linux-kernel - Re: [PATCH v3 6/6] hisi_acc_vfio_pci: Add support for VFIO live migration

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Message-ID: <20210927150119.GB964074@nvidia.com>
Date:   Mon, 27 Sep 2021 12:01:19 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        Leon Romanovsky <leonro@...dia.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "mgurtovoy@...dia.com" <mgurtovoy@...dia.com>,
        liulongfang <liulongfang@...wei.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>,
        "Wangzhou (B)" <wangzhou1@...ilicon.com>
Subject: Re: [PATCH v3 6/6] hisi_acc_vfio_pci: Add support for VFIO live
 migration

On Mon, Sep 27, 2021 at 01:46:31PM +0000, Shameerali Kolothum Thodi wrote:

> > > > Nope, this is locked wrong and has no lifetime management.
> > >
> > > Ok. Holding the device_lock() sufficient here?
> > 
> > You can't hold a hisi_qm pointer with some kind of lifecycle
> > management of that pointer. device_lock/etc is necessary to call
> > pci_get_drvdata()
> 
> Since this migration driver only supports VF devices and the PF
> driver will not be removed until all the VF devices gets removed,
> is the locking necessary here?

Oh.. That is really busted up. pci_sriov_disable() is called under the
device_lock(pf) and obtains the device_lock(vf).

This means a VF driver can never use the device_lock(pf), otherwise it
can deadlock itself if PF removal triggers VF removal.

But you can't access these members without using the device_lock(), as
there really are no safety guarentees..

The mlx5 patches have this same sketchy problem.

We may need a new special function 'pci_get_sriov_pf_devdata()' that
confirms the vf/pf relationship and explicitly interlocks with the
pci_sriov_enable/disable instead of using device_lock()

Leon, what do you think?

Jason