panic: mutex ncnegl not owned at /usr/src/sys/kern/vfs_cache.c:743 in 12.0-CURRENT r308447

Yesterday I upgraded my ports builder box from r306349 to r308477.  It
crashed overnight during a poudriere run.

panic: mutex ncnegl not owned at /usr/src/sys/kern/vfs_cache.c:743
cpuid = 2
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe085c2a4100
vpanic() at vpanic+0x182/frame 0xfffffe085c2a4180
panic() at panic+0x43/frame 0xfffffe085c2a41e0
__mtx_assert() at __mtx_assert+0xc1/frame 0xfffffe085c2a41f0
cache_negative_remove() at cache_negative_remove+0x65/frame 0xfffffe085c2a4230
cache_zap_locked() at cache_zap_locked+0x1ca/frame 0xfffffe085c2a4250
cache_enter_time() at cache_enter_time+0x1c20/frame 0xfffffe085c2a4330
tmpfs_lookup() at tmpfs_lookup+0x47a/frame 0xfffffe085c2a4390
VOP_CACHEDLOOKUP_APV() at VOP_CACHEDLOOKUP_APV+0xda/frame 0xfffffe085c2a43c0
vfs_cache_lookup() at vfs_cache_lookup+0xd6/frame 0xfffffe085c2a4420
VOP_LOOKUP_APV() at VOP_LOOKUP_APV+0xda/frame 0xfffffe085c2a4450
lookup() at lookup+0x6a2/frame 0xfffffe085c2a44e0
namei() at namei+0x57e/frame 0xfffffe085c2a45a0
vn_open_cred() at vn_open_cred+0x25b/frame 0xfffffe085c2a4710
kern_openat() at kern_openat+0x25c/frame 0xfffffe085c2a48a0
ia32_syscall() at ia32_syscall+0x330/frame 0xfffffe085c2a4a30
Xint0x80_syscall() at Xint0x80_syscall+0x95/frame 0xfffffe085c2a4a30
--- syscall (499, FreeBSD ELF32, sys_openat), rip = 0x97b2f57, rsp = 0xffff7698, rbp = 0xffff76b0 ---
KDB: enter: panic


I was able to get a crash dump.  This is the kgdb backtrace:

