[Bug 205725] Stack backtrace on `zpool export`

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=205725

            Bug ID: 205725
           Summary: Stack backtrace on `zpool export`
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: florian.ermisch at alumni.tu-berlin.de
                CC: freebsd-amd64 at FreeBSD.org
                CC: freebsd-amd64 at FreeBSD.org

Hi *,

since I've upgraded my laptop from 10.2-RELEASE to 11-CURRENT (base r292536, 
now base r292755) I see this stack backtrace when a zpool is exported:

Dec 27 18:44:02 fuchi-cyber220 kernel: lock order reversal:
Dec 27 18:44:02 fuchi-cyber220 kernel: 1st 0xfffff800c7472418 zfs (zfs) @
/usr/src/sys/kern/vfs_mount.c:1224
Dec 27 18:44:02 fuchi-cyber220 kernel: 2nd 0xfffff800c73fad50 zfs_gfs (zfs_gfs)
@ /usr/src/sys/cddl/contrib/opensolaris/uts/common/fs/gfs.c:494
Dec 27 18:44:02 fuchi-cyber220 kernel: stack backtrace:
Dec 27 18:44:02 fuchi-cyber220 kernel: #0 0xffffffff80a7d6f0 at
witness_debugger+0x70
Dec 27 18:44:02 fuchi-cyber220 kernel: #1 0xffffffff80a7d5f1 at
witness_checkorder+0xe71
Dec 27 18:44:02 fuchi-cyber220 kernel: #2 0xffffffff809fedcb at
__lockmgr_args+0xd3b
Dec 27 18:44:02 fuchi-cyber220 kernel: #3 0xffffffff80ac55ec at
vop_stdlock+0x3c
Dec 27 18:44:02 fuchi-cyber220 kernel: #4 0xffffffff80fbb220 at
VOP_LOCK1_APV+0x100
Dec 27 18:44:02 fuchi-cyber220 kernel: #5 0xffffffff80ae653a at _vn_lock+0x9a
Dec 27 18:44:02 fuchi-cyber220 kernel: #6 0xffffffff8209db13 at
gfs_file_create+0x73
Dec 27 18:44:02 fuchi-cyber220 kernel: #7 0xffffffff8209dbbd at
gfs_dir_create+0x1d
Dec 27 18:44:02 fuchi-cyber220 kernel: #8 0xffffffff821649e7 at
zfsctl_mknode_snapdir+0x47
Dec 27 18:44:02 fuchi-cyber220 kernel: #9 0xffffffff8209e135 at
gfs_dir_lookup+0x185
Dec 27 18:44:02 fuchi-cyber220 kernel: #10 0xffffffff8209e61d at
gfs_vop_lookup+0x1d
Dec 27 18:44:02 fuchi-cyber220 kernel: #11 0xffffffff82163a05 at
zfsctl_root_lookup+0xf5
Dec 27 18:44:02 fuchi-cyber220 kernel: #12 0xffffffff821648a3 at
zfsctl_umount_snapshots+0x83
Dec 27 18:44:02 fuchi-cyber220 kernel: #13 0xffffffff8217d5ab at
zfs_umount+0x7b
Dec 27 18:44:02 fuchi-cyber220 kernel: #14 0xffffffff80acf0b0 at
dounmount+0x530
Dec 27 18:44:02 fuchi-cyber220 kernel: #15 0xffffffff80aceaed at
sys_unmount+0x35d
Dec 27 18:44:02 fuchi-cyber220 kernel: #16 0xffffffff80e6e13b at
amd64_syscall+0x2db
Dec 27 18:44:02 fuchi-cyber220 kernel: #17 0xffffffff80e4dd8b at
Xfast_syscall+0xfb

(From /var/log/messages)

First I've only seen this on the console at shutdown or reboot but later
found I can reproduce it by exporting a zpool.
While the only pools I can trigger this without a shutdown/reboot are 
connected via USB(3) I still see it just before poweroff at shutdown 
after the "All synced." message.

When I try to export a zpool under heavy load (`make -C /use/src -j 4
buildworld`
on a 2 core CPU w/ HT) the system locks up completely. I don't think it's
related
to memory pressure as I haven't seen swap being used during a buildworld (with 
8 gigs of RAM).

PS for cross-referencing: I've posted this issue on the current@ list
yesterday,
see
https://lists.freebsd.org/pipermail/freebsd-current/2015-December/059117.html

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