On 9/10/20 4:11 PM, Jens Axboe wrote: > On 9/10/20 3:01 PM, Jens Axboe wrote: >> On 9/10/20 12:18 PM, 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. >>> 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. >> >> Sort of half assed attempt... >> >> Idea is to assign a ->files sequence before we grab files, and then >> compare with the current one once we need to use the files. If they >> mismatch, we -ECANCELED the request. >> >> For SQPOLL, don't grab ->files upfront, grab a reference to the task >> instead. Use the task reference to assign files when we need it. >> >> Adding Jann to help poke holes in this scheme. I'd be surprised if it's >> solid as-is, but hopefully we can build on this idea and get rid of the >> fcheck(). > > Split it into two, to make it easier to reason about. Added a few > comments, etc. Pushed to a temp branch, made a few more edits (forgot to wire up open). https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-files_struct -- Jens Axboe
