Re: CVS commit: src/sys/kern

["Tom Spindler (moof)" <dogcow%babymeat.com@localhost>]

I believe it's this change that's made my vbox image panic at the drop of
a hat; while it occasionally panics before it even hits single user, it
also consistently panics when starting syslogd (even from single-user):

panic: kqueue_scan,1491: kq=0xffffc779aeff6dc0 kq->kq_count(1) != count(0), nmarker=1

and the call chain is syscall -> sys___kevent50 -> kevent1 -> kqueue_check

it also causes a panic while trying to dump more than half the time,
but I have managed to get an image or two. ("panic: atastart: channel 0
busy, xfer not possible", if you're curious.)

On Wed, Jan 20, 2021 at 09:39:09PM +0000, Jaromir Dolecek wrote:
> Module Name:  src
> Committed By: jdolecek
> Date:         Wed Jan 20 21:39:09 UTC 2021
>
> Modified Files:
>       src/sys/kern: kern_event.c
>
> Log Message:
> fix a race in kqueue_scan() - when multiple threads check the same
> kqueue, it could happen other thread seen empty kqueue while kevent
> was being checked for re-firing and re-queued
>
> make sure to keep retrying if there are outstanding kevents even
> if no kevent is found on first pass through the queue, and only
> drop the KN_QUEUED flag and kq_count when actually completely done
> with the kevent
>
> change is inspired by the FreeBSD in-flux handling, but without
> introducing the reference counting
>
> PR kern/50094 by Christof Meerwald
>
>
> To generate a diff of this commit:
> cvs rdiff -u -r1.110 -r1.111 src/sys/kern/kern_event.c
>
> Please note that diffs are not public domain; they are subject to the
> copyright notices on the relevant files.
>