linux-kernel - Re: the printk problem

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Date:	Sat, 5 Jul 2008 20:41:39 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Jan Engelhardt" <jengelh@...ozas.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Matthew Wilcox" <matthew@....cx>, "Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-ia64@...r.kernel.org, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org
Subject: Re: the printk problem

On Sat, Jul 5, 2008 at 7:56 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Sat, 5 Jul 2008, Vegard Nossum wrote:
>> On Sat, Jul 5, 2008 at 1:33 PM, Jan Engelhardt <jengelh@...ozas.de> wrote:
>> > How about %p{feature}?
>
> No.
>
> I _expressly_ chose '%p[alphanumeric]*' because it's basically
> totally insane to have that in a *real* printk() string: the end result
> would be totally unreadable.
>
> In contrast, '%p[specialchar]' is not unreadable, and in fact we have lots
> of those already in the kernel. In fact, there are 40 occurrences of '%p{'
> in the kernel, just grep for it (especially the AFS code seems to be very
> happy to use that kind of printout in its debug statements).
>
> So it makes perfect sense to have a _real_ printk string that says
>
>        "BUG: Dentry %p{i=%lx,n=%s}"
>
> where we have that '%p{...' sequence: the end result is easily parseable.
> In contrast, anybody who uses '%pS' or something like that and expects a
> pointer name to be immediately followed by teh letter 'S' is simply
> insane, because the end result is an unreadable mess.

That's really a truly hideous hack :-)

Single letters are bad because it hurts readability and limits the
usefulness of the extension.</MHO>

If it's just the {} that is the problem, use something else. I'm sure
we can find something that isn't used already.

>
>> (It's hard on the stack, yes, I know. We should fix kallsyms.)
>
> Not just that, but it's broken when KALLSYMS is disabled. Look at what
> sprint_symbol() becomes.

Oops. Not hard to mend, though.

> The patch I already sent out is about a million times better, because it
> avoids all these issues, and knows about subtle issues like the difference
> between a direct pointer and a pointer to a function descriptor.

Right, right. I found it now:

http://ozlabs.org/pipermail/linuxppc-dev/2008-July/059257.html

Argh... Some pointer to original thread would be nice when adding new Ccs.


Vegard

PS: Your version has exactly the same stack problem. Will send a patch
for kallsyms later.

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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