On Wed, Feb 12, 2025 at 10:29:32AM +0800, Ming Lei wrote: > It is explained in the following links: > > https://lore.kernel.org/linux-block/b6211101-3f74-4dea-a880-81bb75575dbd@xxxxxxxxx/ > > - node kbuffer is registered in ublk uring_cmd's ->issue(), but lookup > in RW_FIXED OP's ->prep(), and ->prep() is always called before calling > ->issue() when the two are submitted in same io_uring_enter(), so you > need to move io_rsrc_node_lookup() & buffer importing from RW_FIXED's ->prep() > to ->issue() first. I don't think that's accurate, at least in practice. In a normal flow, we'll have this sequence: io_submit_sqes io_submit_sqe (uring_cmd ublk register) io_init_req ->prep() io_queue_sqe ->issue() io_submit_sqe (read/write_fixed) io_init_req ->prep() io_queue_sqe ->issue() The first SQE is handled in its entirety before even looking at the subsequent SQE. Since the register is first, then the read/write_fixed's prep will have a valid index. Testing this patch series appears to show this reliably works. > - secondly, ->issue() order is only respected by IO_LINK, and io_uring > can't provide such guarantee without using IO_LINK: > > Pavel explained it in the following link: > > https://lore.kernel.org/linux-block/68256da6-bb13-4498-a0e0-dce88bb32242@xxxxxxxxx/ > > There are also other examples, such as, register buffer stays in one > link chain, and the consumer OP isn't in this chain, the consumer OP > can still be issued before issuing register_buffer. Yep, I got that. Linking is just something I was hoping to avoid. I understand there are conditions that can break the normal flow I'm relying on regarding the ordering. This hasn't appeared to be a problem in practice, but I agree this needs to be handled.
