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Wild DMA (was Re: Unidentified subject!)


> My Debian box hangs on boot if you don't physicaly shut it of and on.  The
> reboot comand or even ctl+alt+del are a sure to crash when it reboots.
> Any sujestions?

I had this EXACT problem about six months ago and people in this list
(Bruce Perens specially) helped me to solve it. Well, this applies to
you if you are are connected to the network through an Ethernet adapter
that uses Direct Memory Access:

My problem was that when reboot time had arrived (either via the reboot
or shutdown commands or Ctrl-Alt-Del - they're all the same actually) the
Ethernet adapter was not being shutdown properly so it continued to
receive data from the network and putting in via DMA in system RAM. Well,
since the OS wasn't controlling RAM anymore, data was being placed who
knows where so very weird things like Power On Self Test (POST) failures,
computer crashes after all file system were unmounted, CRC errors after
decompressing the kernel at boot time, etc. The only way to reboot was doing
a hard (cold) reboot or by cycling power to the computer.

Summarizing, the Ethernet card was doing what is called "wild DMA".

To solve the problem, Bruce Perenhs told me to shutdown the Ethernet
card properly by placing this in /etc/reboot:

mount -n -o remount,ro /

echo "Shutting down Ethernet interface eth0"
/sbin/ifconfig eth0 down

echo -n "Rebooting... "
reboot -d -f

The key here is the "/sbin/ifconfig eth0 down" command. It shutdowns
the Ethernet interface completely.

If you do not have an Ethernet interface then you should try to identify
what other device can be doing wild DMA and shut it down properly before

I hope this helps. Good luck.



Eloy A. Paris
Information Technology Department
Rockwell Automation de Venezuela
Telephone: +58-2-9432311 Fax: +58-2-9430323

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