[Bug 195458] Hang on shutdown/root unmount after FreeBSD 10.1R upgrade

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=195458

--- Comment #3 from Walter Hop <walter at lifeforms.nl> ---
I’ve tried if the hang is apparent in a 11-CURRENT snapshot too, and it is.
(Tried the iso snapshot available today, which is r273635). WITNESS indicates a
lock order reversal, which looks related to me:

root at current:~ # chflags noschg /sbin/init
root at current:~ # cp -Rp /sbin/init /sbin/init2
lock order reversal:
1st 0xfffffe007b842fa0 bufwait (bufwait) @ /usr/src/sys/kern/vfs_bio.c:3093
2nd 0xfffff80002b9ea00 dirhash (dirhash) @
/usr/src/sys/ufs/ufs/ufs_dirhash.c:284
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe000025c270
kdb_backtrace() at kdb_backtrace+0x39/frame 0xfffffe000025c320
witness_checkorder() at witness_checkorder+0xdad/frame 0xfffffe000025c3b0
_sx_xlock() at _sx_xlock+0x75/frame 0xfffffe000025c3f0
ufsdirhash_add() at ufsdirhash_add+0x3a/frame 0xfffffe000025c430
ufs_direnter() at ufs_direnter+0x6a0/frame 0xfffffe000025c4f0
ufs_makeinode() at ufs_makeinode+0x560/frame 0xfffffe000025c6a0
VOP_CREATE_APV() at VOP_CREATE_APV+0xf1/frame 0xfffffe000025c6d0
vn_open_cred() at vn_open_cred+0x29d/frame 0xfffffe000025c820
kern_openat() at kern_openat+0x26f/frame 0xfffffe000025c9a0
amd64_syscall() at amd64_syscall+0x25a/frame 0xfffffe000025cab0
Xfast_syscall() at Xfast_syscall+0xfb/frame 0xfffffe000025cab0
--- syscall (5, FreeBSD ELF64, sys_open), rip = 0x80094f01a, rsp =
0x7fffffffe958, rbp = 0x7fffffffe9c0 ---

Finally, when rebooting, another lock order reversal appears and the system
hangs.
I don’t have a text log of this, so I’ll copy the first few lines:

Syncing disks, vnodes remaining...1 0 0 done
All buffers synced.
lock order reversal:
1st 0xfffff80002e65d50 ufs (ufs) @ /usr/src/sys/kern/vfs_mount.c:1223
2nd 0xfffff80002e665f0 devfs (devfs) @ /usr/src/sys/kern/vfs_subr.c:2144

Screenshot is here: http://lf.ms/current-r273635-hang-2.png

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