On Jul 21, 2005, at 12:16 PM, Robert Watson wrote:
On Thu, 21 Jul 2005, Eirik Øverby wrote:
The above panic will show up occasionally when logging out from a
serial console (i.e. ctrl-D, logout, exit, whatever). This is
EXTREMELY BAD, as it will crash an otherwise perfectly healthy
box at
random - and renders the serial console useless.
Robert Watson confirmed this to be an issue on the 10th of April.
You might have to wait until 6.0-R since fixing it seems to
require infrastructure changes that cannot easily be backported
to 5.x.
With all due respect - if this is (and I'm assuming it is, because
it happens on all the servers I'm serial-controlling) an
omnipresent problem on 5.x, I daresay it should warrant some more
attention. Having unsafe serial terminal support that can bring
down your system like that defies much of the point of having
serial terminal support in the first place.
However, since I seem to be the only one who has noticed this,
perhaps I'm the last person on earth to routinely use serial
terminal switches instead of KVM switches to do my admin work?
The concern about the 5.x backport is that it will break parts of
the device driver ABI, and is a significant change that involves a
lot of risk.
Regarding the general prevalence of the problem -- I've seen a
small number of people reporting it's a big problem. Since I know
of a great many people running with serial consoles (other than a
workstation, I never run FreeBSD boxes any other way), this leads
me to believe it's something that shows up in fairly specific
conditions -- perhaps relating to precise timing of a race
condition. This means that if we introduce a generally
destabilizing change, it may impact more people than the problem as
it exists (a nasty trade-off).
I've only seen the issue when logging out of a serial console
session, and had previously hypothesized that it had to do with the
simultaneous timing of a console message from syslog and the
opening/closing of the console's tty due to logging out and getty
restarting, resulting in a reference count improperly hitting zero.
I did indeed make some changes to my syslog configuration after
getting the serials online. Your theory might not be entirely off.
Let me know if I should post my syslog.conf file or anything else
here or elsewhere...
Thanks,
/Eirik
I thought Doug White had come up with a work-around patch that
prevented the reference count from being allowed to hit 0 for the
console by artificially elevating it, which would prevent the
panic, so either (a) the work around wasn't committed, or (b) it
didn't work.
I can attempt to take another look at this problem in a week or so,
but have a number of things I need to finish up for FreeBSD 6.0
before then that will be occupying my time.
Robert N M Watson
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