Re: nfsd hung on ufs vnode lock

On Mon, Jan 31, 2011 at 12:00:29PM -0500, John Baldwin wrote:
> On Friday, January 28, 2011 8:10:41 pm John Hickey wrote:
> > There was a previous thread about this, but it doesn't look like there was any resolution:
> > 
> > http://lists.freebsd.org/pipermail/freebsd-stable/2010-May/056986.html
> > 
> > I run a fileserver for an Emulab (www.emulab.net) system.  As such, the exports table is constantly modified as experiments are swapped in and 
> out.  We also get a lot of researchers using NFS for strange things.  In this case, the exclusive lock was for a cache directory shared by about 
> 36 machines running Ubuntu 8.04 and mounting with NFSv2.  Eventually, all our nfsd processes get stuck since the exclusive lock for the directory 
> is never released.  I could use any and all pointers on getting this fixed.
> > 
> > What I am running:
> > 
> > jjh@users: ~$ uname -a
> > FreeBSD users.isi.deterlab.net 7.3-RELEASE-p2 FreeBSD 7.3-RELEASE-p2 #9: Tue Sep 14 16:24:57 PDT 2010     
> root@users.isi.deterlab.net:/usr/obj/usr/src/sys/USERS7  i386
> > 
> > Here are the sleepchains for my system (note that 0xd1f72678 appears twice):
> > 
> > 0xce089cf0: tag syncer, type VNON
> >  usecount 1, writecount 0, refcount 2 mountedhere 0
> >  flags ()
> >   lock type syncer: EXCL (count 1) by thread 0xcdb4b000 (pid 46)
> > 
> > 0xd1f72678: tag ufs, type VDIR
> >  usecount 2, writecount 0, refcount 67 mountedhere 0
> >  flags ()
> >  v_object 0xd1e90e80 ref 0 pages 1
> >   lock type ufs: EXCL (count 1) by thread 0xce1146c0 (pid 866) with 62 pending
> >      ino 143173560, on dev mfid0s1f
> 
> From the stack trace, this vnode is the directory vnode that is the parent
> of the new file being created.
> 
> > (kgdb) bt
> > #0  sched_switch (td=0xce1146c0, newtd=Variable "newtd" is not available.
> > ) at /usr/src/sys/kern/sched_ule.c:1936
> > #1  0xc080a4a6 in mi_switch (flags=Variable "flags" is not available.
> > ) at /usr/src/sys/kern/kern_synch.c:444
> > #2  0xc0837aab in sleepq_switch (wchan=Variable "wchan" is not available.
> > ) at /usr/src/sys/kern/subr_sleepqueue.c:497
> > #3  0xc08380f6 in sleepq_wait (wchan=0xd4176394) at /usr/src/sys/kern/subr_sleepqueue.c:580
> > #4  0xc080a92a in _sleep (ident=0xd4176394, lock=0xc0ceb498, priority=80, wmesg=0xc0bb656e "ufs", timo=0) at /usr/src/sys/kern/kern_synch.c:230
> > #5  0xc07ea9fa in acquire (lkpp=0xcd7375a0, extflags=Variable "extflags" is not available.
> > ) at /usr/src/sys/kern/kern_lock.c:151
> > #6  0xc07eb2ec in _lockmgr (lkp=0xd4176394, flags=8194, interlkp=0xd41763c4, td=0xce1146c0, file=0xc0bc20c8 "/usr/src/sys/kern/vfs_subr.c", 
> line=2062)
> >  at /usr/src/sys/kern/kern_lock.c:384
> > #7  0xc0a24765 in ffs_lock (ap=0xcd737608) at /usr/src/sys/ufs/ffs/ffs_vnops.c:377
> > #8  0xc0b26876 in VOP_LOCK1_APV (vop=0xc0ca4740, a=0xcd737608) at vnode_if.c:1618
> > #9  0xc0896d76 in _vn_lock (vp=0xd417633c, flags=8194, td=0xce1146c0, file=0xc0bc20c8 "/usr/src/sys/kern/vfs_subr.c", line=2062) at 
> vnode_if.h:851
> 
> Note that, this vnode (vp) doesn't show up in your list above.  You can try
> using my gdb scripts at www.freebsd.org/~jhb/gdb (you want gdb6* and do
> 'source gdb6').  You can then do 'vprint vp' at this frame and should see
> lock details about who holds this lock.  However, I would not expect the
> vnode lock for a new i-node to be already held.

jjh@users: ~$ kgdb /boot/kernel/kernel vmcore.0 
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-marcel-freebsd"...

