System hanging during dump

[heliocentric at gmail.com]

>> Based on progress reports from both dump and my fifo process, the
>> snapshot removal began about 10 hours _after_ the system froze
>> (during this time, dump wrote about 143GB).  Given the timeline,
>> it's fairly clear that neither mksnap_ffs nor the 'rm snapshot'
>> were running at the time the system froze.  I am therefore quite
>> confident that the problem I saw is not related to either creation
>> or removal of snapshots.
>>
>> I have been using FreeBSD snapshots for many years and am quite
>> familiar with their quirks.  I have never seen this particular
>> problem before.  (And FWIW, I _am_ using Doug Ambrisko's patch to
>> ffs_snapshot.c).

After upgrading my backup server to 8-CURRENT, I've noticed LORs
during write activity that happens while a snapshot is being read. I
believe these are responsible for some of the deadlocks in a subset of
filesystem operations during some usages of snapshots I've
encountered, but I can't confirm (as I am inexperienced at deeply
debugging locking issues). There is also two LORs involving the same
locks that occur during a snapshot removal, which is most likely the
cause of the 'hang' that seems to be reported for that case.

Perhaps a 'show locks' with an 8-CURRENT kernel with WITNESS enabled
will shed light on the problem? As most of the filesystem locking
doesn't use lockmgr in 7-STABLE, it would be silent with that kernel.