On Fri, 8 Nov 2024 10:42:56 +0100
"Christian König" <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
> The function silently assumed that signaling was already enabled for the
> dma_fence_array. This meant that without enabling signaling first we would
> never see forward progress.
>
> Fix that by falling back to testing each individual fence when signaling
> isn't enabled yet.
>
> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> ---
> drivers/dma-buf/dma-fence-array.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c
> index 46ac42bcfac0..01203796827a 100644
> --- a/drivers/dma-buf/dma-fence-array.c
> +++ b/drivers/dma-buf/dma-fence-array.c
> @@ -103,10 +103,22 @@ static bool dma_fence_array_enable_signaling(struct dma_fence *fence)
> static bool dma_fence_array_signaled(struct dma_fence *fence)
> {
> struct dma_fence_array *array = to_dma_fence_array(fence);
> + unsigned int i, num_pending;
>
> - if (atomic_read(&array->num_pending) > 0)
> + num_pending = atomic_read(&array->num_pending);
> + if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &array->base.flags)) {
> + if (!num_pending)
> + goto signal;
> return false;
> + }
> +
> + for (i = 0; i < array->num_fences; ++i) {
> + if (dma_fence_is_signaled(array->fences[i]) && !--num_pending)
> + goto signal;
> + }
> + return false;
>
> +signal:
> dma_fence_array_clear_pending_error(array);
> return true;
> }
It would be good to have comments explaining what happens here. I think
I figured it out, but it's far from obvious:
- we need to read array->num_pending before checking the enable_signal
bit to avoid racing with the enable_signaling() implementation,
which might decrement the counter, and cause a partial check.
- the !--num_pending is here to account for the any_signaled case
- if we race with enable_signaling(), that means the !num_pending
check in the is_signalling_enabled branch might be outdated
(num_pending might have been decremented), but that's fine. The user
will get the right value when testing again later
With this explained in comments, the patch is
`Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>`
