Re: [PATCH v7 1/6] fuse: decouple fuse_ring creation from ent registration

[Baokun Li <libaokun@linux.alibaba.com>]

On 2026/8/21 01:20, Bernd Schubert wrote:
>
> On 8/20/26 18:16, Joanne Koong wrote:
>> On Thu, Aug 20, 2026 at 1:02 AM Baokun Li <libaokun@linux.alibaba.com> wrote:
>>> Hi all,
>>>
>>> On 2026/8/20 04:05, Bernd Schubert wrote:
>>>> On 8/19/26 19:56, Joanne Koong wrote:
>>>>> On Wed, Aug 19, 2026 at 4:35 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
>>>>>> On Fri, 14 Aug 2026 at 21:00, Joanne Koong <joannelkoong@gmail.com> wrote:
>>>>>>> Currently, the connection's fuse_ring is created lazily on the first
>>>>>>> FUSE_IO_URING_CMD_REGISTER command. A server registers entries from one
>>>>>>> thread per queue (one per CPU) and those threads issue their first
>>>>>>> REGISTER command concurrently. They then race to create the single
>>>>>>> per-connection fuse_ring, which required open-coded handling in
>>>>>>> fuse_uring_create() to detect and protect against concurrent creations.
>>>>>>>
>>>>>>> Decouple fuse_ring creation from ent registration and move it to
>>>>>>> FUSE_INIT reply processing after a server has negotiated and set
>>>>>>> FUSE_OVER_IO_URING. The ring is published before the connection is
>>>>>>> marked initialized. fuse_uring_register() no longer creates the ring and
>>>>>>> it instead uses the ring set up at init time.
>>>>>> I tested this with loraw (a "raw" loopback tester that doesn't use
>>>>>> libfuse) and it fails with
>>>>>>
>>>>>> root@kvm:~# ./loraw -u  /mnt/fuse
>>>>>> loraw: loraw.c:1010: lo_start_uring: Assertion `!cqe->res' failed.
>>>>>>
>>>>>> cqe->res is -22 (EINVAL).
>>>>>>
>>>>>> Attaching the reproducer.  To compile:
>>>>>>
>>>>>> cp $(KERNEL_TREE)/include/uapi/linux/fuse.h fuse_kernel.h
>>>>>> gcc loraw.c -oloraw -luring
>>>>>>
>>>>> Thanks for attaching the repro.
>>>>>
>>>>> This is happening because this patch uses the FUSE_OVER_IO_URING init
>>>>> reply as a signal that the ring should be created, but I missed that
>>>>> the FUSE_OVER_IO_URING reply is *optional*.
>>>>>
>>>>> Prior to this patch, there's two scenarios:
>>>>> a) server sets FUSE_OVER_IO_URING reply at init time - requests will
>>>>> automatically block until fuse-io-uring is completely set up
>>>>> b) server does not set FUSE_OVER_IO_URING but later sends uring
>>>>> register request - requests will continue along /dev/fuse path until
>>>>> fuse-io-uring is completely set up
>>>>>
>>>>> Libfuse sets FUSE_OVER_IO_URING in the reply, but the loraw.c server does not.
>>>>>
>>>>> I think the best way to fix this is to have the ring creation happen
>>>>> when the kernel receives the first io-uring command instead of at
>>>>> FUSE_INIT or at FUSE_IO_URING_CMD_REGISTER ent creation time, given
>>>>> that FUSE_IO_URING_ADD_QUEUE needs the ring to exist:
>>>> I don't think we should allow io-uring without FUSE_OVER_IO_URING and
>>>> I really thought that was disabled.
>>> I share Bernd's concern here. Allowing io-uring without
>>> FUSE_OVER_IO_URING means enabling a capability beyond what was
>>> negotiated. We should honor the negotiated feature set, and print
>>> the negotiated flags to dmesg at INIT time so issues like this are
>>> easy to spot.
>> Not sure if you missed this reply [1], but will copy and paste it here:
>>
>> This is pre-existing behavior that's been there since the beginning
>> (kernel version 6.14). I don't think we can change this now, or
>> it'll break backwards compatibility, like Miklos's loraw program.
>>
> I think we need to discuss this. I had replied that the current
> accidental scheme we
>
> - deadlock (lock order), with bg_lock being one issue, but I bet there
> is more
> - module option bypass
> - bypass of what fuse-client/kernel announces
>
> I.e. if a fuse-server did implement the accidental scheme, it was broken
> anyway.

Agreed. A server that skips FUSE_OVER_IO_URING was never in a
well-defined state — no blocking guarantee, no lock ordering
protection, nothing. There is nothing to preserve compatibility
with.


Thanks,
Baokun