Attention pf/ipfw users with uid/gid/jail rules (Re: Reminder:
NET_NEEDS_GIANT, debug.mpsafenet going away in 7.0)
rwatson at FreeBSD.org
Fri Jul 27 08:00:29 UTC 2007
On Fri, 27 Jul 2007, Max Laier wrote:
>>> does "problem" include a LOR message, or only a deadlock?
>>> I've seen plenty of the first, but not the second.
>> Various users have reported definite deadlocks relating to uid/gid
> ^------^ ^------^
>> firewall rules in the past.
> I don't think the strong wording is true. I have seen a few reports of
> deadlocks in the past where debug.mpsafenet "fixed" the issue, but none of
> the reporters where able to provide enough debugging details to actually
> identify the culprit.
There was a period when there were definitely deadlocks being reported, but I
believe quite a few cases were resolved by Christian's work to push the inpcb
down the transmit path in more cases, allowing firewalls to avoid acquiring
pcbinfo/pcb locks, which in turn avoided a lock order reversable between the
pcbinfo and pcb locks (pcb locks are held over transmit from the IP layer, and
the lock order is pcbinfo -> pcb). So to expand on Max's thoughts -- the
worry here is that we've corrected the deadlock but not the witness warnings
associated with a non-deadlockable reversal. Witness is reasonable to get
upset about lock order, but current reasoning is that because this lock is
only acquired readably when used in combination with other locks, there isn't
deadlock potential, since the read acquire will be non-blocking when involved
with those locks, preventing a cycle of waiting. It's good reasoning, and
possibly correct :-).
Robert N M Watson
University of Cambridge
> Also note that a lot has changed since the early reports. What WITNESS is
> warning about now is something like:
> It's obvious that this can't cause a deadlock unless there is a third
> codepath that does either:
> I have an idea how to teach WITNESS about this, but it's an awful hack,
> /"\ Best regards, | mlaier at freebsd.org
> \ / Max Laier | ICQ #67774661
> X http://pf4freebsd.love2party.net/ | mlaier at EFnet
> / \ ASCII Ribbon Campaign | Against HTML Mail and News
More information about the freebsd-arch