Re: [RFC PATCH 00/11] printk: safe printing in NMI context

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Jun 10, 2014 at 9:46 AM, Frederic Weisbecker <fweisbec@gmail.com> wrote:
>
> There is also a big risk that if we push back this bugfix, nobody will actually do
> that desired rewrite.
>
> 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.

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.

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.

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.

             Linus