NFS (& amd?) dysfunction descending a hierarchy

David Wolfskill david at
Thu Dec 11 14:53:50 PST 2008

On Wed, Dec 10, 2008 at 07:06:20PM +0200, Kostik Belousov wrote:
> ...
> > What concerns me is that even if the attempted unmount gets EBUSY, the
> > user-level process descending the directory hierarchy is getting ENOENT
> > trying to issue fstatfs() against an open file descriptor.
> > 
> > I'm having trouble figuring out any way that makes any sense.
> Basically, the problem is that NFS uses shared lookup, and this allows
> for the bug where several negative namecache entries are created for
> non-existent node. Then this node gets created, removing only the first
> negative namecache entry. For some reasons, vnode is reclaimed; amd'
> tasting of unmount is a good reason for vnode to be reclaimed.
> Now, you have existing path and a negative cache entry. This was
> reported by Peter Holm first, I listed relevant revisions that
> should fix this in previous mail.

Well, I messed up the machine I had been using for testing, and needed
to wait for IT to do something to it since I don't have physical or
console access to it.

So after I happened to demonstrate the effect using my desktop  -- which
had been running RELENG_7_1, sources updated as of around 0400 hrs.
US/Pacific -- I decided to go ahead and update the desktop to RELENG_7_1
as of this morning (which had the commit to sys/kern/vfs_cache.c), then

It still failed, apparently in the same way; details below.

First, here's a list of the files that were changed:

U lib/libarchive/archive_read_support_format_iso9660.c
U lib/libarchive/archive_string.c
U lib/libarchive/archive_string.h
U lib/libc/gen/times.3
U lib/libc/i386/sys/pipe.S
U lib/libc/i386/sys/reboot.S
U lib/libc/i386/sys/setlogin.S
U lib/libutil/Makefile
U lib/libutil/kinfo_getfile.c
U lib/libutil/kinfo_getvmmap.c
U lib/libutil/libutil.h
U share/man/man4/bce.4
U share/man/man5/Makefile
U share/man/man5/fstab.5
U share/man/man5/nullfs.5
U sys/amd64/Makefile
U sys/boot/forth/loader.conf.5
U sys/dev/ale/if_ale.c
U sys/dev/bce/if_bce.c
U sys/dev/cxgb/cxgb_main.c
U sys/dev/cxgb/common/cxgb_ael1002.c
U sys/dev/cxgb/common/cxgb_t3_hw.c
U sys/dev/cxgb/common/cxgb_xgmac.c
U sys/dev/re/if_re.c
U sys/fs/nullfs/null_vnops.c
U sys/kern/
U sys/kern/kern_descrip.c
U sys/kern/kern_proc.c
U sys/kern/vfs_cache.c
U sys/netinet/in_pcb.h
U sys/pci/if_rlreg.h
U sys/sys/sysctl.h
U sys/sys/user.h
U sys/ufs/ufs/ufs_quota.c
U usr.bin/procstat/Makefile
U usr.bin/procstat/procstat_files.c
U usr.bin/procstat/procstat_vm.c
U usr.bin/tar/util.c
U usr.bin/tar/test/Makefile
U usr.bin/tar/test/test_strip_components.c
U usr.bin/tar/test/test_symlink_dir.c
U usr.bin/xargs/xargs.1
U usr.sbin/mtree/mtree.c

We see that sys/kern/vfs_cache.c is, indeed, among them.  And:

dwolf-bsd(7.1-P)[5] grep '\$FreeBSD' /sys/kern/vfs_cache.c
__FBSDID("$FreeBSD: src/sys/kern/vfs_cache.c,v 2008/12/09 16:20:58 kib Exp $");

That should correspond to the desired version of the file.

Here we see an excerpt from the ktrace output for the amd(8) process and
its children; this is a point when amd(8) is trying an unmount() to see
if it can get away with it:

