Am 17.08.19 um 16:47 schrieb Chris Wilson:
> Currently dma_fence_signal() tries to avoid the spinlock and only takes
> it if absolutely required to walk the callback list. However, to allow
> for some users to surreptitiously insert lazy signal callbacks that
> do not depend on enabling the signaling mechanism around every fence,
> we always need to notify the callbacks on signaling. As such, we will
> always need to take the spinlock and dma_fence_signal() effectively
> becomes a clone of dma_fence_signal_locked().
>
> v2: Update the test_and_set_bit() before entering the spinlock.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Christian König <christian.koenig@xxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> drivers/dma-buf/dma-fence.c | 43 +++++++++++--------------------------
> 1 file changed, 13 insertions(+), 30 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index ff0cd6eae766..89d96e3e6df6 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -129,25 +129,16 @@ EXPORT_SYMBOL(dma_fence_context_alloc);
> int dma_fence_signal_locked(struct dma_fence *fence)
> {
> struct dma_fence_cb *cur, *tmp;
> - int ret = 0;
>
> lockdep_assert_held(fence->lock);
>
> - if (WARN_ON(!fence))
> + if (unlikely(test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> + &fence->flags)))
> return -EINVAL;
>
> - if (test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
> - ret = -EINVAL;
> -
> - /*
> - * we might have raced with the unlocked dma_fence_signal,
> - * still run through all callbacks
> - */
> - } else {
> - fence->timestamp = ktime_get();
> - set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
> - trace_dma_fence_signaled(fence);
> - }
> + fence->timestamp = ktime_get();
> + set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
> + trace_dma_fence_signaled(fence);
>
> if (!list_empty(&fence->cb_list)) {
> list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
> @@ -156,7 +147,8 @@ int dma_fence_signal_locked(struct dma_fence *fence)
> }
> INIT_LIST_HEAD(&fence->cb_list);
> }
> - return ret;
> +
> + return 0;
> }
> EXPORT_SYMBOL(dma_fence_signal_locked);
>
> @@ -176,28 +168,19 @@ EXPORT_SYMBOL(dma_fence_signal_locked);
> int dma_fence_signal(struct dma_fence *fence)
> {
> unsigned long flags;
> + int ret;
>
> if (!fence)
> return -EINVAL;
>
> - if (test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> + if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
> return -EINVAL;
Actually I think we can completely drop this extra test. Signaling a
fence twice shouldn't be the fast path we should optimize for.
Apart from that it looks good to me,
Christian.
>
> - fence->timestamp = ktime_get();
> - set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);
> - trace_dma_fence_signaled(fence);
> -
> - if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &fence->flags)) {
> - struct dma_fence_cb *cur, *tmp;
> + spin_lock_irqsave(fence->lock, flags);
> + ret = dma_fence_signal_locked(fence);
> + spin_unlock_irqrestore(fence->lock, flags);
>
> - spin_lock_irqsave(fence->lock, flags);
> - list_for_each_entry_safe(cur, tmp, &fence->cb_list, node) {
> - list_del_init(&cur->node);
> - cur->func(fence, cur);
> - }
> - spin_unlock_irqrestore(fence->lock, flags);
> - }
> - return 0;
> + return ret;
> }
> EXPORT_SYMBOL(dma_fence_signal);
>
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