Re: relative openat dirfd reference on submit

Stefan Metzmacher
2020-11-05T08:45:49Z

[-- Attachment #1.1: Type: text/plain, Size: 3099 bytes --]

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?
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.

metze

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]