alq_open_flags() panics in _mtx_lock_flags()

[Lev Serebryakov]

Hello, FreeBSD.

 I'm trying to create logging queue with alq kernel API. I call
 alq_open_flags() like this in my module:

error = alq_open_flags(&sc->sc_alq, sc->sc_vnode_name,
    curthread->td_ucred, ALQ_DEFAULT_CMODE,
    sizeof(struct g_log_entry), ALQ_ORDERED);

 and my system (10-CURRENT) panics with this stack trace (top frames
 are DDB-related, so I omit them):

#5  0xc06101c3 in kdb_trap (type=12, code=0, tf=0xc4e29990)
    at /usr/home/lev/FreeBSD-head/sys/kern/subr_kdb.c:620
#6  0xc08290af in trap_fatal (frame=0xc4e29990, eva=136)
    at /usr/home/lev/FreeBSD-head/sys/i386/i386/trap.c:958
#7  0xc08292c0 in trap_pfault (frame=0xc4e29990, usermode=0, eva=136)
    at /usr/home/lev/FreeBSD-head/sys/i386/i386/trap.c:880
#8  0xc0829f46 in trap (frame=0xc4e29990)
    at /usr/home/lev/FreeBSD-head/sys/i386/i386/trap.c:555
#9  0xc0812e7c in calltrap ()
    at /usr/home/lev/FreeBSD-head/sys/i386/i386/exception.s:168
#10 0xc05cafc0 in _mtx_lock_flags (m=0x78, opts=0, 
    file=0xc088c140 "/usr/home/lev/FreeBSD-head/sys/kern/vfs_subr.c", 
    line=2169) at /usr/home/lev/FreeBSD-head/sys/kern/kern_mutex.c:194
#11 0xc0672eb2 in vref (vp=0x0)
    at /usr/home/lev/FreeBSD-head/sys/kern/vfs_subr.c:2169
#12 0xc066932f in namei (ndp=0xc4e29b74)
    at /usr/home/lev/FreeBSD-head/sys/kern/vfs_lookup.c:264
#13 0xc0682900 in vn_open_cred (ndp=0xc4e29b74, flagp=0xc4e29be8, cmode=384, 
    vn_open_flags=0, cred=0xc50fee80, fp=0x0)
    at /usr/home/lev/FreeBSD-head/sys/kern/vfs_vnops.c:137
#14 0xc5c42609 in alq_open_flags (alqp=0xc550dc08, 
    file=0xc5108d40 "/usr/ada4.log", cred=0xc50fee80, cmode=384, size=28,
    flags=16)
    at /usr/home/lev/FreeBSD-head/sys/modules/alq/../../kern/kern_alq.c:451

  It seems, that vref() get NULL instead of valid pointer to struct
 vnode. But I have no idea -- why?! Yes, I have no such file created,
 but man alq(9) says, that it will create file for me. And if I point
 to existed file, it panic anyway.

  What do I do wrong?!

-- 
// Black Lion AKA Lev Serebryakov <lev at FreeBSD.org>