hard lockup on multiple CPUs in spl_cache_flush

Photo of failed machine, and video of a second lockup at https://photos.app.goo.gl/9qUxysZ4LN1NRdrA7

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-4.15.0-43-generic 4.15.0-43.46
ProcVersionSignature: Ubuntu 4.15.0-43.46-generic 4.15.18
Uname: Linux 4.15.0-43-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/hwC1D0', '/dev/snd/pcmC1D9p', '/dev/snd/pcmC1D8p', '/dev/snd/pcmC1D7p', '/dev/snd/pcmC1D3p', '/dev/snd/hwC2D0', '/dev/snd/pcmC2D7p', '/dev/snd/controlC1', '/dev/snd/pcmC2D3p', '/dev/snd/controlC2', '/dev/snd/by-path', '/dev/snd/hwC0D0', '/dev/snd/pcmC0D2c', '/dev/snd/pcmC0D1p', '/dev/snd/pcmC0D0c', '/dev/snd/pcmC0D0p', '/dev/snd/controlC0', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
Date: Wed Jan 2 20:14:38 2019
InstallationDate: Installed on 2013-12-09 (1850 days ago)
InstallationMedia: Ubuntu-Server 13.10 "Saucy Salamander" - Release amd64 (20131016)
MachineType: System manufacturer System Product Name
ProcEnviron:
 LANGUAGE=en_NZ:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_NZ.UTF-8
 SHELL=/bin/bash
ProcFB: 0 nouveaufb
ProcKernelCmdLine: BOOT_IMAGE=/ROOT/ubuntu@/boot/vmlinuz-4.15.0-43-generic root=ZFS=tr2srv/ROOT/ubuntu ro nomdmonddf nomdmonisw
PulseList:
 Error: command ['pacmd', 'list'] failed with exit code 1: Home directory not accessible: Permission denied
 No PulseAudio daemon running, or not running as session daemon.
RelatedPackageVersions:
 linux-restricted-modules-4.15.0-43-generic N/A
 linux-backports-modules-4.15.0-43-generic N/A
 linux-firmware 1.173.2
RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill'
SourcePackage: linux
UpgradeStatus: Upgraded to bionic on 2018-06-26 (189 days ago)
dmi.bios.date: 10/12/2018
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 0808
dmi.board.asset.tag: Default string
dmi.board.name: PRIME X399-A
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: Default string
dmi.chassis.type: 3
dmi.chassis.vendor: Default string
dmi.chassis.version: Default string
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr0808:bd10/12/2018:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKCOMPUTERINC.:rnPRIMEX399-A:rvrRev1.xx:cvnDefaultstring:ct3:cvrDefaultstring:
dmi.product.family: To be filled by O.E.M.
dmi.product.name: System Product Name
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer