Date:   Thu, 2 Feb 2023 16:13:07 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
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        "pmorel@...ux.ibm.com" <pmorel@...ux.ibm.com>,
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        "Christopherson,, Sean" <seanjc@...gle.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
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        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] vfio: fix deadlock between group lock and kvm lock

On Thu, 2 Feb 2023 23:04:10 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > From: Alex Williamson <alex.williamson@...hat.com>
> > Sent: Friday, February 3, 2023 3:42 AM
> > 
> > On Thu,  2 Feb 2023 11:24:42 -0500
> > Matthew Rosato <mjrosato@...ux.ibm.com> wrote:
> >   
> > > After 51cdc8bc120e, we have another deadlock scenario between the
> > > kvm->lock and the vfio group_lock with two different codepaths acquiring
> > > the locks in different order.  Specifically in vfio_open_device, vfio
> > > holds the vfio group_lock when issuing device->ops->open_device but  
> > some  
> > > drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> > > routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> > > before calling vfio_file_set_kvm which will acquire the vfio group_lock.
> > >
> > > To resolve this, let's remove the need for the vfio group_lock from the
> > > kvm_vfio_release codepath.  This is done by introducing a new spinlock to
> > > protect modifications to the vfio group kvm pointer, and acquiring a kvm
> > > ref from within vfio while holding this spinlock, with the reference held
> > > until the last close for the device in question.
> > >
> > > Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> > > Reported-by: Anthony Krowiak <akrowiak@...ux.ibm.com>
> > > Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> > > Signed-off-by: Matthew Rosato <mjrosato@...ux.ibm.com>
> > > ---
> > > Changes from v2:
> > > * Relocate the new functions back to vfio_main and externalize to call
> > >   from group (Kevin) since cdev will need this too
> > > * s/vfio_kvm_*_kvm/vfio_device_*_kvm/ and only pass device as input.
> > >   Handle new kvm_ref_lock directly inside vfio_device_get_kvm (Alex)
> > > * Add assert_lockdep_held for dev_set lock (Alex)
> > > * Internalize error paths for vfio_device_get_kvm_safe and now return
> > >   void - either device->kvm is set with a ref taken or is NULL (Alex)
> > > * Other flow suggestions to make the call path cleaner - Thanks! (Alex)
> > > * Can't pass group->kvm to vfio_device_open, as it references the value
> > >   outside of new lock.  Pass device->kvm to minimize changes in this
> > >   fix (Alex, Yi)
> > > Changes from v1:
> > > * use spin_lock instead of spin_lock_irqsave (Jason)
> > > * clear device->kvm_put as part of vfio_kvm_put_kvm (Yi)
> > > * Re-arrange code to avoid referencing the group contents from within
> > >   vfio_main (Kevin) which meant moving most of the code in this patch
> > >   to group.c along with getting/dropping of the dev_set lock
> > > ---
> > >  drivers/vfio/group.c     | 32 ++++++++++++++----
> > >  drivers/vfio/vfio.h      | 14 ++++++++
> > >  drivers/vfio/vfio_main.c | 70 ++++++++++++++++++++++++++++++++++++--  
> > --  
> > >  include/linux/vfio.h     |  2 +-
> > >  4 files changed, 103 insertions(+), 15 deletions(-)  
> > 
> > LGTM.  I'm not sure moving the functions to vfio_main really buys us
> > anything since we're making so much use of group fields.  The cdev
> > approach will necessarily be different, so the bulk of the get code will
> > likely need to move back to group.c anyway.
> >   
> 
> well my last comment was based on Matthew's v2 where the get code
> gets a kvm passed in instead of implicitly retrieving group ref_lock
> internally. In that case the get/put helpers only contain device logic
> thus fit in vfio_main.c.
> 
> with v3 then they have to be in group.c since we don't want to use
> group fields in vfio_main.c.
> 
> but I still think v2 of the helpers is slightly better. The only difference
> between cdev and group when handling this race is using different
> ref_lock. the symbol get/put part is exactly same. So even if we
> merge v3 like this, very likely Yi has to change it back to v2 style
> to share the get/put helpers while just leaving the ref_lock part
> handled differently between the two path.

I'm not really a fan of the asymmetry of the v2 version where the get
helper needs to be called under the new kvm_ref_lock, but the put
helper does not.  Having the get helper handle that makes the caller
much cleaner.  Thanks,

Alex

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