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

[Alex Williamson <alex.williamson@nvidia.com>]

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(-)
> >   
>