Date: Tue, 11 May 1999 10:50:02 -0700 (PDT) From: Bob Willcox <bob@luke.pmr.com> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/10872: Panic in sorecieve() Message-ID: <199905111750.KAA62876@freefall.freebsd.org>
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The following reply was made to PR kern/10872; it has been noted by GNATS.
From: Bob Willcox <bob@luke.pmr.com>
To: Pierre Beyssac <beyssac@enst.fr>
Cc: freebsd-bugs@freebsd.org, bob@pmr.com,
FreeBSD-gnats-submit@freebsd.org
Subject: Re: kern/10872: Panic in sorecieve()
Date: Tue, 11 May 1999 12:41:17 -0500
On Tue, May 11, 1999 at 06:59:56PM +0200, Pierre Beyssac wrote:
> I was looking into PR kern/10872, hoping to find an easily fixable
> occurence of NULL mbuf pointer. But it doesn't seem to be.
>
> It's labelled "Panic in sorecieve() due to NULL mbuf pointer", but
> from the debug data filed with the PR it seems the actual problem
> is with so_rcv.sb_cc being 0, which triggers a KASSERT in uipc_socket.c:
>
> if (m == 0 || (((flags & MSG_DONTWAIT) == 0 &&
> so->so_rcv.sb_cc < uio->uio_resid) &&
> (so->so_rcv.sb_cc < so->so_rcv.sb_lowat ||
> ((flags & MSG_WAITALL) && uio->uio_resid <= so->so_rcv.sb_hiwat)) &&
> m->m_nextpkt == 0 && (pr->pr_flags & PR_ATOMIC) == 0)) {
> KASSERT(m != 0 || !so->so_rcv.sb_cc, ("receive 1"));
>
> (more data can be found in the PR)
Hmm, I haven't looked at this in a few weeks (I downgraded my amanda
backup server to 2.2.8 to work around the problem till I could find a
fix). The problem as I have seen it is that the mbuf chain pointer (m)
is NULL and so_rcv.sb_cc is not zero. Its as though somewhere either
the mbuf chain pointer gets zapped with NULL or something fails to
properly update so_rcv.sb_cc as mbufs are processed.
I believe one can expand the KASSERT macro and rewrite the line:
KASSERT(m != 0 || !so->so_rcv.sb_cc, ("receive 1"));
as
do {
if (!(m != 0 || !so->so_rcv.sb_cc))
panic("receive 1");
} while (0);
which can be simplified into:
do {
if (m == 0 && so->so_rcv.sb_cc != 0)
panic("receive 1");
} while (0);
by removing the ! from the expression and adjusting it accordingly.
>
> I can't seem to be able to reproduce the problem on -current with
> the script provided by Bob, and I don't have a -stable box to try
> it on either.
I have been able to reproduce it on both -stable and -current (but not
2.2.8). I have a full-duplex 100Mb ethernet switch that my systems
are on. On slower networks it may not fail. It seems to be timing
dependent.
>
> Plus, I don't have (yet) much of a clue regarding the semantics of
> sb_cc. I continue investigating this stuff, but if anyone has more
> clue than I have, he's welcome to send me some directions to look
> into :-)
> --
> Pierre Beyssac pb@enst.fr
--
Bob Willcox The man who follows the crowd will usually get no
bob@luke.pmr.com further than the crowd. The man who walks alone is
Austin, TX likely to find himself in places no one has ever
been. -- Alan Ashley-Pitt
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