7.1 hangs in cache_lookup mutex?

[Guy Helmer]

I think I may have found a clue regarding some of the hangs I'm seeing 
on FreeBSD 7.1.
I have a program (kvoop), compiled under FreeBSD 6 and using 
compatibility libraries under FreeBSD 7, that seems to be consistently 
involved during these hangs.  This time, I noticed that many processes 
are stopped, waiting on the ufs lock.  I can't tell for certain, but I 
assume this kvoop process 33203 is blocking the other processes.  The 
kvoop process looks to me like it is waiting for a mutex, but nothing 
shows up being deadlocked.  Am I on the right track?  Is there something 
else I should be looking for?

(kgdb) proc 33203
[Switching to thread 129 (Thread 100241)]#0  sched_switch (
    td=0xffffff0019109a50, newtd=0x0, flags=1)
    at ../../../kern/sched_ule.c:1944
1944                    cpuid = PCPU_GET(cpuid);
(kgdb) where
#0  sched_switch (td=0xffffff0019109a50, newtd=0x0, flags=1)
    at ../../../kern/sched_ule.c:1944
#1  0xffffffff803a573b in mi_switch (flags=1, newtd=0x0)
    at ../../../kern/kern_synch.c:440
#2  0xffffffff803d1214 in turnstile_wait (ts=Variable "ts" is not available.
)
    at ../../../kern/subr_turnstile.c:748
#3  0xffffffff80392db0 in _mtx_lock_sleep (m=0xffffffff8083c220,
    tid=18446742974618442320, opts=Variable "opts" is not available.
) at ../../../kern/kern_mutex.c:420
#4  0xffffffff80392ea6 in _mtx_lock_flags (m=0xffffffff8083c220, opts=0,
    file=0xffffffff805bd438 "../../../kern/vfs_cache.c", line=327)
    at ../../../kern/kern_mutex.c:186
#5  0xffffffff80403bc6 in cache_lookup (dvp=0xffffff00013e0dc8,
    vpp=0xffffffffa2d93a18, cnp=0xffffffffa2d93a40)
    at ../../../kern/vfs_cache.c:327
#6  0xffffffff80404093 in vfs_cache_lookup (ap=Variable "ap" is not 
available.
)
    at ../../../kern/vfs_cache.c:674
#7  0xffffffff805628a0 in VOP_LOOKUP_APV (vop=0xffffffff8076e440,
    a=0xffffffffa2d936b0) at vnode_if.c:99
#8  0xffffffff80409afd in lookup (ndp=0xffffffffa2d939f0) at vnode_if.h:57
#9  0xffffffff8040a824 in namei (ndp=0xffffffffa2d939f0)
    at ../../../kern/vfs_lookup.c:219
#10 0xffffffff8041dc4f in vn_open_cred (ndp=0xffffffffa2d939f0,
    flagp=0xffffffffa2d9393c, cmode=0, cred=0xffffff0001588600,
    fp=0xffffff0001964200) at ../../../kern/vfs_vnops.c:188
#11 0xffffffff8041ccc7 in kern_open (td=0xffffff0019109a50,
    path=0xffffac30 <Address 0xffffac30 out of bounds>, pathseg=Variable 
"pathseg" is not available.
)
    at ../../../kern/vfs_syscalls.c:1032
#12 0xffffffff80544660 in ia32_syscall (frame=0xffffffffa2d93c80)
    at ../../../amd64/ia32/ia32_syscall.c:182
#13 0xffffffff805014d0 in Xint0x80_syscall () at ia32_exception.S:65
#14 0x0000000028284037 in ?? ()

(kgdb) frame 4
#4  0xffffffff80392ea6 in _mtx_lock_flags (m=0xffffffff8083c220, opts=0,
    file=0xffffffff805bd438 "../../../kern/vfs_cache.c", line=327)
    at ../../../kern/kern_mutex.c:186
186             _get_sleep_lock(m, curthread, opts, file, line);
(kgdb) list
181                 ("mtx_lock() of spin mutex %s @ %s:%d", 
m->lock_object.lo_name,
182                 file, line));
183             WITNESS_CHECKORDER(&m->lock_object, opts | LOP_NEWORDER 
| LOP_EXCLUSIVE,
184                 file, line);
185
186             _get_sleep_lock(m, curthread, opts, file, line);
187             LOCK_LOG_LOCK("LOCK", &m->lock_object, opts, 
m->mtx_recurse, file,
188                 line);
189             WITNESS_LOCK(&m->lock_object, opts | LOP_EXCLUSIVE, 
file, line);
190             curthread->td_locks++;

(kgdb) print *m
$2 = {lock_object = {lo_name = 0xffffffff805bd5d2 "Name Cache",
    lo_type = 0xffffffff805bd5d2 "Name Cache", lo_flags = 16973824,
    lo_witness_data = {lod_list = {stqe_next = 0xffffffff807cd600},
      lod_witness = 0xffffffff807cd600}}, mtx_lock = 4, mtx_recurse = 0}