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. 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. -- Jens Axboe
