On Tue, 10 Jun 2014, Linus Torvalds wrote:
> > Lets be crazy and Cc Linus on that.
>
> Quite frankly, I hate seeing something like this:
>
> kernel/printk/printk.c | 1218
> +++++++++++++++++++++++++----------
>
> for something that is stupid and broken. Printing from NMI context
> isn't really supposed to work, and we all *know* it's not supposed to
> work.
It's OTOH rather useful in a few scenarios -- particularly it's the only
way to dump stacktraces from remote CPUs in order to obtain traces that
actually make sense (in situations like RCU stall); using workqueue-based
dumping is useless there.
> I'd much rather disallow it, and if there is one or two places that
> really want to print a warning and know that they are in NMI context,
> have a special workaround just for them, with something that does
> *not* try to make printk in general work any better.
Well, that'd mean that at least our stack dumping mechanism would need to
know both ways of printing; but yes, it'll still probably be less than 880
lines added.
> Dammit, NMI context is special. I absolutely refuse to buy into the
> broken concept that we should make more stuff work in NMI context.
> Hell no, we should *not* try to make more crap work in NMI. NMI people
> should be careful.
In parallel, I'd for the sake of argument propose to just drop the whole
_CONT printing (and all the things that followed on top) as that made
printk() a complete hell to maintain for a disputable gain IMO.
> Make a trivial "printk_nmi()" wrapper that tries to do a trylock on
> logbuf_lock, and *maybe* the existing sequence of
>
> if (console_trylock_for_printk())
> console_unlock();
>
> then works for actually triggering the printout. But the wrapper
> should be 15 lines of code for "if possible, try to print things", and
> *not* a thousand lines of changes.
Well, we are carrying much simpler fix for this whole braindamage in our
enterprise kernel that is from pre-7ff9554bb578 era, and it was rather
simple fix in principle (the diffstat is much larger than it had to be due
to code movements):
http://kernel.suse.com/cgit/kernel/commit/?h=SLE11-SP3&id=8d62ae68ff61d77ae3c4899f05dbd9c9742b14c9
But after the scary 7ff9554bb578 and its successors, things got a lot more
complicated.
--
Jiri Kosina
SUSE Labs
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