Running 12.0-CURRENT, r322543 LOR: lock order reversal: 1st 0xfffff80042f839a0 zfs (zfs) @ /usr/src/sys/kern/vfs_mount.c:1271 2nd 0xfffff80042f745f0 devfs (devfs) @ /usr/src/sys/fs/msdosfs/msdosfs_vfsops.c:928 stack backtrace: #0 0xffffffff80ac3870 at witness_debugger+0x70 #1 0xffffffff80ac36f2 at witness_checkorder+0xe02 #2 0xffffffff80a3785e at lockmgr_lock_fast_path+0x1ae #3 0xffffffff81066610 at VOP_LOCK1_APV+0xe0 #4 0xffffffff80b3c3c6 at _vn_lock+0x66 #5 0xffffffff8093bcc0 at msdosfs_sync+0x1a0 #6 0xffffffff80b231fd at dounmount+0x5fd #7 0xffffffff80b22b7f at sys_unmount+0x30f #8 0xffffffff80eeb40b at amd64_syscall+0x79b #9 0xffffffff80ecabeb at Xfast_syscall+0xfb Attach, detach: da0 at umass-sim0 bus 0 scbus8 target 0 lun 0 da0: <SanDisk Cruzer Edge 1.27> Removable Direct Access SPC-4 SCSI device da0: Serial Number 4C530001520818118305 da0: 40.000MB/s transfers da0: 7485MB (15330304 512 byte sectors) da0: quirks=0x2<NO_6_BYTE> ugen3.2: <SanDisk Cruzer Edge> at usbus3 (disconnected) umass0: at uhub4, port 2, addr 2 (disconnected) da0 at umass-sim0 bus 0 scbus8 target 0 lun 0 da0: <SanDisk Cruzer Edge 1.27> s/n 4C530001520818118305 detached (da0:umass-sim0:0:0:0): Periph destroyed umass0: detached The disk is formatted as: $ gpart show da0 => 1 15330303 da0 MBR (7.3G) 1 31 - free - (16K) 32 15330272 1 fat32 (7.3G)
fixed in https://cgit.freebsd.org/src/commit?id=dbc689cdef0cc8ff11171642cdcf107dfbc3fb41