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. -- Jens Axboe
