linux-kernel - Re: [RFC PATCH v4 02/10] vfio/pci: macros to generate module_init and module

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Message-ID: <20200604170106.561db9ad.cohuck@redhat.com>
Date:   Thu, 4 Jun 2020 17:01:06 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        alex.williamson@...hat.com, zhenyuw@...ux.intel.com,
        zhi.a.wang@...el.com, kevin.tian@...el.com, shaopeng.he@...el.com,
        yi.l.liu@...el.com, xin.zeng@...el.com, hang.yuan@...el.com
Subject: Re: [RFC PATCH v4 02/10] vfio/pci: macros to generate module_init
 and module_exit for vendor modules

On Sun, 17 May 2020 22:45:10 -0400
Yan Zhao <yan.y.zhao@...el.com> wrote:

> vendor modules call macro module_vfio_pci_register_vendor_handler to
> generate module_init and module_exit.
> It is necessary to ensure that vendor modules always call
> vfio_pci_register_vendor_driver() on driver loading and
> vfio_pci_unregister_vendor_driver on driver unloading,
> because
> (1) at compiling time, there's only a dependency of vendor modules on
> vfio_pci.
> (2) at runtime,
> - vendor modules add refs of vfio_pci on a successful calling of
>   vfio_pci_register_vendor_driver() and deref of vfio_pci on a
>   successful calling of vfio_pci_unregister_vendor_driver().
> - vfio_pci only adds refs of vendor module on a successful probe of vendor
>   driver.
>   vfio_pci derefs vendor module when unbinding from a device.
> 
> So, after vfio_pci is unbound from a device, the vendor module to that
> device is free to get unloaded. However, if that vendor module does not
> call vfio_pci_unregister_vendor_driver() in its module_exit, vfio_pci may
> hold a stale pointer to vendor module.
> 
> Cc: Kevin Tian <kevin.tian@...el.com>
> Suggested-by: Alex Williamson <alex.williamson@...hat.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
>  include/linux/vfio.h | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index 3e53deb012b6..f3746608c2d9 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -223,4 +223,31 @@ struct vfio_pci_vendor_driver_ops {
>  };
>  int __vfio_pci_register_vendor_driver(struct vfio_pci_vendor_driver_ops *ops);
>  void vfio_pci_unregister_vendor_driver(struct vfio_device_ops *device_ops);
> +
> +#define vfio_pci_register_vendor_driver(__name, __probe, __remove,	\
> +					__device_ops)			\
> +static struct vfio_pci_vendor_driver_ops  __ops ## _node = {		\
> +	.owner		= THIS_MODULE,					\
> +	.name		= __name,					\
> +	.probe		= __probe,					\
> +	.remove		= __remove,					\
> +	.device_ops	= __device_ops,					\
> +};									\
> +__vfio_pci_register_vendor_driver(&__ops ## _node)
> +
> +#define module_vfio_pci_register_vendor_handler(name, probe, remove,	\
> +						device_ops)		\
> +static int __init device_ops ## _module_init(void)			\
> +{									\
> +	vfio_pci_register_vendor_driver(name, probe, remove,		\
> +					device_ops);			\

What if this function fails (e.g. with -ENOMEM)?

> +	return 0;							\
> +};									\
> +static void __exit device_ops ## _module_exit(void)			\
> +{									\
> +	vfio_pci_unregister_vendor_driver(device_ops);			\
> +};									\
> +module_init(device_ops ## _module_init);				\
> +module_exit(device_ops ## _module_exit)
> +
>  #endif /* VFIO_H */