linux-kernel - Re: [RFC PATCH 1/2] Marker probes in futex.c

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Date:	Wed, 16 Apr 2008 07:24:16 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	prasad@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [RFC PATCH 1/2] Marker probes in futex.c

On Wed, 16 Apr 2008 10:00:09 -0400
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:


> > > > If we want to support NMI context and have the ability to
> > > > instrument preemptable code without too much headache, we must
> > > > insure that every modification will leave the code in a
> > > > "correct" state and that we do not grow the size of any
> > > > reachable instruction.  Also, we must insure gcc did not put
> > > > code between these instructions. Modifying non-relocatable
> > > > instructions would also be a pain, since we would have to deal
> > > > with instruction pointer relocation in the breakpoint code when
> > > > the code modification is being done.
> > 
> > you also need to make sure no cpu is executing that code ever.. 
> > but you already deal with that right?
> > 
> 
> By "insure that every modification will leave the code in a "correct"
> state", I mean that at any given time before, during or after the code
> modification, if an NMI comes on any CPU and try to run the modified
> code, it should have a valid version of the code to execute. Does it
> make more sense ?

I understand your words. My concern is that I don't quite understand how you
guarantee that you'll not be executing the code you're modifying.
Just saying "it's consistent before and after" sounds nice but probably isn't
enough to be safe.



> Not only does the compare and jmp need to be consecutive, but the movb
> $0x0,%al also does. I *could* try to detect specific code inserted in
> between, but I really have to make sure I don't get burned by the
> compiler inserting a jmp there.

I wonder if just sticking in 2 barriers around your code make gcc stop moving stuff too much

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