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

Alex Williamson
2026-04-15T15:40:14Z

On Wed, 15 Apr 2026 18:12:50 +0300
Yishai Hadas <yishaih@nvidia.com> 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,

Alex

> > 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@nvidia.com>
>
> 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(-)
> >
>