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

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Message-ID: <20080416142951.GF24383@elte.hu>
Date:	Wed, 16 Apr 2008 16:29:51 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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

* Arjan van de Ven <arjan@...radead.org> wrote:

> > 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

hm, an extra optimization barrier might have worse effects than even an 
extra instruction. I think if the detection and patching can be made 
100% safe, we dont care about the remaining 4% of markers that gcc 
somehow reorders.

and in parallel gcc folks might want to start helping us achieve 
single-instruction branch points? Currently there's no way to get flags 
values out of inline assembly, except via a register intermediary which 
adds another instruction. For flags that are unaffected by gcc's 
input/output constraint generation code it would make sense to allow 
them to be exported out of inline assembly.

	Ingo
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