   977 amd      1229033597.269612 CALL  gettimeofday(0x807ad48,0)
   977 amd      1229033597.269620 RET   gettimeofday 0
   977 amd      1229033597.269630 CALL  sigprocmask(SIG_BLOCK,0xbfbfeaec,0xbfbfeadc)
   977 amd      1229033597.269637 RET   sigprocmask 0
   977 amd      1229033597.269645 CALL  fork
   977 amd      1229033597.273810 RET   fork 1712/0x6b0
  1712 amd      1229033597.273811 RET   fork 0
   977 amd      1229033597.273836 CALL  sigprocmask(SIG_SETMASK,0xbfbfeadc,0)
  1712 amd      1229033597.273845 CALL  getpid
   977 amd      1229033597.273850 RET   sigprocmask 0
  1712 amd      1229033597.273855 RET   getpid 1712/0x6b0
   977 amd      1229033597.273864 CALL  gettimeofday(0x807ad48,0)
   977 amd      1229033597.273874 RET   gettimeofday 0
  1712 amd      1229033597.273878 CALL  unmount(0x2832c610,<invalid>0)
  1712 amd      1229033597.352643 RET   unmount -1 errno 16 Device busy
  1712 amd      1229033597.352695 CALL  sigprocmask(SIG_BLOCK,0x28097c00,0xbfbfea0c)
  1712 amd      1229033597.352728 RET   sigprocmask 0
  1712 amd      1229033597.352751 CALL  sigprocmask(SIG_SETMASK,0x28097c10,0)
  1712 amd      1229033597.352769 RET   sigprocmask 0
  1712 amd      1229033597.352781 CALL  sigprocmask(SIG_BLOCK,0x28097c00,0xbfbfe9dc)
  1712 amd      1229033597.352790 RET   sigprocmask 0
  1712 amd      1229033597.352801 CALL  sigprocmask(SIG_SETMASK,0x28097c10,0)
  1712 amd      1229033597.352805 RET   sigprocmask 0
  1712 amd      1229033597.352815 CALL  exit(0x10)
   977 amd      1229033597.353085 RET   select -1 errno 4 Interrupted system call
   977 amd      1229033597.353093 PSIG  SIGCHLD caught handler=0x805de50 mask=0x0 code=0x0
   977 amd      1229033597.353103 CALL  wait4(0xffffffff,0xbfbfe83c,WNOHANG,0)
   977 amd      1229033597.353116 RET   wait4 1712/0x6b0
   977 amd      1229033597.353122 CALL  wait4(0xffffffff,0xbfbfe83c,WNOHANG,0)
   977 amd      1229033597.353127 RET   wait4 -1 errno 10 No child processes

So amd(8) master process (pid 977) jorks off a child (pid 1712) to
try an umount(), which it initiates at 1229033597.273878.  At
1229033597.352643 the child gets control back, as well as an EBUSY,
which I would expect to mean that the attempt failed.  The child
exits at 1229033597.352815 with a status code of 16.

Armed with that, we look at a ktrace excerpt from "rm -fr":

  1660 rm       1229033597.283277 CALL  rmdir(0x2822b388)
  1660 rm       1229033597.283283 NAMI  "stvef-paks"
  1660 rm       1229033597.285599 RET   rmdir 0
  1660 rm       1229033597.285620 CALL  stat(0x2822b3e8,0xbfbfe8dc)
  1660 rm       1229033597.285626 NAMI  "stvef-server"
  1660 rm       1229033597.286071 STRU  struct stat {dev=83951372, ino=20124614, mode=drwxr-xr-x , nlink=3, uid=9874, gid=929, rdev=0, atime=1228844788, stime=1227555769, ctime=1228845828.326650000, birthtime=0, size=4096, blksize=4096, blocks=8, flags=0x0 }
  1660 rm       1229033597.286078 RET   stat 0
  1660 rm       1229033597.286084 CALL  open(0x2822b3e8,O_NONBLOCK,<unused>0x1)
  1660 rm       1229033597.286091 NAMI  "stvef-server"
  1660 rm       1229033597.287145 RET   open 4
  1660 rm       1229033597.287154 CALL  fstat(0x4,0xbfbfe8dc)
  1660 rm       1229033597.287161 STRU  struct stat {dev=83951372, ino=20124614, mode=drwxr-xr-x , nlink=3, uid=9874, gid=929, rdev=0, atime=1228844788, stime=1227555769, ctime=1228845828.326650000, birthtime=0, size=4096, blksize=4096, blocks=8, flags=0x0 }
  1660 rm       1229033597.287166 RET   fstat 0
  1660 rm       1229033597.287171 CALL  fcntl(0x4,F_SETFD,FD_CLOEXEC)
  1660 rm       1229033597.287177 RET   fcntl 0
  1660 rm       1229033597.287187 CALL  fstatfs(0x4,0xbfbfe704)
  1660 rm       1229033597.287195 RET   fstatfs -1 errno 2 No such file or directory
  1660 rm       1229033597.287202 CALL  close(0x4)
  1660 rm       1229033597.287211 RET   close 0

[Sorry for the long lines....]

Here we see that the "rm" process (pid 1660) removed a directory
named stvef-paks sucessfully in the interval between 1229033597.283277
(when the request was made) and 1229033597.285599 (when the 0 return
occurred).  The "rm" process proceeds to process a directory named

* At 1229033597.285620 it issues a stat(); the successful return
  is at 1229033597.286078.

* At 1229033597.286084 it issues an open(); the successful return
  is at 1229033597.287145.  The FD is 4.

* At 1229033597.287154 it issues an fstat() against FD 4; the successful
  return is at 1229033597.287166.

* At 1229033597.287171 it issues an fcntl() against FD 4; the
  successful return is at 1229033597.287177.

* At 1229033597.287187 it issues an fstatfs() against FD 4; the
  unsuccessful return is at 1229033597.287195, claiming ENOENT.

Say WHAT??!?

I expect to be able to test a bit more promptly now.

David H. Wolfskill				david at
Depriving a girl or boy of an opportunity for education is evil.

See for my public key.
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