Re: [PATCH v2] drm/nouveau: Prevent signalled fences in pending list

Christian König
2025-04-03T12:22:49Z

Am 03.04.25 um 12:25 schrieb Danilo Krummrich:
> On Thu, Apr 03, 2025 at 12:17:29PM +0200, Philipp Stanner wrote:
>> On Thu, 2025-04-03 at 12:13 +0200, Philipp Stanner wrote:
>>> -static int
>>> -nouveau_fence_signal(struct nouveau_fence *fence)
>>> +static void
>>> +nouveau_fence_cleanup_cb(struct dma_fence *dfence, struct
>>> dma_fence_cb *cb)
>>>  {
>>> - int drop = 0;
>>> + struct nouveau_fence_chan *fctx;
>>> + struct nouveau_fence *fence;
>>> +
>>> + fence = container_of(dfence, struct nouveau_fence, base);
>>> + fctx = nouveau_fctx(fence);
>>>
>>> - dma_fence_signal_locked(&fence->base);
>>>   list_del(&fence->head);
>>>   rcu_assign_pointer(fence->channel, NULL);
>>>
>>>   if (test_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags))
>>> {
>>> - struct nouveau_fence_chan *fctx =
>>> nouveau_fctx(fence);
>>> -
>>>   if (!--fctx->notify_ref)
>>> - drop = 1;
>>> + nvif_event_block(&fctx->event);
>>>   }
>>>
>>>   dma_fence_put(&fence->base);
>> What I realized while coding this v2 is that we might want to think
>> about whether we really want the dma_fence_put() in the fence callback?
>>
>> It should work fine, since it's exactly identical to the previous
>> code's behavior – but effectively it means that the driver's reference
>> will be dropped whenever it signals that fence.
> Not quite, it's the reference of the fence context's pending list.
>
> When the fence is emitted, dma_fence_init() is called, which initializes the
> reference count to 1. Subsequently, another reference is taken, when the fence
> is added to the pending list. Once the fence is signaled and hence removed from
> the pending list, we can (and have to) drop this reference.

The general idea is that the caller must hold the reference until the signaling is completed.

So for signaling from the interrupt handler it means that you need to call dma_fence_put() for the list reference *after* you called dma_fence_signal_locked().

For signaling from the .enable_signaling or .signaled callback you need to remove the fence from the linked list and call dma_fence_put() *before* you return (because the caller is holding the potential last reference).

That's why I'm pretty sure that the approach with installing the callback won't work. As far as I know no other DMA fence implementation is doing that.

Regards,
Christian.