I'm not certain it's related, but on a head build at r303767 I see a
LOR and a reproducible panic that involve the snapdir code.

First, the LOR:
$ zpool destroy foo

lock order reversal:
 1st 0xfffff800404c8b78 zfs (zfs) @
 2nd 0xfffff800404c85f0 zfs_gfs (zfs_gfs) @
stack backtrace:
#0 0xffffffff80aa90b0 at witness_debugger+0x70
#1 0xffffffff80aa8fa4 at witness_checkorder+0xe54
#2 0xffffffff80a22072 at __lockmgr_args+0x4c2
#3 0xffffffff80af8e7c at vop_stdlock+0x3c
#4 0xffffffff81018880 at VOP_LOCK1_APV+0xe0
#5 0xffffffff80b19f2a at _vn_lock+0x9a
#6 0xffffffff821b9c53 at gfs_file_create+0x73
#7 0xffffffff821b9cfd at gfs_dir_create+0x1d
#8 0xffffffff8228aa07 at zfsctl_mknode_snapdir+0x47
#9 0xffffffff821ba1a5 at gfs_dir_lookup+0x185
#10 0xffffffff821ba68d at gfs_vop_lookup+0x1d
#11 0xffffffff82289a42 at zfsctl_root_lookup+0xf2
#12 0xffffffff8228a8c3 at zfsctl_umount_snapshots+0x83
#13 0xffffffff822a1d2b at zfs_umount+0x7b
#14 0xffffffff80b02a14 at dounmount+0x6f4
#15 0xffffffff80b0228d at sys_unmount+0x35d
#16 0xffffffff80ebbb7b at amd64_syscall+0x2db
#17 0xffffffff80e9b72b at Xfast_syscall+0xfb

Here's the panic:
$ zpool create testpool da0
$ touch /testpool/testfile
$ zfs snapshot testpool@testsnap
$ cd /testpool/.zfs/snapshots

Fatal trap 12: page fault while in kernel mode
cpuid = 2; apic id = 04
fault virtual address   = 0x8
fault code              = supervisor read data, page not present
instruction pointer     = 0x20:0xffffffff80b19f1c
stack pointer           = 0x28:0xfffffe0b54bf7430
frame pointer           = 0x28:0xfffffe0b54bf74a0
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 966 (bash)
trap number             = 12
panic: page fault
cpuid = 2
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe0b54bf6fc0
vpanic() at vpanic+0x182/frame 0xfffffe0b54bf7040
panic() at panic+0x43/frame 0xfffffe0b54bf70a0
trap_fatal() at trap_fatal+0x351/frame 0xfffffe0b54bf7100
trap_pfault() at trap_pfault+0x1fd/frame 0xfffffe0b54bf7160
trap() at trap+0x284/frame 0xfffffe0b54bf7370
calltrap() at calltrap+0x8/frame 0xfffffe0b54bf7370
--- trap 0xc, rip = 0xffffffff80b19f1c, rsp = 0xfffffe0b54bf7440, rbp
= 0xfffffe0b54bf74a0 ---
_vn_lock() at _vn_lock+0x8c/frame 0xfffffe0b54bf74a0
zfs_lookup() at zfs_lookup+0x50d/frame 0xfffffe0b54bf7540
zfs_freebsd_lookup() at zfs_freebsd_lookup+0x91/frame 0xfffffe0b54bf7680
VOP_CACHEDLOOKUP_APV() at VOP_CACHEDLOOKUP_APV+0xda/frame 0xfffffe0b54bf76b0
vfs_cache_lookup() at vfs_cache_lookup+0xd6/frame 0xfffffe0b54bf7710
VOP_LOOKUP_APV() at VOP_LOOKUP_APV+0xda/frame 0xfffffe0b54bf7740
lookup() at lookup+0x5a2/frame 0xfffffe0b54bf77d0
namei() at namei+0x5b2/frame 0xfffffe0b54bf7890
kern_statat() at kern_statat+0xa8/frame 0xfffffe0b54bf7a40
sys_stat() at sys_stat+0x2d/frame 0xfffffe0b54bf7ae0
amd64_syscall() at amd64_syscall+0x2db/frame 0xfffffe0b54bf7bf0
Xfast_syscall() at Xfast_syscall+0xfb/frame 0xfffffe0b54bf7bf0

I can provide core files, test scripts, whatever you need.  Thanks for
tackling this difficult problem.


On Fri, Aug 5, 2016 at 12:36 AM, Andriy Gapon <a...@freebsd.org> wrote:
> On 03/08/2016 17:25, Andriy Gapon wrote:
>> Another change that was not strictly required and which is probably too
>> intrusive is killing the support for case insensitive operations.   My
>> thinking was that FreeBSD VFS does not provide support for those anyway.
>>  But I'll probably restore the code, at least in the bottom half of the
>> ZPL, before committing the change.
> It turned out that most of the removed code was dead anyway and it took
> just a few lines of code to restore support for case-insensitive
> filesystems.  Filesystems with mixed case sensitivity behave exactly the
> same as case-sensitive filesystem as it has always been the case on FreeBSD.
> Anyway the big change has just been committed:
> https://svnweb.freebsd.org/changeset/base/303763
> Please test away.
> Another note is that the filesystem name cache is now disabled for case
> insensitive filesystems and filesystems with normalization other than
> none.  That may hurt the lookup performance, but should ensure
> correctness of operations.
> --
> Andriy Gapon
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