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
