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

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

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.

>
> <... checking the code ...>
>
> I'm on a ublk branch without your commits a applied, i.e. plain upstream 7.2 fuse
>
> fuse_uring_cmd()
> 	/* Once a connection has io-uring enabled on it, it can't be disabled */
> 	if (!enable_uring && !fch->io_uring) {
> 		pr_info_ratelimited("fuse-io-uring is disabled\n");
> 		return -EOPNOTSUPP;
> 	}

BTW, the current code clears fch->io_uring on REGISTER failure:

                if (err) {
                        fch->io_uring = 0;
                        wake_up_all(&fch->blocked_waitq);
                        return err;
                }

But by then other entries may have already registered successfully,
fiq->ops is switched to fuse_io_uring_ops, and requests are flowing
through the uring path. Clearing fch->io_uring here may reject all
subsequent io_uring commands with -EOPNOTSUPP while the data path
is still running on uring ops.

fch->io_uring is a protocol fact — the daemon negotiated
FUSE_OVER_IO_URING at INIT time. A single REGISTER failure should
not regress it, at least not once the ring is ready. The daemon
can retry; the negotiated capability should be permanent.


Thanks,
Baokun