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

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Date:	Wed, 20 Sep 2006 11:39:55 +0200
From:	Helge Hafting <helge.hafting@...el.hist.no>
To:	prasanna@...ibm.com
CC:	Martin Bligh <mbligh@...gle.com>, Andrew Morton <akpm@...l.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Linux Kernel Markers

S. P. Prasanna wrote:
>>
>> Yes, that's simple. but slower, as you have a double jump. Probably
>> a damned sight faster than int3 though.
>>
>> M.
>>     
>
> The advantage of using int3 over jmp to launch the instrumented
> module is that int3 (or breakpoint in most architectures) is an
> atomic operation to insert.
>   
Yes, 5 bytes is not an atomic write except on 64-bit. So a race is possible.

How about this workaround:
1. Overwrite the start of the function with a hlt, which is atomic.
2. Write that 5-byte jump after the hlt.
3. Overwrite the hlt with nop so things will work
4. interrupt any cpus that got stuck on the hlt - or just wait for the 
timer.

Helge Hafting

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