linux-kernel - Re: [PATCH] Linux Kernel Markers

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Date:	Wed, 20 Sep 2006 06:38:52 +0530
From:	"S. P. Prasanna" <prasanna@...ibm.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Martin Bligh <mbligh@...gle.com>,
	Andrew Morton <akpm@...l.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com
Subject: Re: [PATCH] Linux Kernel Markers

Hi Alan,

On Wed, Sep 20, 2006 at 01:08:45AM +0100, Alan Cox wrote:
> Ar Maw, 2006-09-19 am 13:54 -0400, ysgrifennodd Mathieu Desnoyers:
> > Very good idea.. However, overwriting the second instruction with a jump could
> > be dangerous on preemptible and SMP kernels, because we never know if a thread
> > has an IP in any of its contexts that would return exactly at the middle of the
> > jump. 
> 
> No: on x86 it is the *same* case for all of these even writing an int3.
> One byte or a megabyte,
> 
> You MUST ensure that every CPU executes a serializing instruction before
> it hits code that was modified by another processor. Otherwise you get
> CPU errata and the CPU produces results which vendors like to describe
> as "undefined".

Are you referring to Intel erratum "unsynchronized cross-modifying code"
- where it refers to the practice of modifying code on one processor
where another has prefetched the unmodified version of the code. 

Thanks
Prasanna

> 
> Thus you have to serialize, and if you are serializing it really doesn't
> matter if you write a byte, a paragraph or a page.
> 

-- 
Prasanna S.P.
Linux Technology Center
India Software Labs, IBM Bangalore
Email: prasanna@...ibm.com
Ph: 91-80-41776329
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