On 10/8/19 4:05 PM, Pavel Begunkov wrote: > On 09/10/2019 00:22, Jens Axboe wrote: >> On 10/8/19 2:58 PM, Pavel Begunkov wrote: >>> On 08/10/2019 20:00, Jens Axboe wrote: >>>> On 10/8/19 10:43 AM, Pavel Begunkov wrote: >>>>> On 08/10/2019 06:16, Jens Axboe wrote: >>>>>> On 10/7/19 5:18 PM, Pavel Begunkov (Silence) wrote: >>>>>>> From: Pavel Begunkov <asml.silence@xxxxxxxxx> >>>>>>> >>>>>>> Any changes interesting to tasks waiting in io_cqring_wait() are >>>>>>> commited with io_cqring_ev_posted(). However, io_ring_drop_ctx_refs() >>>>>>> also tries to do that but with no reason, that means spurious wakeups >>>>>>> every io_free_req() and io_uring_enter(). >>>>>>> >>>>>>> Just use percpu_ref_put() instead. >>>>>> >>>>>> Looks good, this is a leftover from when the ctx teardown used >>>>>> the waitqueue as well. >>>>>> >>>>> BTW, is there a reason for ref-counting in struct io_kiocb? I understand >>>>> the idea behind submission reference, but don't see any actual part >>>>> needing it. >>>> >>>> In short, it's to prevent the completion running before we're done with >>>> the iocb on the submission side. >>> >>> Yep, that's what I expected. Perhaps I missed something, but what I've >>> seen following code paths all the way down, it either >>> 1. gets error / completes synchronously and then frees req locally >>> 2. or passes it further (e.g. async list) and never accesses it after >> >> As soon as the IO is passed on, it can complete. In fact, it can complete >> even _before_ that call returns. That's the issue. Obviously this isn't >> true for purely polled IO, but it is true for IRQ based IO. > > And the idea was to not use io_kiocb after submission. Except when we know, > that it won't complete asynchronously (e.g. error), that could be checked > with return code, I guess. I think you're still missing the point. During the submission it can go away, it can be deep in a call chain. So it's not enough to say "we won't touch it after completion returns", we need to hold a reference to ensure it doesn't go away WHILE being submitted. Hope that helps! -- Jens Axboe
