all the pointers in ops.
>> Will double check, since I don't see this in other virtio transport drivers
>> (PCI or MMIO).
> pci_register_driver is a macro that provides a THIS_MODULE, and the
> pci core code sets driver.owner, then the rest of the stuff related to
> driver ops is supposed to work against that to protect the driver ops.
>
> For the device module refcounting you either need to ensure that
> 'unregister' is a strong fence and guanentees that no device ops are
> called past unregister (noting that this is impossible for release),
> or you need to hold the module lock until release.
>
> It is common to see non-core subsystems get this stuff wrong.
>
> Jason


Ok. I see.

Thanks