#0  __curthread () at ./machine/pcpu.h:222
#1  doadump (textdump=1546271680) at /usr/src/sys/kern/kern_shutdown.c:298
#2  0xffffffff80396db6 in db_fncall_generic (nargs=0, addr=<optimized out>, 
    rv=<optimized out>, args=<optimized out>)
    at /usr/src/sys/ddb/db_command.c:581
#3  db_fncall (dummy1=<optimized out>, dummy2=<optimized out>, 
    dummy3=<optimized out>, dummy4=<optimized out>)
    at /usr/src/sys/ddb/db_command.c:629
#4  0xffffffff80396919 in db_command (last_cmdp=<optimized out>, 
    cmd_table=<optimized out>, dopager=<optimized out>)
    at /usr/src/sys/ddb/db_command.c:453
#5  0xffffffff80396674 in db_command_loop ()
    at /usr/src/sys/ddb/db_command.c:506
#6  0xffffffff8039972f in db_trap (type=<optimized out>, code=<optimized out>)
    at /usr/src/sys/ddb/db_main.c:248
#7  0xffffffff80aa0a13 in kdb_trap (type=<optimized out>, 
    code=<optimized out>, tf=<optimized out>)
    at /usr/src/sys/kern/subr_kdb.c:654
#8  0xffffffff80ed8cc4 in trap (frame=0xfffffe085c2a4030)
    at /usr/src/sys/amd64/amd64/trap.c:537
#9  <signal handler called>
#10 kdb_enter (why=0xffffffff8140e5d1 "panic", 
    msg=0x80 <error: Cannot access memory at address 0x80>)
    at /usr/src/sys/kern/subr_kdb.c:444
#11 0xffffffff80a5eb1f in vpanic (fmt=<optimized out>, ap=0xfffffe085c2a41c0)
    at /usr/src/sys/kern/kern_shutdown.c:752
#12 0xffffffff80a5eb83 in panic (fmt=0xffffffff81bf1de0 <cnputs_mtx+24> "\004")
    at /usr/src/sys/kern/kern_shutdown.c:690
#13 0xffffffff80a40191 in __mtx_assert (c=<optimized out>, 
    what=<optimized out>, file=0xfffffe085c2a3fe0 "", line=0)
    at /usr/src/sys/kern/kern_mutex.c:937
#14 0xffffffff80b0d3d5 in cache_negative_remove (ncp=0xfffff8062bb8dee0, 
    neg_locked=true) at /usr/src/sys/kern/vfs_cache.c:743
#15 0xffffffff80b0d87a in cache_zap_locked (ncp=0xfffff8062bb8dee0, 
    neg_locked=true) at /usr/src/sys/kern/vfs_cache.c:886
#16 0xffffffff80b0d210 in cache_negative_zap_one ()
    at /usr/src/sys/kern/vfs_cache.c:835
#17 cache_enter_time (dvp=<optimized out>, vp=<optimized out>, 
    cnp=<optimized out>, tsp=<optimized out>, dtsp=<optimized out>)
    at /usr/src/sys/kern/vfs_cache.c:1741
#18 0xffffffff82893b3a in tmpfs_lookup (v=<optimized out>)
    at /usr/src/sys/modules/tmpfs/../../fs/tmpfs/tmpfs_vnops.c:199
#19 0xffffffff8103ee4a in VOP_CACHEDLOOKUP_APV (vop=<optimized out>, 
    a=<optimized out>) at vnode_if.c:195
#20 0xffffffff80b0e9d6 in VOP_CACHEDLOOKUP (dvp=0xfffff8023f3693b0, 
    vpp=0xfffffe085c2a47d8, cnp=0xfffffe085c2a4800) at ./vnode_if.h:80
#21 vfs_cache_lookup (ap=<optimized out>) at /usr/src/sys/kern/vfs_cache.c:2022
#22 0xffffffff8103ecea in VOP_LOOKUP_APV (vop=<optimized out>, 
    a=<optimized out>) at vnode_if.c:127
#23 0xffffffff80b17df2 in VOP_LOOKUP (vpp=0xfffffe085c2a47d8, 
    cnp=0xfffffe085c2a4800, dvp=<optimized out>) at ./vnode_if.h:54
#24 lookup (ndp=<optimized out>) at /usr/src/sys/kern/vfs_lookup.c:830
#25 0xffffffff80b173ae in namei (ndp=<optimized out>)
    at /usr/src/sys/kern/vfs_lookup.c:397
#26 0xffffffff80b333db in vn_open_cred (ndp=<optimized out>, 
    flagp=0xfffffe085c2a485c, cmode=0, vn_open_flags=<optimized out>, 
    cred=<optimized out>, fp=0x1d0) at /usr/src/sys/kern/vfs_vnops.c:277
#27 0xffffffff80b2c72c in kern_openat (td=0xfffff8001d68d000, fd=-100, 
    path=0x2c9e71e8 <error: Cannot access memory at address 0x2c9e71e8>, 
    pathseg=UIO_USERSPACE, 
    flags=<error reading variable: Cannot access memory at address 0x80>, 
    mode=<optimized out>) at /usr/src/sys/kern/vfs_syscalls.c:1016
#28 0xffffffff80ff88a0 in syscallenter (td=0xfffff8001d68d000, 
    sa=<optimized out>)
    at /usr/src/sys/amd64/ia32/../../kern/subr_syscall.c:135
#29 ia32_syscall (frame=0xfffffe085c2a4a40)
    at /usr/src/sys/amd64/ia32/ia32_syscall.c:187
#30 <signal handler called>
#31 0x00000000097b2f57 in ?? ()
Backtrace stopped: Cannot access memory at address 0xffff7698


I don't see any obvious issues in the code.