linux-ext4 - Re: [PATCH 1/1] x86: fix text

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Message-ID: <20080425155000.GA4438@elte.hu>
Date:	Fri, 25 Apr 2008 17:50:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jiri Slaby <jirislaby@...il.com>,
	David Miller <davem@...emloft.net>, zdenek.kabelac@...il.com,
	rjw@...k.pl, paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-ext4@...r.kernel.org, herbert@...dor.apana.org.au,
	penberg@...helsinki.fi, clameter@....com,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Andi Kleen <andi@...stfloor.org>, pageexec@...email.hu,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH 1/1] x86: fix text_poke


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> No. That whole code sequence is total and utter crap. It needs to be 
> rewritten.
> 
> It first does a BUG_ON() if it's not naturally aligned (because that 
> wouldn't be atomic), and then it has code for page crossing! What a 
> TOTAL PIECE OF SH*T!
> 
> Hint:
>  - if it's naturally aligned, it couldn't be page crossing ANYWAY
>  - and if it was a page-crosser, it sure as hell couldn't be atomic!
> 
> The code is just crap, crap, crap. It needs to be rewritten from 
> scratch. I'll have a patch soonish.

yeah :(

it seems that this code only worked because text_poke_early() [which can 
take arbitrary length and alignment] does most of the patching, it is 
the real code-patching machinery that is used during early bootup - and 
that's not used later on.

text_poke() itself only applies/unapplies the LOCK prefix - a single 
byte. We shouldnt be doing that at all: the cost of LOCK is 
insignificant (a few cycles) and most systems are SMP anyway.

any other type of code patching should use stop_machine_run(), where 
every CPU is stopped with irqs disabled.

	Ingo
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