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

[Bernd Schubert <bernd@bsbernd.com>]

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.

If wanted to be paranoid, we would switch to FUSE_OVER_IO_URING2 flag
and ignore FUSE_OVER_IO_URING. I hope you don't insist on allowing
fuse-server to set flags that fuse-server doesn't even announce...

Thanks,
Bernd