Kernel panic on FreeBSD 9.0-beta2

[dave jones]

2011/10/4 Mikolaj Golub :
>
> On Sat, 1 Oct 2011 14:15:45 +0800 dave jones wrote:
>
>  dj> On Fri, Sep 30, 2011 at 9:41 PM, Robert Watson wrote:
>  >>
>  >> On Wed, 28 Sep 2011, Mikolaj Golub wrote:
>  >>
>  >>> On Mon, 26 Sep 2011 16:12:55 +0200 K. Macy wrote:
>  >>>
>  >>> KM> Sorry, didn't look at the images (limited bw), I've seen something KM>
>  >>> like this before in timewait. This "can't happen" with UDP so will be KM>
>  >>> interested in learning more about the bug.
>  >>>
>  >>> The panic can be easily triggered by this:
>  >>
>  >> Hi:
>  >>
>  >> Just catching up on this thread.  I think the analysis here is generally
>  >> right: in 9.0, you're much more likely to see an inpcb with its in_socket
>  >> pointer cleared in the hash list than in prior releases, and
>  >> in_pcbbind_setup() trips over this.
>  >>
>  >> However, at least on first glance (and from the perspective of invariants
>  >> here), I think the bug is actualy that in_pcbbind_setup() is asking
>  >> in_pcblookup_local() for an inpcb and then access the returned inpcb's
>  >> in_socket pointer without acquiring a lock on the inpcb.  Structurally, it
>  >> can't acquire this lock for lock order reasons -- it already holds the lock
>  >> on its own inpcb.  Therefore, we should only access fields that are safe to
>  >> follow in an inpcb when you hold a reference via the hash lock and not a
>  >> lock on the inpcb itself, which appears generally OK (+/-) for all the
>  >> fields in that clause but the t->inp_socket->so_options dereference.
>  >>
>  >> A preferred fix would cache the SO_REUSEPORT flag in an inpcb-layer field,
>  >> such as inp_flags2, giving us access to its value without having to walk
>  >> into the attached (or not) socket.
>  >>
>  >> This raises another structural question, which is whether we need a new
>  >> inp_foo flags field that is protected explicitly by the hash lock, and not
>  >> by the inpcb lock, which could hold fields relevant to address binding.  I
>  >> don't think we need to solve that problem in this context, as a slightly
>  >> race on SO_REUSEPORT is likely acceptable.
>  >>
>  >> The suggested fix does perform the desired function of explicitly detaching
>  >> the inpcb from the hash list before the socket is disconnected from the
>  >> inpcb. However, it's incomplete in that the invariant that's being broken is
>  >> also relied on for other protocols (such as raw sockets).  The correct
>  >> invariant is that inp_socket is safe to follow unconditionally if an inpcb
>  >> is locked and INP_DROPPED isn't set -- the bug is in "locked" not in
>  >> "INP_DROPPED", which is why I think this is the wrong fix, even though it
>  >> prevents a panic :-).
>
>  dj> Hello Robert,
>
>  dj> Thank you for taking your valuable time to find out the problem.
>  dj> Since I don't have idea about network internals, would you have a patch
>  dj> about this? I'd be glad to test it, thanks again.
>
> Here is the patch that implements what Robert suggests.
>
> Dave, could you test it?

Sure. Thanks for cooking the patch.
Machines have been running two days now without panic.

> --
> Mikolaj Golub

Best regards,
Dave.