linux-kernel - Re: [PATCH 3/7] vfio/pci: Use vfio_device_unmap_mapping

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Message-ID: <20210806141745.1d8c3e0a.alex.williamson@redhat.com>
Date:   Fri, 6 Aug 2021 14:17:45 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        peterx@...hat.com
Subject: Re: [PATCH 3/7] vfio/pci: Use vfio_device_unmap_mapping_range()

On Thu, 5 Aug 2021 22:04:18 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Thu, Aug 05, 2021 at 11:07:35AM -0600, Alex Williamson wrote:
> > @@ -2281,15 +2143,13 @@ static int vfio_pci_try_zap_and_vma_lock_cb(struct pci_dev *pdev, void *data)
> >  
> >  	vdev = container_of(device, struct vfio_pci_device, vdev);
> >  
> > -	/*
> > -	 * Locking multiple devices is prone to deadlock, runaway and
> > -	 * unwind if we hit contention.
> > -	 */
> > -	if (!vfio_pci_zap_and_vma_lock(vdev, true)) {
> > +	if (!down_write_trylock(&vdev->memory_lock)) {
> >  		vfio_device_put(device);
> >  		return -EBUSY;
> >  	}  
> 
> Now that this is simplified so much, I wonder if we can drop the
> memory_lock and just use the dev_set->lock?
> 
> That avoids the whole down_write_trylock thing and makes it much more
> understandable?

Hmm, that would make this case a lot easier, but using a mutex,
potentially shared across multiple devices, taken on every non-mmap
read/write doesn't really feel like a good trade-off when we're
currently using a per device rwsem to retain concurrency here.  Thanks,

Alex