Re: [PATCH 0/4] vfio/virtio: Fix list_lock type and modernize locking

[Yishai Hadas]

On 15/04/2026 18:39, Alex Williamson wrote:
> On Wed, 15 Apr 2026 18:12:50 +0300
> Yishai Hadas <yishaih@xxxxxxxxxx> wrote:
>
> > On 14/04/2026 23:06, Alex Williamson wrote:
> > > Jinhui Guo reported a mismatched spin_lock()/spin_unlock_irq() pair
> > > in virtiovf_read_device_context_chunk() where spin_unlock_irq() would
> > > unconditionally enable interrupts despite spin_lock() never having
> > > disabled them.  On closer inspection, the list_lock spinlock with IRQ
> > > disabling was copied from the mlx5 variant driver where a hardirq
> > > completion callback justifies it, but the virtio driver has no
> > > interrupt context usage of list_lock.  Patch 1 converts list_lock to
> > > a mutex, fixing the mismatch and aligning with peer vfio-pci variant
> > > drivers.
> >
> > Alex,
> > How about staying with spin_lock but without the 'irq' variant, instead
> > of replacing to mutex ?
> >
> > The scope of the lock is very small which can fit spin.
> > We may potentially get a performance degradation compared to mutex as
> > part of the hot path of STOP_COPY where this lock is used.
> >
> > Note:
> > I don't see that other peer vfio-pci variant drivers maintain a list of
> > buffers as of this driver, unless I missed that.
>
> The argument doesn't make sense to me, we use a spinlock if we have an
> operation that cannot be preempted and a spinlock-irq if we need to
> manage that from a hardirq context.  I think we just need mutual
> exclusion here.  Stealing the CPU because you want the absolute best
> performance for a little bit of list manipulation is not valid
> justification.
>
> Documentation/locking/mutex-design.rst:
>
>    When to use mutexes
>    -------------------
>
>    Unless the strict semantics of mutexes are unsuitable and/or the critical
>    region prevents the lock from being shared, always prefer them to any other
>    locking primitive.
>
> Can you cite specific requirements for a spinlock in the critical
> section here?  Thanks,

I see

In our use case, we mostly expect a single task handling the migration, 
so real contention is unlikely and we don’t anticipate the overhead of 
sleeping and waking to be triggered.

That said, I’m fine with using a mutex here as of your patch.

So,
Reviewed-by: Yishai Hadas <yishaih@xxxxxxxxxx>

For the full series.

Yishai

>   
> > > Patch 2 converts the list_lock acquisitions to guard()/scoped_guard()
> > > where the lock scope aligns naturally with function or block scope.
> > >    
> >
> > This patch might not be applicable if we'll stay with spin_lock.
> >
> > > Patches 3 and 4 extend the same guard() conversion to the remaining
> > > two mutexes in the driver (migf->lock and bar_mutex).  These are
> > > relatively independent of the list_lock fix but complete the
> > > conversion across the driver.  Thanks,
> > >    
> >
> > Those 2 patches seem fine to me.
> > Reviewed-by: Yishai Hadas <yishaih@xxxxxxxxxx>
> >
> > Yishai
> >
> > > Alex
> > >
> > > Alex Williamson (4):
> > >     vfio/virtio: Convert list_lock from spinlock to mutex
> > >     vfio/virtio: Use guard() for list_lock where applicable
> > >     vfio/virtio: Use guard() for migf->lock where applicable
> > >     vfio/virtio: Use guard() for bar_mutex in legacy I/O
> > >
> > >    drivers/vfio/pci/virtio/common.h    |  2 +-
> > >    drivers/vfio/pci/virtio/legacy_io.c | 17 +++---
> > >    drivers/vfio/pci/virtio/migrate.c   | 90 ++++++++++++-----------------
> > >    3 files changed, 46 insertions(+), 63 deletions(-)
> > >