On 9/10/20 6:37 AM, Pavel Begunkov wrote: > On 09/09/2020 19:07, Jens Axboe wrote: >> On 9/9/20 9:48 AM, Pavel Begunkov wrote: >>> On 09/09/2020 16:10, Jens Axboe wrote: >>>> On 9/9/20 1:09 AM, Pavel Begunkov wrote: >>>>> On 09/09/2020 01:54, Jens Axboe wrote: >>>>>> On 9/8/20 3:22 PM, Jens Axboe wrote: >>>>>>> On 9/8/20 2:58 PM, Pavel Begunkov wrote: >>>>>>>> On 08/09/2020 20:48, Jens Axboe wrote: >>>>>>>>> Fd instantiating commands like IORING_OP_ACCEPT now work with SQPOLL, but >>>>>>>>> we have an error in grabbing that if IOSQE_ASYNC is set. Ensure we assign >>>>>>>>> the ring fd/file appropriately so we can defer grab them. >>>>>>>> >>>>>>>> IIRC, for fcheck() in io_grab_files() to work it should be under fdget(), >>>>>>>> that isn't the case with SQPOLL threads. Am I mistaken? >>>>>>>> >>>>>>>> And it looks strange that the following snippet will effectively disable >>>>>>>> such requests. >>>>>>>> >>>>>>>> fd = dup(ring_fd) >>>>>>>> close(ring_fd) >>>>>>>> ring_fd = fd >>>>>>> >>>>>>> Not disagreeing with that, I think my initial posting made it clear >>>>>>> it was a hack. Just piled it in there for easier testing in terms >>>>>>> of functionality. >>>>>>> >>>>>>> But the next question is how to do this right...> >>>>>> Looking at this a bit more, and I don't necessarily think there's a >>>>>> better option. If you dup+close, then it just won't work. We have no >>>>>> way of knowing if the 'fd' changed, but we can detect if it was closed >>>>>> and then we'll end up just EBADF'ing the requests. >>>>>> >>>>>> So right now the answer is that we can support this just fine with >>>>>> SQPOLL, but you better not dup and close the original fd. Which is not >>>>>> ideal, but better than NOT being able to support it. >>>>>> >>>>>> Only other option I see is to to provide an io_uring_register() >>>>>> command to update the fd/file associated with it. Which may be useful, >>>>>> it allows a process to indeed to this, if it absolutely has to. >>>>> >>>>> Let's put aside such dirty hacks, at least until someone actually >>>>> needs it. Ideally, for many reasons I'd prefer to get rid of >>>> >>>> BUt it is actually needed, otherwise we're even more in a limbo state of >>>> "SQPOLL works for most things now, just not all". And this isn't that >>>> hard to make right - on the flush() side, we just need to park/stall the >>> >>> I understand that it isn't hard, but I just don't want to expose it to >>> the userspace, a) because it's a userspace API, so couldn't probably be >>> killed in the future, b) works around kernel's problems, and so >>> shouldn't really be exposed to the userspace in normal circumstances. >>> >>> And it's not generic enough because of a possible "many fds -> single >>> file" mapping, and there will be a lot of questions and problems. >>> >>> e.g. if a process shares a io_uring with another process, then >>> dup()+close() would require not only this hook but also additional >>> inter-process synchronisation. And so on. >> >> I think you're blowing this out of proportion. Just to restate the > > I just think that if there is a potentially cleaner solution without > involving userspace, we should try to look for it first, even if it > would take more time. That was the point. Regardless of whether or not we can eliminate that need, at least it'll be a relaxing of the restriction, not an increase of it. It'll never hurt to do an extra system call for the case where you're swapping fds. I do get your point, I just don't think it's a big deal. >>>>> fcheck(ctx->ring_fd) in favour of synchronisation in io_grab_files(), >>>>> but I wish I knew how. >>>> >>>> That'd be nice, and apply equally to all cases as the SQPOLL case isn't >>>> special at all anymore. >>> >>> I miss the whole story, have you asked fs guys about the problem? >>> Or is it known that nothing would work? >> >> I haven't looked into it. > > Any chance you have someone in mind who can take a look? I don't > think I have a chance to get to anyone in fs. I'll take a look. -- Jens Axboe
