Re: Kernel lockup, might be helpful log.

[Hugo Mills <hugo@carfax.org.uk>]

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On Mon, Dec 14, 2015 at 06:51:41AM +0000, Duncan wrote:
> Birdsarenice posted on Sun, 13 Dec 2015 22:55:19 +0000 as excerpted:
> 
> > Meanwhile, I did get lucky: At one crash I happened to be logged in and
> > was able to hit dmesg seconds before it went completely. So what I have
> > here is information that looks like it'll help you track down a
> > rarely-encountered and hard-to-reproduce bug which can cause the system
> > to lock up completely in event of certain types of hard drive failure.
> > It might be nothing, but perhaps someone will find it of use - because
> > it'd be a tricky one to both reproduce and get a good error report if it
> > did occur.
> > 
> > I see an 'invalid opcode' error in here, that's pretty unusual
> 
> Disclaimer:  I'm a list regular and (small-scale) sysadmin, not a dev, 
> and most certainly not a btrfs dev.  Take what I saw with that in mind, 
> tho I've been active on-list for over a year and thus now have a 
> reasonable level of practical sysadmin configuration and crisis recovery 
> level btrfs experience.
> 
> You could well be quite correct with the unusual crash log and its value, 
> I'll leave that up to the devs to decide, but that "invalid opcode: 0000" 
> bit is in fact not at all unusual on btrfs.  Tho I can say it fooled me 
> originally as well, because it certainly /looks/ both suspicious and in 
> general unusual.
> 
> Based on how a dev explained it to me, I believe btrfs actually 
> deliberately uses opcode 0000 to trigger a semi-controlled crash in 
> instances where code that "should never happen" actually gets executed 
> for some reason, leaving the kernel is an unknown and thus not 
> trustworthy enough to reliably write to storage devices and do a 
> controlled shutdown.  That's of course why the tracebacks are there, to 
> help the devs figure out where it was and what triggered it, but the 0000 
> opcode itself is actually quite frequently found in these tracebacks, 
> because it's the method chosen to deliberately trigger them.

   It's not just btrfs. Invalid opcode is the way that the kernel's
BUG and BUG_ON macro is implemented.

   Hugo.

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