Unread portion of the kernel message buffer:
panic: kdb_sysctl_panic
cpuid = 1
Uptime: 4d3h36m42s
Physical memory: 3566 MB
Dumping 437 MB: 422 406 390 374 358 342 326 310 294 278 262 246 230 214 198 182 166 150 134 118 102 86 70 54 38 22 6

#0  doadump () at pcpu.h:196
196	pcpu.h: No such file or directory.
	in pcpu.h
(kgdb) source gdb6
(kgdb) proc 866
[Switching to thread 66 (Thread 100104)]#0  sched_switch (td=0xce1146c0, newtd=Variable "newtd" is not available.
) at /usr/src/sys/kern/sched_ule.c:1936
1936			cpuid = PCPU_GET(cpuid);
(kgdb) frame 9
#9  0xc0896d76 in _vn_lock (vp=0xd417633c, flags=8194, td=0xce1146c0, file=0xc0bc20c8 "/usr/src/sys/kern/vfs_subr.c", line=2062) at vnode_if.h:851
warning: Source file is more recent than executable.

851		return (VOP_LOCK1_APV(vp->v_op, &a));
(kgdb) vprint vp
0xd417633c: tag none, type VBAD
    usecount 0, writecount 0, refcount 1 mountedhere 0x0
    flags (VI_DOOMED)
    lock type ufs: UNLOCKED with 1 pending
(kgdb) 

> There's a chance though you are tripping over the bug fixed by these two
> changes:

I should be able to sync up with 7.4 without much pain in a few days.

> Author: jhb
> Date: Fri Jul 16 20:23:24 2010
> New Revision: 210173
> URL: http://svn.freebsd.org/changeset/base/210173
> 
> Log:
>   When the MNTK_EXTENDED_SHARED mount option was added, some filesystems were
>   changed to defer the setting of VN_LOCK_ASHARE() (which clears LK_NOSHARE
>   in the vnode lock's flags) until after they had determined if the vnode was
>   a FIFO.  This occurs after the vnode has been inserted into a VFS hash or
>   some similar table, so it is possible for another thread to find this vnode
>   via vget() on an i-node number and block on the vnode lock.  If the lockmgr
>   interlock (vnode interlock for vnode locks) is not held when clearing the
>   LK_NOSHARE flag, then the lk_flags field can be clobbered.  As a result
>   the thread blocked on the vnode lock may never get woken up.  Fix this by
>   holding the vnode interlock while modifying the lock flags in this case.
>   
>   The softupdates code also toggles LK_NOSHARE in one function to close a
>   race with snapshots.  Fix this code to grab the interlock while fiddling
>   with lk_flags.
> 
> Author: jhb
> Date: Fri Aug 20 20:58:57 2010
> New Revision: 211533
> URL: http://svn.freebsd.org/changeset/base/211533
> 
> Log:
>   Revert 210173 as it did not properly fix the bug.  It assumed that the
>   VI_LOCK() for a given vnode was used as the internal interlock for that
>   vnode's v_lock lockmgr lock.  This is not the case.  Instead, add dedicated
>   routines to toggle the LK_NOSHARE and LK_CANRECURSE flags.  These routines
>   lock the lockmgr lock's internal interlock to synchronize the updates to
>   the flags member with other threads attempting to acquire the lock.  The
>   VN_LOCK_A*() macros now invoke these routines, and the softupdates code
>   uses these routines to temporarly enable recursion on buffer locks.
>   
>   Reviewed by:  kib
> 
> -- 
> John Baldwin
> 
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