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

[Danilo Krummrich <dakr@kernel.org>]

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.