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

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Date:   Mon, 27 Sep 2021 21:30:23 +0300
From:   Leon Romanovsky <leonro@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        "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 03:22:24PM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 27, 2021 at 09:17:19PM +0300, Leon Romanovsky wrote:
> 
> > > The point is to all out a different locking regime that relies on the
> > > sriov enable/disable removing the VF struct devices
> > 
> > You can't avoid trylock, because this pci_get_sriov_pf_devdata() will be
> > called in VF where it already holds lock, so attempt to take PF lock
> > will cause to deadlock.
> 
> My whole point is we cannot use the device_lock *at all* and a
> pci_get_sriov_pf_devdata() would not have it.

Right

> 
> Instead it would have some test to confirm that the 'current' struct
> device is a VF of the 'target' struct device and thus the drvdata
> must be valid so long as the 'current' struct device hasn't completed
> remove.

I'm curious to see how can you implement it without holding VF lock.

> 
> It is a completely different locking scheme than device lock. It also
> relies on the PF driver placing the sriov enable/disable 'locks' in
> the correct place relative to their drvdata's.
> 
> Jason