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 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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