linux-kernel - Re: [PATCH RESEND 1/3] printk: convert byte-buffer to variable-length record buffer

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Date:	Thu, 03 May 2012 22:02:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Kay Sievers <kay@...y.org>, Greg Kroah-Hartmann <greg@...ah.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 1/3] printk: convert byte-buffer to
 variable-length record buffer

On Thu, 2012-05-03 at 12:56 -0700, Linus Torvalds wrote:
> On Thu, May 3, 2012 at 12:48 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Thu, 2012-05-03 at 02:29 +0200, Kay Sievers wrote:
> >> + * This is printk(). It can be called from any context. We want it to work.
> >
> > This is not true..
> 
> Well, it is very very nearly true, and people should basically expect it to be.
> 
> There are exceptions, but they are ridiculously specific
>  - no NMI context

With the new record design you could actually make the thing lockless
and allow writing to it from NMI context. IIRC vsnprintf() doesn't do
any allocations/locks.

>  - we've had trouble with the percpu runqueue lock (due to the stupid wakeup)

Yep :-(

>  - for similar reasons, we've had trouble with printk called from
> within some terminal locks.
> 
> But basically, it damn well should be considered a hard rule that you
> can printk from pretty much anything. The few cases where that is not
> true are big problems, and should be considered to be real problems,
> not "oh, you can't printk from here".

Thing is, with bonkers stuff like usb-console and kms/drm that's a _lot_
of code running under the logbuf/console locks.
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