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

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Date:	Tue, 10 Jun 2014 09:57:36 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Jiri Kosina <jkosina@...e.cz>, Petr Mladek <pmladek@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dave Anderson <anderson@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kay Sievers <kay@...y.org>, Michal Hocko <mhocko@...e.cz>,
	Jan Kara <jack@...e.cz>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/11] printk: safe printing in NMI context

On Tue, Jun 10, 2014 at 9:46 AM, Frederic Weisbecker <fweisbec@...il.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
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