Re: [PATCH v9 10/17] fuse: Add io-uring sqe commit and fetch support

[Bernd Schubert <bernd@bsbernd.com>]

On 1/22/25 01:45, Joanne Koong wrote:
> On Tue, Jan 21, 2025 at 4:18 PM Bernd Schubert <bernd@bsbernd.com> wrote:
>>
> ...
>>>>
>>>>>
>>>>>> +
>>>>>> +       err = fuse_ring_ent_set_commit(ring_ent);
>>>>>> +       if (err != 0) {
>>>>>> +               pr_info_ratelimited("qid=%d commit_id %llu state %d",
>>>>>> +                                   queue->qid, commit_id, ring_ent->state);
>>>>>> +               spin_unlock(&queue->lock);
>>>>>> +               return err;
>>>>>> +       }
>>>>>> +
>>>>>> +       ring_ent->cmd = cmd;
>>>>>> +       spin_unlock(&queue->lock);
>>>>>> +
>>>>>> +       /* without the queue lock, as other locks are taken */
>>>>>> +       fuse_uring_commit(ring_ent, issue_flags);
>>>>>> +
>>>>>> +       /*
>>>>>> +        * Fetching the next request is absolutely required as queued
>>>>>> +        * fuse requests would otherwise not get processed - committing
>>>>>> +        * and fetching is done in one step vs legacy fuse, which has separated
>>>>>> +        * read (fetch request) and write (commit result).
>>>>>> +        */
>>>>>> +       fuse_uring_next_fuse_req(ring_ent, queue, issue_flags);
>>>>>
>>>>> If there's no request ready to read next, then no request will be
>>>>> fetched and this will return. However, as I understand it, once the
>>>>> uring is registered, userspace should only be interacting with the
>>>>> uring via FUSE_IO_URING_CMD_COMMIT_AND_FETCH. However for the case
>>>>> where no request was ready to read, it seems like userspace would have
>>>>> nothing to commit when it wants to fetch the next request?
>>>>
>>>> We have
>>>>
>>>> FUSE_IO_URING_CMD_REGISTER
>>>> FUSE_IO_URING_CMD_COMMIT_AND_FETCH
>>>>
>>>>
>>>> After _CMD_REGISTER the corresponding ring-entry is ready to get fuse
>>>> requests and waiting. After it gets a request assigned and handles it
>>>> by fuse server the _COMMIT_AND_FETCH scheme applies. Did you possibly
>>>> miss that _CMD_REGISTER will already have it waiting?
>>>>
>>>
>>> Sorry for the late reply. After _CMD_REGISTER and _COMMIT_AND_FETCH,
>>> it seems possible that there is no fuse request waiting until a later
>>> time? This is the scenario I'm envisioning:
>>> a) uring registers successfully and fetches request through _CMD_REGISTER
>>> b) server replies to request and fetches new request through _COMMIT_AND_FETCH
>>> c) server replies to request, tries to fetch new request but no
>>> request is ready, through _COMMIT_AND_FETCH
>>>
>>> maybe I'm missing something in my reading of the code, but how will
>>> the server then fetch the next request once the request is ready? It
>>> will have to commit something in order to fetch it since there's only
>>> _COMMIT_AND_FETCH which requires a commit, no?
>>>
>>
>> The right name would be '_COMMIT_AND_FETCH_OR_WAIT'. Please see
>> fuse_uring_next_fuse_req().
>>
>> retry:
>>         spin_lock(&queue->lock);
>>         fuse_uring_ent_avail(ent, queue);           --> entry available
>>         has_next = fuse_uring_ent_assign_req(ent);
>>         spin_unlock(&queue->lock);
>>
>>         if (has_next) {
>>                 err = fuse_uring_send_next_to_ring(ent, issue_flags);
>>                 if (err)
>>                         goto retry;
>>         }
>>
>>
>> If there is no available request, the io-uring cmd stored in ent->cmd is
>> just queued/available.
> 
> Could you point me to where the wait happens?  I think that's the part
> I'm missing. In my reading of the code, if there's no available
> request (eg queue->fuse_req_queue is empty), then I see that has_next
> will return false and fuse_uring_next_fuse_req() /
> fuse_uring_commit_fetch() returns without having fetched anything.
> Where does the "if there is no available request, the io-uring cmd is
> just queued/available" happen?
>

You need to read it the other way around, without "has_next" the 
avail/queued entry is not removed from the list - it is available 
whenever a new request comes in. Looks like we either need refactoring 
or at least a comment.


Thanks,
Bernd