[Bug 225795] panic: found unreferenced mountpoint when accessing and unmounting snapshots in parallel

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=225795

            Bug ID: 225795
           Summary: panic: found unreferenced mountpoint when accessing
                    and unmounting snapshots in parallel
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: asomers at FreeBSD.org

Found by snapshot_019_pos from the ZFS test suite:

panic: found unreferenced mountpoint
cpuid = 8
time = 1518201424
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe00cef3f320
vpanic() at vpanic+0x18d/frame 0xfffffe00cef3f380
vpanic() at vpanic/frame 0xfffffe00cef3f400
zfsctl_snapdir_lookup() at zfsctl_snapdir_lookup+0x461/frame 0xfffffe00cef3f680
VOP_LOOKUP_APV() at VOP_LOOKUP_APV+0xd3/frame 0xfffffe00cef3f6b0
lookup() at lookup+0x682/frame 0xfffffe00cef3f750
namei() at namei+0x52b/frame 0xfffffe00cef3f810
vn_open_cred() at vn_open_cred+0x213/frame 0xfffffe00cef3f950
kern_openat() at kern_openat+0x20c/frame 0xfffffe00cef3fac0
amd64_syscall() at amd64_syscall+0x79b/frame 0xfffffe00cef3fbf0
fast_syscall_common() at fast_syscall_common+0xfc/frame 0x7fffffffb240
Uptime: 11h59m40s
Dumping 5340 out of 49055 MB:..1%..11%..21%..31%..41%..51%..61%..71%..81%..91%

__curthread () at ./machine/pcpu.h:229
229             __asm("movq %%gs:%1,%0" : "=r" (td)
(kgdb) #0  __curthread () at ./machine/pcpu.h:229
#1  doadump (textdump=1)
    at /usr/home/alans/freebsd/head/sys/kern/kern_shutdown.c:346
#2  0xffffffff80ac7042 in kern_reboot (howto=260)
    at /usr/home/alans/freebsd/head/sys/kern/kern_shutdown.c:415
#3  0xffffffff80ac760d in vpanic (fmt=<optimized out>, ap=0xfffffe00cef3f3c0)
    at /usr/home/alans/freebsd/head/sys/kern/kern_shutdown.c:811
#4  0xffffffff80ac7420 in kassert_panic (
    fmt=0xffffffff8224ea68 "found unreferenced mountpoint")
    at /usr/home/alans/freebsd/head/sys/kern/kern_shutdown.c:697
#5  0xffffffff821eae71 in zfsctl_snapdir_lookup (ap=0xfffffe00cef3f6f0)
    at
/usr/home/alans/freebsd/head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ctldir.c:893
#6  0xffffffff810f50a3 in VOP_LOOKUP_APV (vop=<optimized out>,
    a=0xfffffe00cef3f6f0) at vnode_if.c:127
#7  0xffffffff80b89c92 in VOP_LOOKUP (vpp=0xfffffe00cef3f9c8,
    cnp=0xfffffe00cef3f9f0, dvp=<optimized out>) at ./vnode_if.h:54
#8  lookup (ndp=0xfffffe00cef3f968)
    at /usr/home/alans/freebsd/head/sys/kern/vfs_lookup.c:888
#9  0xffffffff80b8921b in namei (ndp=0xfffffe00cef3f968)
    at /usr/home/alans/freebsd/head/sys/kern/vfs_lookup.c:450
#10 0xffffffff80ba6d13 in vn_open_cred (ndp=0xfffffe00cef3f968,
    flagp=0xfffffe00cef3fa94, cmode=1, vn_open_flags=0,
    cred=0xfffff80133087c00, fp=0xfffff807b332e9b0)
    at /usr/home/alans/freebsd/head/sys/kern/vfs_vnops.c:279
#11 0xffffffff80b9f5ec in kern_openat (td=0xfffff804ca09d000, fd=-100,
    path=0x800cde000 <error: Cannot access memory at address 0x800cde000>,
    pathseg=UIO_USERSPACE, flags=1179653, mode=<optimized out>)
    at /usr/home/alans/freebsd/head/sys/kern/vfs_syscalls.c:1084
#12 0xffffffff80f7937b in syscallenter (td=0xfffff804ca09d000)
    at
/usr/home/alans/freebsd/head/sys/amd64/amd64/../../kern/subr_syscall.c:134
#13 amd64_syscall (td=0xfffff804ca09d000, traced=0)
    at /usr/home/alans/freebsd/head/sys/amd64/amd64/trap.c:935
#14 0xffffffff80f55e38 in fast_syscall_common ()
    at /usr/home/alans/freebsd/head/sys/amd64/amd64/exception.S:468

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