Hmm, you are indeed right, it's not the same panic. The backtrace I got just now with INVARIANTS is the one you quoted above. I still get the "_sx_xlock (sx=Variable "sx" is not available.)" and "_sx_xlock_hard (sx=0xffffff00090d5018, ..., opts=Variable "opts" is not available.)" though. Am I missing some option (I've got GENERIC, minus WITNESS plus DTRACE, now that INVARIANTS is back in place), or does this "just happen"?

Here's the "full" backtrace (minus the panic(), trap() etc.):

#10 0xffffffff8057dfe7 in calltrap () at /usr/src/sys/amd64/amd64/exception.S:224 #11 0xffffffff80342b99 in _sx_xlock_hard (sx=0xffffff00090d5018, tid=18446742974952890368, opts=Variable "opts" is not available. ) at /usr/src/sys/kern/kern_sx.c:575 #12 0xffffffff8034350e in _sx_xlock (sx=Variable "sx" is not available. ) at sx.h:155 #13 0xffffffff80af7596 in zfs_freebsd_reclaim () from /boot/kernel/ zfs.ko #14 0xffffffff805c5c2a in VOP_RECLAIM_APV (vop=0xffffff00090d5018, a=0xffffff00090d5000) at vnode_if.c:1926 #15 0xffffffff803c839e in vgonel (vp=0xffffff0009252588) at vnode_if.h:830 #16 0xffffffff803cc958 in vflush (mp=0xffffff0002cd7bc0, rootrefs=0, flags=0, td=0xffffff002cffe000) at /usr/src/sys/kern/vfs_subr.c:2449 #17 0xffffffff80af2038 in zfs_umount () from /boot/kernel/zfs.ko #18 0xffffffff803c55ca in dounmount (mp=0xffffff0002cd7bc0, flags=47020992, td=Variable "td" is not available. ) at /usr/src/sys/kern/vfs_mount.c:1289 #19 0xffffffff803c5df8 in unmount (td=0xffffff002cffe000, uap=0xffffff803e98bbf0) at /usr/src/sys/kern/vfs_mount.c:1174 #20 0xffffffff805980bf in syscall (frame=0xffffff803e98bc80) at /usr/src/sys/amd64/amd64/trap.c:984 #21 0xffffffff8057e2c1 in Xfast_syscall () at /usr/src/sys/amd64/amd64/exception.S:373 #22 0x000000080104e9ec in ?? () Previous frame inner to this frame (corrupt stack?)