On Wed, 15 Apr 2026 18:12:50 +0300 Yishai Hadas <[email protected]> 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 <[email protected]> > > 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(-) > > >
