[PATCH] drm/syncobj: Deal with signalled fences in transfer.

[Bas Nieuwenhuizen]

On Tue, Dec 7, 2021 at 8:21 AM Christian König <christian.koenig at amd.com> wrote:
>
> Am 07.12.21 um 08:10 schrieb Lionel Landwerlin:
> > On 07/12/2021 03:32, Bas Nieuwenhuizen wrote:
> >> See the comments in the code. Basically if the seqno is already
> >> signalled then we get a NULL fence. If we then put the NULL fence
> >> in a binary syncobj it counts as unsignalled, making that syncobj
> >> pretty much useless for all expected uses.
> >>
> >> Not 100% sure about the transfer to a timeline syncobj but I
> >> believe it is needed there too, as AFAICT the add_point function
> >> assumes the fence isn't NULL.
> >>
> >> Fixes: ea569910cbab ("drm/syncobj: add transition iotcls between
> >> binary and timeline v2")
> >> Cc: stable at vger.kernel.org
> >> Signed-off-by: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
> >> ---
> >>   drivers/gpu/drm/drm_syncobj.c | 26 ++++++++++++++++++++++++++
> >>   1 file changed, 26 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/drm_syncobj.c
> >> b/drivers/gpu/drm/drm_syncobj.c
> >> index fdd2ec87cdd1..eb28a40400d2 100644
> >> --- a/drivers/gpu/drm/drm_syncobj.c
> >> +++ b/drivers/gpu/drm/drm_syncobj.c
> >> @@ -861,6 +861,19 @@ static int
> >> drm_syncobj_transfer_to_timeline(struct drm_file *file_private,
> >>                        &fence);
> >>       if (ret)
> >>           goto err;
> >> +
> >> +    /* If the requested seqno is already signaled
> >> drm_syncobj_find_fence may
> >> +     * return a NULL fence. To make sure the recipient gets
> >> signalled, use
> >> +     * a new fence instead.
> >> +     */
> >> +    if (!fence) {
> >> +        fence = dma_fence_allocate_private_stub();
> >> +        if (!fence) {
> >> +            ret = -ENOMEM;
> >> +            goto err;
> >> +        }
> >> +    }
> >> +
> >
> >
> > Shouldn't we fix drm_syncobj_find_fence() instead?
>
> Mhm, now that you mention it. Bas, why do you think that
> dma_fence_chain_find_seqno() may return NULL when the fence is already
> signaled?
>
> Double checking the code that should never ever happen.

Well, I tested the patch with
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14097/diffs?commit_id=d4c5c840f4e3839f9f5c1747a9034eb2b565f5c0
so I'm pretty sure it happens, and this patch fixes  it, though I may
have misidentified what the code should do.

My reading is that the dma_fence_chain_for_each in
dma_fence_chain_find_seqno will never visit a signalled fence (unless
the top one is signalled), as dma_fence_chain_walk will never return a
signalled fence (it only returns on NULL or !signalled).

Happy to move this to drm_syncobj_find_fence.
>
> Regards,
> Christian.
>
> >
> > By returning a stub fence for the timeline case if there isn't one.
> >
> >
> > Because the same NULL fence check appears missing in amdgpu (and
> > probably other drivers).
> >
> >
> > Also we should have tests for this in IGT.
> >
> > AMD contributed some tests when this code was written but they never
> > got reviewed :(
> >
> >
> > -Lionel
> >
> >
> >>       chain = kzalloc(sizeof(struct dma_fence_chain), GFP_KERNEL);
> >>       if (!chain) {
> >>           ret = -ENOMEM;
> >> @@ -890,6 +903,19 @@ drm_syncobj_transfer_to_binary(struct drm_file
> >> *file_private,
> >>                        args->src_point, args->flags, &fence);
> >>       if (ret)
> >>           goto err;
> >> +
> >> +    /* If the requested seqno is already signaled
> >> drm_syncobj_find_fence may
> >> +     * return a NULL fence. To make sure the recipient gets
> >> signalled, use
> >> +     * a new fence instead.
> >> +     */
> >> +    if (!fence) {
> >> +        fence = dma_fence_allocate_private_stub();
> >> +        if (!fence) {
> >> +            ret = -ENOMEM;
> >> +            goto err;
> >> +        }
> >> +    }
> >> +
> >>       drm_syncobj_replace_fence(binary_syncobj, fence);
> >>       dma_fence_put(fence);
> >>   err:
> >
> >
>