rapid file creation on snapshotted filesystems panics lockmgr

[John Kozubik]

(the below refers exclusively to 5.2.1-RELEASE)

Rapid file creation on filesystems with as few as one snapshot on them
will panic lockmgr:

panic: lockmgr: locking against myself
cpuid = 0;

syncing disks, buffers remaining... panic: ffs_copyonwrite: recursive call
cpuid = 0;
uptime: 5m10s
Shutting down ACPI

(crash)

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I have repeated this problem on both a laptop and a server, with both IDE
and SCSI disks, and with both a GENERIC and a pared down kernel (no
additions, simply subtractions).  Further, disabling ACPI by adding
hint.acpi.0.disabled="1" to the file /boot/loader.conf did not help in any
way.  All tested systems were single processor Pentium-3 systems.

Here is how you can quickly recreate this problem:

mksnap_ffs /mnt/mount1 /mnt/mount1/snapshot_1
cd /mnt/mount1
tar cvf /mnt/mount1/devel.tar /usr/ports/devel
tar xvf devel.tar

You will panic the system and (possibly) receive the above output within
seconds.

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If you do not immediately panic the system, the tar process will be
blocked:

 procs      memory      page                    disks
 r b w     avm    fre  flt  re  pi  po  fr  sr da0 fd0
 0 1 0   37816  29160    3   0   0   0  33  25   0   0

and will be in state "snaplk" :

  PID USERNAME PRI NICE   SIZE    RES STATE    TIME   WCPU    CPU COMMAND
  598 root      -4    0  1660K  1088K snaplk   1:00  0.00%  0.00% tar

It will be impossible to kill the tar process.  Further, any additional
tar processes will immediately block (the counter will increment to 2 in
`vmstat` output).  Any attempt to `sync` will also result in a blocked
`sync` process in state "snaplk".  You will not be able to successfully
reboot, as the system will lock when it the shutdown process attempts to
sync.

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To reproduce this error, it is necessary to create small files rapidly, as
I did above by tarring and untarring the /usr/ports/devel directory.  If
you dd several very large files and tar them up, and then untar them in
the filesystem that has been snapshotted, this panic will not occur - even
though the total amount of data is much larger than what is contained in a
tarball of /usr/ports/devel.

I have submitted this as PR misc/68757.


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John Kozubik - john at kozubik.com - http://www.kozubik.com