DragonFlyBSD

I forgot to mention that the kernel and cores are *.3 and *.4.

:New submission from Peter Avalos <pavalos@theshell.com>:
:
:Here's a panic I'm getting with some pretty serious network (www) load, then
:doing a netstat -an:
:
:Unread portion of the kernel message buffer:
:panic: m_copydata, negative off -1
:mp_lock = 00000000; cpuid = 0; lapic.id = 00000000
:boot() called on cpu#0
:
:syncing disks... 5
:done
:Uptime: 12d22h0m32s

Woa.  You mean the panic occurs only when you do the netstat -an command
    under heavy network load?  It doesn't happen any other time?

This is a really odd crash.  Somehow tp->snd_nxt has become less
    then tp->snd_una, causing 'off' to be calculated as -1.

-Matt

Correct. Once I execute "netstat -an" it panics. Any ideas?

--Peter

:On Wed, Apr 11, 2007 at 11:38:48AM -0700, Matthew Dillon wrote:
:>=20
:> Woa. You mean the panic occurs only when you do the netstat -an comm=
:and
:> under heavy network load? It doesn't happen any other time?
:>=20
:
:Correct. Once I execute "netstat -an" it panics. Any ideas?
:
:--Peter

That's really odd.  I looked at the code and found one possible
    place where the field could get out of whack.  Try turning off the
    tcp limited transmit code:

(in /etc/sysctl.conf):
    net.inet.tcp.limitedtransmit=0

and reboot to clean out any preexisting tcp connections (or otherwise
    clean them out manually by killing and restarting the services).

I'm a bit at a loss a to why netstat -an would trigger the problem,
    though.  We do know that anything that accesses /dev/kmem heavily,
    like fstat, can crash the machine while chasing down stale pointers
    in kernel memory.  But this panic seems a bit at odds with the sort
    of crash I would expect from stale pointer chasing.

-Matt

netstat -an uses a sysctl interface though.

Joerg

:On Wed, Apr 11, 2007 at 03:52:30PM -0700, Matthew Dillon wrote:
:> I'm a bit at a loss a to why netstat -an would trigger the problem,
:> though. We do know that anything that accesses /dev/kmem heavily,
:> like fstat, can crash the machine while chasing down stale pointers
:> in kernel memory. But this panic seems a bit at odds with the sort
:> of crash I would expect from stale pointer chasing.
:
:netstat -an uses a sysctl interface though.
:
:Joerg

That would make more sense.  I was scratching my head at how a KVM
    access could cause this, a direct sysctl interface is more likely.

I don't see a whole lot in the sysctl code either, unfortunately.
    e.g. tcp_pcblist() in tcp_subr.c.  There is one likely possibility.
    Because the sysctl is dumping its huge, huge list in one large go
    and holding the big giant lock while it does it, it could be
    preventing the TCP stack's callout's (which is where the panic occured)
    from running during that period.  There could be a race condition there
    that we are not handling properly.

so, e.g. some sort of race in softclock_handler() in kern_timeout.c
    related to the acquisition of the big giant lock.

-Matt
                    Matthew Dillon 
                    <dillon@backplane.com>

Any luck turning off that sysctl?

-Matt

I have turned off the sysctl, but I'm having a hard time generating the
traffic needed that originally caused the panic. (I'm estimating it was
about 6000 simultaneous http connections, but I'm not exactly sure since
netstat wasn't working.)

--Peter

:traffic needed that originally caused the panic. (I'm estimating it was
:about 6000 simultaneous http connections, but I'm not exactly sure since
:netstat wasn't working.)
:
:--Peter

Ok, for now please continue with the sysctl turned off (put it in your
    /etc/sysctl.conf).  That way if the crash occurs again in the future
    we can discount that part of the tcp stack.

I am going to go ahead and commit a fix to the possible bug related
    to the code in question.

-Matt
                    Matthew Dillon 
                    <dillon@backplane.com>

net.inet.tcp.limitedtransmit=1 definitely causes instability, even on a latest
master:

DragonFly ylem.theshell.com 2.9-DEVELOPMENT DragonFly v2.9.1.321.gadb6af-
DEVELOPMENT #31: Tue Dec 21 13:12:36 HST 2010
root@ylem.theshell.com:/usr/obj/usr/src/sys/YLEM i386

Unfortunately I keep getting secondary panics, so I'm not able to get a vmcore.