[PATCH v3 4/5] drm/amdgpu: validate the eviction fence before attaching/detaching

[Liang, Prike]

[Public]

> From: Koenig, Christian <Christian.Koenig at amd.com>
> Sent: Tuesday, May 6, 2025 4:39 PM
> To: Liang, Prike <Prike.Liang at amd.com>; amd-gfx at lists.freedesktop.org
> Cc: Deucher, Alexander <Alexander.Deucher at amd.com>
> Subject: Re: [PATCH v3 4/5] drm/amdgpu: validate the eviction fence before
> attaching/detaching
>
> On 5/6/25 10:22, Liang, Prike wrote:
> >>> -   /* attach gfx eviction fence */
> >>> +   /* attach gfx the validated eviction fence */
> >>>     r = amdgpu_eviction_fence_attach(&fpriv->evf_mgr, abo);
> >>>     if (r) {
> >>>             DRM_DEBUG_DRIVER("Failed to attach eviction fence to BO\n");
> >>> +           amdgpu_bo_unreserve(abo);
> >> Adding this here looks like the only valid fix in the patch.
> > As the eviction fence will be invalidated until the user queue is created from the
> user space, here it requires validating the eviction fence before trying to attach
> and detach it to the reservation.
> > I will try to draft a patch for validating the eviction fence at attach/detach
> separately with this attach error handler change.
>
>
> No, that is clearly incorrect.
>
> See the eviction fence works like this:
>
> Validating thread
> * Create new eviction fence
> * Publish eviction fence
> * Lock all BOs
> * Replace eviction fence
>
> Attaching:
> * Lock BO
> * Attach current eviction fence
> * Unlock BO
>
> Detaching:
> * Lock BO
> * Unconditionally detach all possible eviction fences, no matter if new or old.
> * Unlock BO
>
> This order is necessary or otherwise you break the logic here.
>
> Any additional check will completely mess that up because it makes the operation
> racy.
As the user queue eviction fence doesn't create until user queue submission, the eviction fence will be NULL without userq submission. So do we still try to attach/detach the null eviction fence for the kernel queue case?
It's ok without validating the eviction fence or userqueue work before attach/detach the eviction fence, but it will cost cycles for walking over the reservation fences array in the dma_resv_reserve_fences() and dma_resv_replace_fences().

> Regards,
> Christian.
>
> >
> > Thanks,
> > Prike
> >
> >>
> >> Regards,
> >> Christian.
> >>
> >>>             return r;
> >>>     }
> >>>