Re: relative openat dirfd reference on submit

Jens Axboe
2020-11-05T14:09:24Z

On 11/5/20 1:45 AM, Stefan Metzmacher wrote:
> Am 05.11.20 um 00:43 schrieb Jens Axboe:
>> On 11/2/20 5:41 PM, Pavel Begunkov wrote:
>>> On 03/11/2020 00:34, Jens Axboe wrote:
>>>> On 11/2/20 5:17 PM, Pavel Begunkov wrote:
>>>>> On 03/11/2020 00:05, Jens Axboe wrote:
>>>>>> On 11/2/20 1:52 PM, Vito Caputo wrote:
>>>>>>> Hello list,
>>>>>>>
>>>>>>> I've been tinkering a bit with some async continuation passing style
>>>>>>> IO-oriented code employing liburing. This exposed a kind of awkward
>>>>>>> behavior I suspect could be better from an ergonomics perspective.
>>>>>>>
>>>>>>> Imagine a bunch of OPENAT SQEs have been prepared, and they're all
>>>>>>> relative to a common dirfd. Once io_uring_submit() has consumed all
>>>>>>> these SQEs across the syscall boundary, logically it seems the dirfd
>>>>>>> should be safe to close, since these dirfd-dependent operations have
>>>>>>> all been submitted to the kernel.
>>>>>>>
>>>>>>> But when I attempted this, the subsequent OPENAT CQE results were all
>>>>>>> -EBADFD errors. It appeared the submit didn't add any references to
>>>>>>> the dependent dirfd.
>>>>>>>
>>>>>>> To work around this, I resorted to stowing the dirfd and maintaining a
>>>>>>> shared refcount in the closures associated with these SQEs and
>>>>>>> executed on their CQEs. This effectively forced replicating the
>>>>>>> batched relationship implicit in the shared parent dirfd, where I
>>>>>>> otherwise had zero need to. Just so I could defer closing the dirfd
>>>>>>> until once all these closures had run on their respective CQE arrivals
>>>>>>> and the refcount for the batch had reached zero.
>>>>>>>
>>>>>>> It doesn't seem right. If I ensure sufficient queue depth and
>>>>>>> explicitly flush all the dependent SQEs beforehand
>>>>>>> w/io_uring_submit(), it seems like I should be able to immediately
>>>>>>> close(dirfd) and have the close be automagically deferred until the
>>>>>>> last dependent CQE removes its reference from the kernel side.
>>>>>>
>>>>>> We pass the 'dfd' straight on, and only the async part acts on it.
>>>>>> Which is why it needs to be kept open. But I wonder if we can get
>>>>>> around it by just pinning the fd for the duration. Since you didn't
>>>>>> include a test case, can you try with this patch applied? Totally
>>>>>> untested...
>>>>>
>>>>> afaik this doesn't pin an fd in a file table, so the app closes and
>>>>> dfd right after submit and then do_filp_open() tries to look up
>>>>> closed dfd. Doesn't seem to work, and we need to pass that struct
>>>>> file to do_filp_open().
>>>>
>>>> Yeah, I just double checked, and it's just referenced, but close() will
>>>> still make it NULL in the file table. So won't work... We'll have to
>>>> live with it for now, I'm afraid.
>>>
>>> Is there a problem with passing in a struct file? Apart from it
>>> being used deep in open callchains?
>>
>> No technical problems as far as I can tell, just needs doing...
>
> That would also allow fixed files to be used as dirfd, correct?

Correct

> If that's the case it would be great to have a way to install the resulting
> fd also (or maybe only) as fixed file.

That might be handy, yes.

--
Jens Axboe