dmesg empty after shutdown -r

[Mel]

On Thursday 01 May 2008 13:58:45 A. Hamilton-Wright wrote:
> > On Thursday 01 May 2008 01:58:46 A Hamilton-Wright wrote:
> >> After "shutdown -r now" and the subsequent reboot, I have
>
>    (... no dmesg)
>
> On Thu, 1 May 2008, Mel wrote:
> > dmesg -M doesn't show anything either?
>
> Wish I'd thought to try that last night.  I eventually shut it
> down again (shutdown -p) until I could come in this morning and
> take a look at the console while booting -- and now everything
> is fine.  I have now tried a few reboots (shutdown -r) and halts
> (shutdown -h), and I have a dmesg every time it recovers.

Just for the record, I asked because by default, dmesg uses kvm_read(3) to 
read the kernel's message buffer. -M tries to read the sysctl kern.msgbuf, so 
in the event kvm_read screwed up somewhere without noting an error, 
the 'backdoor' might just work. Pure speculation without having looked in 
detail at the kernel code, but I wanted to rule it out. What I did suspect, 
is that kernel's message buffer didn't get filled to begin with and that's a 
whole different ballgame.

FYI: unless you really need dmesg to work for normal operations, you could've 
just let it up running and use scroll-lock+arrows/pgup|down to go all the way 
back up to btx loader. If it was the kernel message buffer not getting 
filled, I'd suspect some kind of message complaining about that.
I know that doesn't help now, but for next time and googlers ;)
-- 
Mel

Problem with today's modular software: they start with the modules
    and never get to the software part.