[Bug 271292] kernel panic while dd'ing a USB disk to a ZFS directory

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271292

            Bug ID: 271292
           Summary: kernel panic while dd'ing a USB disk to a ZFS
                    directory
           Product: Base System
           Version: CURRENT
          Hardware: riscv
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: dgilbert@eicat.ca

Created attachment 242030
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=242030&action=edit
core.txt file for the crash

I was using my RISC-V unleashed (running -CURRENT) to dd copy an NVME disk
using a USB enclosure.  The crash message was unhelpful:

Unread portion of the kernel message buffer:
panic: buffer modified while frozen!
cpuid = 0
time = 1683224304
KDB: stack backtrace:
db_trace_self() at db_trace_self
db_trace_self_wrapper() at db_trace_self_wrapper+0x38
kdb_backtrace() at kdb_backtrace+0x2c
vpanic() at vpanic+0x126
panic() at panic+0x2a
.LBB9_21() at .LBB9_21+0xc
arc_write_done() at arc_write_done+0x4e
.LBB84_358() at .LBB84_358+0xcc
.LBB41_60() at .LBB41_60+0xbc
taskqueue_run_locked() at taskqueue_run_locked+0x90
taskqueue_thread_loop() at taskqueue_thread_loop+0xd4
fork_exit() at fork_exit+0x68
fork_trampoline() at fork_trampoline+0xa
KDB: enter: panic

With slightly more information coming from the kgdb output from panic() down:

#12 0xffffffc002cc363a in arc_cksum_verify (buf=<optimized out>)
    at /usr/src/sys/contrib/openzfs/module/zfs/arc.c:1475
#13 0xffffffc002ccfc48 in arc_write_done (zio=0xffffffd0df9839c0)
    at /usr/src/sys/contrib/openzfs/module/zfs/arc.c:6725
#14 0xffffffc002dfbd16 in zio_done (zio=0xffffffd0df9839c0)
    at /usr/src/sys/contrib/openzfs/module/zfs/zio.c:4893
#15 0xffffffc002df48ae in __zio_execute (zio=0xffffffd0df9839c0)
    at /usr/src/sys/contrib/openzfs/module/zfs/zio.c:2233
#16 zio_execute (zio=<optimized out>)
    at /usr/src/sys/contrib/openzfs/module/zfs/zio.c:2144
#17 0xffffffc000350bc4 in taskqueue_run_locked (queue=0xffffffd01539c000)
    at /usr/src/sys/kern/subr_taskqueue.c:514
#18 0xffffffc000351a2c in taskqueue_thread_loop (arg=<optimized out>)
    at /usr/src/sys/kern/subr_taskqueue.c:826
#19 0xffffffc0002b8f90 in fork_exit (
    callout=0xffffffc000351954 <taskqueue_thread_loop>, 
    arg=0xffffffd00318df40, frame=0xffffffc1fa399c40)
    at /usr/src/sys/kern/kern_fork.c:1102
#20 0xffffffc0005b069e in fork_trampoline ()
    at /usr/src/sys/riscv/riscv/swtch.S:385
Backtrace stopped: frame did not save the PC

coredump, kernel and debug symbols will appear here as I upload them:

https://v4.nextcloud.towernet.ca/s/NkD6E4Bd7TAQdGH

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