Date: Sat, 12 May 2007 10:59:39 +0530
From: Suparna Bhattacharya <suparna@in.ibm.com>
To: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, Andi Kleen <ak@muc.de>,
       systemtap@sources.redhat.com, prasanna@in.ibm.com,
       anil.s.keshavamurthy@intel.com, akpm@linux-foundation.org,
       linux-kernel@vger.kernel.org, hch@infradead.org,
       richardj_moore@uk.ibm.com
Subject: Re: [patch 05/10] Linux Kernel Markers - i386 optimized version
Message-ID: <20070512052939.GA20085@in.ibm.com>
Reply-To: suparna@in.ibm.com
References: <20070510015555.973107048@polymtl.ca> <20070510020916.508519573@polymtl.ca> <20070510090656.GA57297@muc.de> <20070510155501.GI22424@Krystal> <20070510172843.7aa72237@the-village.bc.nu> <20070510165918.GK22424@Krystal> <20070511045729.GA8143@in.ibm.com>
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On Fri, May 11, 2007 at 10:27:29AM +0530, Ananth N Mavinakayanahalli wrote:
> On Thu, May 10, 2007 at 12:59:18PM -0400, Mathieu Desnoyers wrote:
> > * Alan Cox (alan@lxorguk.ukuu.org.uk) wrote:
> 
> ...
> > > > * Third issue : Scalability. Changing code will stop every CPU on the
> > > >   system for a while. Compared to this, the int3-based approach will run
> > > >   through the breakpoint handler "if" one of the CPU happens to execute
> > > >   this code at the wrong time. The standard case is just an IPI (to
> > > 
> > > If I read the errata right then patching in an int3 will itself trigger
> > > the errata so anything could happen.
> > > 
> > > I believe there are other safe sequences for doing code patching - perhaps
> > > one of the Intel folk can advise ?
> 
> IIRC, when the first implementation of what exists now as kprobes was
> done (as part of the dprobes framework), this question did come up. I
> think the conclusion was that the errata applies only to multi-byte
> modifications and single-byte changes are guaranteed to be atomic.
> Given int3 on Intel is just 1-byte, we are safe.
> 
> > I'll let the Intel guys confirm this, I don't have the reference nearby
> > (I got this information by talking with the kprobe team members, and
> > they got this information directly from Intel developers) but the
> > int3 is the one special case to which the errata does not apply.
> > Otherwise, kprobes and gdb would have a big, big issue.
> 
> Perhaps Richard/Suparna can confirm.

I just tried digging up past discussions on this from Richard, about int3
being safe

http://sourceware.org/ml/systemtap/2005-q3/msg00208.html
http://lkml.org/lkml/2006/9/20/30

Regards
Suparna

> 
> Ananth

-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India

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