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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Arjan van de Ven (arjan@infradead.org) wrote:
> On Wed, 16 Apr 2008 10:00:09 -0400
> Mathieu Desnoyers <mathieu.desnoyers@polymtl.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.
> 
Ah, I see. I insert a breakpoint and execute a bypass rather than the
code being modified. I only put back the 1st instruction byte after the
rest of the instruction has been modified.

> 
> 
> > 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
> 

I'm not sure people would like the side-effect for the rest of
optimizations, but it should be tried.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68