Re: [PATCH net-next v10 02/14] net: page_pool: create hooks for custom page providers

Christian König
2024-06-11T08:22:08Z

Am 11.06.24 um 08:25 schrieb Christoph Hellwig:
> On Mon, Jun 10, 2024 at 02:38:18PM +0200, Christian König wrote:
>> Well there is the fundamental problem that you can't use io_uring to
>> implement the semantics necessary for a dma_fence.
> What is the exact problem there?

It's an intentional design decision that dma_fences can be waited on
with quite a bunch of locks held. Including the DMA-buf reservation
lock, mmap lock, anything page fault related, shrinker etc...

When you give userspace control over the signaling of a dma_fence then
that has the same effect as returning to userspace with those locks held
- you can basically trivially deadlock the system.

I think nearly a dozen implementations fell into that trap:
https://www.kernel.org/doc/html/v5.9/driver-api/dma-buf.html#indefinite-dma-fences

It's well understood and documented by now why this approach doesn't
work. So not much of an issue any more, we just have to reject
implementations from time to time which try doing the same thing again.

Regards,
Christian.