On 4/15/22 3:05 PM, Pavel Begunkov wrote: > On 4/12/22 17:46, Jens Axboe wrote: >> On 4/12/22 10:41 AM, Jens Axboe wrote: >>> On 4/12/22 10:24 AM, Pavel Begunkov wrote: >>>> If all completed requests in io_do_iopoll() were marked with >>>> REQ_F_CQE_SKIP, we'll not only skip CQE posting but also >>>> io_free_batch_list() leaking memory and resources. >>>> >>>> Move @nr_events increment before REQ_F_CQE_SKIP check. We'll potentially >>>> return the value greater than the real one, but iopolling will deal with >>>> it and the userspace will re-iopoll if needed. In anyway, I don't think >>>> there are many use cases for REQ_F_CQE_SKIP + IOPOLL. >>> >>> Ah good catch - yes probably not much practical concern, as the lack of >>> ordering for file IO means that CQE_SKIP isn't really useful for that >>> scenario. >> >> One potential snag is with the change we're now doing >> io_cqring_ev_posted_iopoll() even if didn't post an event. Again >> probably not a practical concern, but it is theoretically a violation >> if an eventfd is used. > Looks this didn't get applied. Are you concerned about eventfd? Yep, was hoping to get a reply back, so just deferred it for now. > Is there any good reason why the userspace can't tolerate spurious > eventfd events? Because I don't think we should care this case I always forget the details on that, but we've had cases like this in the past where some applications assume that if they got N eventfd events, then are are also N events in the ring. Which granted is a bit odd, but it does also make some sense. Why would you have more eventfd events posted than events? So while I don't think it's a huge issue, and particularly because IOPOLL and eventfd would be a nonsensical combo, it would still be nice to generally make sure it's the case. This isn't the only one though, so maybe we just apply this fix and do a full check down the line. Can't see this one making issues. -- Jens Axboe
