panic: userret: Returning with 1 locks held.

[Danny Braniss]

with the latest kernel, the message changed somewhat, but the panic is
still there (this is an amd64):

trap 12: page fault while in kernel mode
cpuid = 1; apic id = 01
fault virtual address   = 0x0
fault code              = supervisor read, page not present
instruction pointer     = 0x8:0xffffffff8038e3f5
stack pointer           = 0x10:0xffffffffb280e7b0
frame pointer           = 0x10:0xffffffffb280e7e0
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         = 241 (sh)
[thread pid 241 tid 100071 ]
Stopped at      _mtx_lock_flags+0x35:   cmpq    $0x80779d40,0(%rdi)
db> tr
Tracing pid 241 tid 100071 td 0xffffff007ae25980
_mtx_lock_flags() at _mtx_lock_flags+0x35
exec_map_first_page() at exec_map_first_page+0x60
kern_execve() at kern_execve+0x2a0
execve() at execve+0x5d
syscall() at syscall+0x4ab
Xfast_syscall() at Xfast_syscall+0xa8
--- syscall (59, FreeBSD ELF64, execve), rip = 0x80090630c, rsp = 
0x7fffffffcbf8, rbp = 0 ---
db> show lockedvnods
Locked vnodes

0xffffff0061a48000: tag union, type VREG
    usecount 1, writecount 0, refcount 1 mountedhere 0
    flags (VV_TEXT)
     lock type union: EXCL (count 1) by thread 0xffffff007ae25980 (pid 241)
        vp=0xffffff0061a48000, uppervp=0, lowervp=0xffffff00626187e0
union: lower
0xffffff00626187e0: tag nfs, type VREG
    usecount 1, writecount 0, refcount 3 mountedhere 0
    flags ()
    v_object 0xffffff007c3a87e0 ref 0 pages 1
    
        fileid 47269 fsid 0x900ff01
db> tr 241
Tracing pid 241 tid 100071 td 0xffffff007ae25980
_mtx_lock_flags() at _mtx_lock_flags+0x35
exec_map_first_page() at exec_map_first_page+0x60
kern_execve() at kern_execve+0x2a0
execve() at execve+0x5d
syscall() at syscall+0x4ab
Xfast_syscall() at Xfast_syscall+0xa8
--- syscall (59, FreeBSD ELF64, execve), rip = 0x80090630c, rsp = 
0x7fffffffcbf8, rbp = 0 ---
db>