[Intel-gfx] [PATCH 01/22] dma-fence: Serialise signal enabling (dma_fence_enable_sw_signaling)

[Chris Wilson]

Quoting Chris Wilson (2019-10-03 10:36:18)
> Make dma_fence_enable_sw_signaling() behave like its
> dma_fence_add_callback() and dma_fence_default_wait() counterparts and
> perform the test to enable signaling under the fence->lock, along with
> the action to do so. This ensure that should an implementation be trying
> to flush the cb_list (by signaling) on retirement before freeing the
> fence, it can do so in a race-free manner.
> 
> See also 0fc89b6802ba ("dma-fence: Simply wrap dma_fence_signal_locked
> with dma_fence_signal").
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/dma-buf/dma-fence.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 2c136aee3e79..587727089134 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -285,19 +285,18 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence)
>  {
>         unsigned long flags;
>  
> +       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> +               return;
> +
> +       spin_lock_irqsave(fence->lock, flags);
>         if (!test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
>                               &fence->flags) &&
> -           !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&

Staring at this in discussion with Tvrtko, we can't drop this check
either. After which this looks almost identical with the other
enable_signaling callers.

>             fence->ops->enable_signaling) {
>                 trace_dma_fence_enable_signal(fence);
> -
> -               spin_lock_irqsave(fence->lock, flags);
> -
>                 if (!fence->ops->enable_signaling(fence))
>                         dma_fence_signal_locked(fence);
> -
> -               spin_unlock_irqrestore(fence->lock, flags);
>         }
> +       spin_unlock_irqrestore(fence->lock, flags);
>  }
>  EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
>  
> -- 
> 2.23.0
>