Re: relative openat dirfd reference on submit

[Pavel Begunkov <asml.silence@gmail.com>]

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?

-- 
Pavel Begunkov