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.
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).
Signed-off-by: Matt Evans matt@ozlabs.org --- drivers/vfio/pci/vfio_pci_core.c | 2 ++ drivers/vfio/pci/vfio_pci_dmabuf.c | 30 +++++++++++++++++++++++++----- include/linux/vfio_pci_core.h | 2 ++ 3 files changed, 29 insertions(+), 5 deletions(-)
diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c index 6757054e9d87..1f6066107831 100644 --- a/drivers/vfio/pci/vfio_pci_core.c +++ b/drivers/vfio/pci/vfio_pci_core.c @@ -663,6 +663,7 @@ int vfio_pci_core_enable(struct vfio_pci_core_device *vdev) vdev->has_vga = true;
vfio_pci_core_map_bars(vdev); + vdev->bars_revoked = false;
return 0;
@@ -2197,6 +2198,7 @@ int vfio_pci_core_init_dev(struct vfio_device *core_vdev) return ret; INIT_LIST_HEAD(&vdev->dmabufs); init_rwsem(&vdev->memory_lock); + init_rwsem(&vdev->dmabuf_lock); xa_init(&vdev->ctx);
return 0; diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c index c16f460c01d6..847ee5cc78e7 100644 --- a/drivers/vfio/pci/vfio_pci_dmabuf.c +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c @@ -90,9 +90,9 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf) * The refcount prevents both. */ if (priv->vdev) { - down_write(&priv->vdev->memory_lock); + down_write(&priv->vdev->dmabuf_lock); list_del_init(&priv->dmabufs_elm); - up_write(&priv->vdev->memory_lock); + up_write(&priv->vdev->dmabuf_lock); vfio_device_put_registration(&priv->vdev->vdev); } kfree(priv->phys_vec); @@ -305,12 +305,27 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
/* dma_buf_put() now frees priv */ INIT_LIST_HEAD(&priv->dmabufs_elm); - down_write(&vdev->memory_lock); + + /* + * dmabuf_lock synchronises access (R) or updates (W) to the + * vdev->dmabufs list and to bars_revoked (see below). The + * revocation state of DMABUF elements in the list is written + * holding both dmabuf_lock(W) and resv, and tested with + * either. + * + * dmabuf_lock -> resv + * + * vdev->bars_revoked tracks the BAR revocation status updated + * via vfio_pci_dma_buf_move(), so the initial DMABUF state + * follows the same criteria that later update the DMABUF + * state (BAR zap, etc.). + */ + down_write(&vdev->dmabuf_lock); dma_resv_lock(priv->dmabuf->resv, NULL); - priv->revoked = !__vfio_pci_memory_enabled(vdev); + priv->revoked = vdev->bars_revoked; list_add_tail(&priv->dmabufs_elm, &vdev->dmabufs); dma_resv_unlock(priv->dmabuf->resv); - up_write(&vdev->memory_lock); + up_write(&vdev->dmabuf_lock);
/* * dma_buf_fd() consumes the reference, when the file closes the dmabuf @@ -340,6 +355,8 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
lockdep_assert_held_write(&vdev->memory_lock);
+ down_write(&vdev->dmabuf_lock); + vdev->bars_revoked = revoked; list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) { if (!get_file_active(&priv->dmabuf->file)) continue; @@ -375,6 +392,7 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked) } fput(priv->dmabuf->file); } + up_write(&vdev->dmabuf_lock); }
void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev) @@ -393,6 +411,7 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev) */ vfio_pci_dma_buf_move(vdev, true);
+ down_write(&vdev->dmabuf_lock); list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) { if (!get_file_active(&priv->dmabuf->file)) continue; @@ -402,5 +421,6 @@ void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev) vfio_device_put_registration(&vdev->vdev); fput(priv->dmabuf->file); } + up_write(&vdev->dmabuf_lock); up_write(&vdev->memory_lock); } diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h index 9a1674c152aa..f99b152edcca 100644 --- a/include/linux/vfio_pci_core.h +++ b/include/linux/vfio_pci_core.h @@ -134,6 +134,7 @@ struct vfio_pci_core_device { bool pm_intx_masked; bool pm_runtime_engaged; bool sriov_active; + bool bars_revoked; struct pci_saved_state *pci_saved_state; struct pci_saved_state *pm_save; int ioeventfds_nr; @@ -148,6 +149,7 @@ struct vfio_pci_core_device { struct vfio_pci_core_device *sriov_pf_core_dev; struct notifier_block nb; struct rw_semaphore memory_lock; + struct rw_semaphore dmabuf_lock; struct list_head dmabufs; };