Re: [PATCH] drm/xe: Wait on external BO kernel fences in exec IOCTL

[Matthew Auld <matthew.auld@intel.com>]

On 02/07/2026 22:58, Matthew Brost wrote:
> Before arming a user job, xe_exec_ioctl() only added the VM's
> dma-resv KERNEL slot as a dependency. That slot covers rebinds and
> the kernel operations of the VM's private BOs, but not external BOs
> (bo->vm == NULL), which carry their kernel operations (evictions,
> moves, ...) in their own dma-resv KERNEL slot.
> 
> The DMA_RESV_USAGE_KERNEL slot is the cross-driver contract for
> memory management operations that must complete before the BO or its
> backing store may be used: any accessor is required to wait on the
> KERNEL fences before touching the resv. By skipping the external BOs'
> KERNEL slots, the exec path violated that contract and could schedule
> a user job while a kernel operation on an external BO mapped by the VM
> was still in flight, racing against it and potentially reading or
> writing memory that was being moved.
> 
> Replace the VM-only dependency with an iteration over every object
> locked by the exec, adding each object's KERNEL slot as a job
> dependency. This covers the VM resv (rebinds and private BOs) as well
> as every external BO, mirroring the drm_gpuvm_resv_add_fence() call
> that later publishes the job fence to the same set of objects.
> Long-running mode continues to skip this, as before.
> 
> Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
> Cc: stable@vger.kernel.org
> Assisted-by: GitHub_Copilot:claude-opus-4.8
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>

Wow, kind of surprised we missed this.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>

> ---
>   drivers/gpu/drm/xe/xe_exec.c | 22 ++++++++++++++++------
>   1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_exec.c b/drivers/gpu/drm/xe/xe_exec.c
> index e05dabfcd43c..d5293bc33a67 100644
> --- a/drivers/gpu/drm/xe/xe_exec.c
> +++ b/drivers/gpu/drm/xe/xe_exec.c
> @@ -292,13 +292,23 @@ int xe_exec_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
>   		goto err_exec;
>   	}
>   
> -	/* Wait behind rebinds */
> +	/*
> +	 * Wait behind rebinds and any kernel operations (evictions, defrag
> +	 * moves, ...) on the VM and all external BOs. The VM's private BOs
> +	 * carry their kernel ops in the VM dma-resv KERNEL slot, while each
> +	 * external BO carries them in its own dma-resv KERNEL slot; both are
> +	 * covered by iterating every object locked by the exec, mirroring the
> +	 * drm_gpuvm_resv_add_fence() below.
> +	 */
>   	if (!xe_vm_in_lr_mode(vm)) {
> -		err = xe_sched_job_add_deps(job,
> -					    xe_vm_resv(vm),
> -					    DMA_RESV_USAGE_KERNEL);
> -		if (err)
> -			goto err_put_job;
> +		struct drm_gem_object *obj;
> +
> +		drm_exec_for_each_locked_object(exec, obj) {
> +			err = xe_sched_job_add_deps(job, obj->resv,
> +						    DMA_RESV_USAGE_KERNEL);
> +			if (err)
> +				goto err_put_job;
> +		}
>   	}
>   
>   	for (i = 0; i < num_syncs && !err; i++)