On 10/09/2020 21:18, Jens Axboe wrote: > On 9/10/20 7:11 AM, Jens Axboe wrote: >> 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. > > BTW, I don't see how we can ever get rid of a need to enter the kernel, > we'd need some chance at grabbing the updated ->files, for instance. Thanks for taking a look. Yeah, agree, it should get it from somewhere, and that reminds me that we have a similar situation with sqo_mm -- it grabs it from the task-creator and keeps it to the end... Do we really need to set ->files of another thread? Retaining to the end seem to work well enough with mm. And we need, then it would be more consistent to replace mm there as well. > Might be possible to hold a reference to the task and grab it from > there, though feels a bit iffy to hold a task reference from the ring on > the task that holds a reference to the ring. Haven't looked too close, > should work though as this won't hold a file/files reference, it's just > a freeing reference. BTW, if the process-creator dies, then its ->files might be killed and ->sqo_files become dangling, so should be invalidated. Your approach with a task's reference probably handles it naturally. -- Pavel Begunkov
