Re: [PATCH v8 1/9] vfio/pci: Remove DMABUF export dependency on vdev->memory_lock

[Alex Williamson]

On Tue,  6 Oct 2026 20:36:29 +0100
Matt Evans <matt@xxxxxxxxxx> wrote:

> The VFIO_DEVICE_FEATURE_DMA_BUF export previously used
> vdev->memory_lock to protect VFIO's list of exported DMABUFs
> (vdev->dmabufs) and their revocation status.  When adding a new
> export, memory_lock was taken for write.
> 
> A future commit will refactor DMABUF export into a function used from
> mmap(), which would create a new mmap_lock -> memory_lock dependency.
> 
> In preparation, this commit introduces a vdev->dmabuf_lock that
> protects the dmabufs list, per-DMABUF revocation state, and a new
> bars_revoked flag.  The flag tracks the device-wide BAR revocation
> state set via vfio_pci_dma_buf_move() under dmabuf_lock, removing the
> test of __vfio_pci_memory_enabled() and dependency on
> vdev->memory_lock.

There's a subtle change here that I'm not sure is worth mentioning, so
I'll at least put it on the list for posterity.  The initial revoked
state of a newly minted DMABUF is now governed by the device-wide BAR
revocation state set via vfio_pci_dma_buf_move().  This is subtly
different from the current behavior where the initial state is based on
instantaneous BAR accessibility tested under vdev->memory_lock.  Thanks,

Alex

> This is a cleanup currently, but allows the future export path to
> avoid holding memory_lock under mmap_lock and a therefore avoid a
> deadlock scenario (a fault handler attempting to take mmap_lock in a
> vfio-pci variant driver thread that holds memory_lock, vs another
> thread doing an mmap/export holding them both in the other order).