Re: [PATCH] dma-buf: Add DMA_BUF_IOCTL_SET_RELEASE_EVENTFD

[Val Packett <val@invisiblethingslab.com>]

On 10/21/26 10:43 AM, Christian König wrote:
> [..]
>> The buffer lifecycle coordination we need this for applies to KVM as well
>> and it generally doesn't make sense to have this as a provider-specific
>> thing since it's not related to the provider in any way, it's just
>> something we want from dmabufs in general.
> I wouldn't put that on the DMA-buf file descriptor where basically anybody can mess with it. That is just an recipe for trouble.
>
> The better approach seems to be putting it into the udmabuf_ioctl(), so that from the very beginning you add the eventfd which signals when the udmabuf is released.
>
> If we need that for more implementations than just udmabuf, we can potentially add it to dma_buf_export_info so that exporters can provide the eventfd file descriptor directly on creation.

Well, we already need it with both udmabuf and gntdev-dmabuf..

Involving the exporter sounds really annoying when it's not something 
exporter dependent really.. What if the policy is just "only one eventfd 
ever" i.e. the first call of the ioctl succeeds but later calls on the 
same dmabuf return an error? That should remove the concern about one 
user messing with another's events (and avoid the concurrent replacement 
issue sashiko-bot warned about)..

Thanks,
~val