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. Anyway, thanks for the explanation! -- Yours sincerely, Pavel Begunkov
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