[Intel-gfx] [PATCH v2] dma-fence: Serialise signal enabling (dma_fence_enable_sw_signaling)

[Chris Wilson]

Quoting Chris Wilson (2019-10-03 14:19:46)
> 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").
> 
> v2: Refactor all 3 enable_signaling paths to use a common function.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>  drivers/dma-buf/dma-fence.c | 78 +++++++++++++++++--------------------
>  1 file changed, 35 insertions(+), 43 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 2c136aee3e79..4a1442a93f04 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -273,6 +273,30 @@ void dma_fence_free(struct dma_fence *fence)
>  }
>  EXPORT_SYMBOL(dma_fence_free);
>  
> +static bool __dma_fence_enable_signaling(struct dma_fence *fence)
> +{
> +       bool was_set;
> +
> +       lockdep_assert_held(fence->lock);
> +
> +       was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> +                                  &fence->flags);
> +
> +       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> +               return true;

Perhaps this should be false for "could not _enable_ signaling as it was
already signaled", which is closer to the original.

> +
> +       if (!was_set && fence->ops->enable_signaling) {
> +               if (!fence->ops->enable_signaling(fence)) {
> +                       dma_fence_signal_locked(fence);
> +                       return true;
> +               }
> +
> +               trace_dma_fence_enable_signal(fence);
> +       }
> +
> +       return false;
> +}
> +
>  /**
>   * dma_fence_enable_sw_signaling - enable signaling on fence
>   * @fence: the fence to enable
> @@ -285,19 +309,12 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence)
>  {
>         unsigned long flags;
>  
> -       if (!test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> -                             &fence->flags) &&
> -           !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&
> -           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);
> +       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> +               return;
>  
> -               spin_unlock_irqrestore(fence->lock, flags);
> -       }
> +       spin_lock_irqsave(fence->lock, flags);
> +       __dma_fence_enable_signaling(fence);
> +       spin_unlock_irqrestore(fence->lock, flags);
>  }
>  EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
>  
> @@ -331,7 +348,6 @@ int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
>  {
>         unsigned long flags;
>         int ret = 0;
> -       bool was_set;
>  
>         if (WARN_ON(!fence || !func))
>                 return -EINVAL;
> @@ -343,25 +359,14 @@ int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
>  
>         spin_lock_irqsave(fence->lock, flags);
>  
> -       was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> -                                  &fence->flags);
> -
> -       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> +       if (__dma_fence_enable_signaling(fence)) {
> +               INIT_LIST_HEAD(&cb->node);
>                 ret = -ENOENT;
> -       else if (!was_set && fence->ops->enable_signaling) {
> -               trace_dma_fence_enable_signal(fence);
> -
> -               if (!fence->ops->enable_signaling(fence)) {
> -                       dma_fence_signal_locked(fence);
> -                       ret = -ENOENT;
> -               }
> -       }
> -
> -       if (!ret) {
> +       } else {
>                 cb->func = func;
>                 list_add_tail(&cb->node, &fence->cb_list);
> -       } else
> -               INIT_LIST_HEAD(&cb->node);
> +       }
> +
>         spin_unlock_irqrestore(fence->lock, flags);
>  
>         return ret;
> @@ -461,7 +466,6 @@ dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout)
>         struct default_wait_cb cb;
>         unsigned long flags;
>         signed long ret = timeout ? timeout : 1;
> -       bool was_set;
>  
>         if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
>                 return ret;
> @@ -473,21 +477,9 @@ dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout)
>                 goto out;
>         }
>  
> -       was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
> -                                  &fence->flags);
> -
> -       if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> +       if (__dma_fence_enable_signaling(fence))
>                 goto out;
>  
> -       if (!was_set && fence->ops->enable_signaling) {
> -               trace_dma_fence_enable_signal(fence);
> -
> -               if (!fence->ops->enable_signaling(fence)) {
> -                       dma_fence_signal_locked(fence);
> -                       goto out;
> -               }
> -       }
> -
>         if (!timeout) {
>                 ret = 0;
>                 goto out;
> -- 
> 2.23.0
>