linux-kernel - Re: [PATCH 1/2] KVM: async kvm_destroy

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Message-ID: <Y8AA8r5MzKQIF8I7@nvidia.com>
Date:   Thu, 12 Jan 2023 08:45:38 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        alex.williamson@...hat.com, pbonzini@...hat.com, cohuck@...hat.com,
        farman@...ux.ibm.com, pmorel@...ux.ibm.com,
        borntraeger@...ux.ibm.com, frankja@...ux.ibm.com,
        imbrenda@...ux.ibm.com, david@...hat.com, akrowiak@...ux.ibm.com,
        jjherne@...ux.ibm.com, pasic@...ux.ibm.com,
        zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: async kvm_destroy_vm for vfio devices

On Wed, Jan 11, 2023 at 08:53:34PM +0000, Sean Christopherson wrote:
> On Wed, Jan 11, 2023, Jason Gunthorpe wrote:
> > On Wed, Jan 11, 2023 at 07:54:51PM +0000, Sean Christopherson wrote:
> > 
> > > Something feels off.  If KVM's refcount is 0, then accessing device->group->kvm
> > > in vfio_device_open() can't happen unless there's a refcounting bug somewhere.
> > 
> > The problem is in close, not open.
> 
> The deadlock problem is, yes.  My point is that if group_lock needs to be taken
> when nullifying group->kvm during kvm_vfio_destroy(), then there is also a refcounting
> prolem with respect to open().  If there is no refcounting problem, then nullifying
> group->kvm during kvm_vfio_destroy() is unnecessary (but again, I doubt this is
> the case).

IIRC the drivers are supposed to use one of the refcount not zero
incrs to counteract this, but I never checked they do..

Yi is working on a patch to change things so vfio drops the kvm
pointer when the kvm file closes, not when the reference goes to 0
to avoid a refcount cycle problem which should also solve that.

> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 6e8804fe0095..b3a84d65baa6 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -772,7 +772,12 @@ static struct file *vfio_device_open(struct vfio_device *device)
>  		 * reference and release it during close_device.
>  		 */
>  		mutex_lock(&device->group->group_lock);
> -		device->kvm = device->group->kvm;
> +
> +		if (device->kvm_ops && device->group->kvm) {
> +			ret = device->kvm_ops->get_kvm(device->group->kvm);

At this point I'd rather just use the symbol get stuff like kvm does
and call the proper functions.

Jason