linux-kernel - Re: [PATCH 3/9] uprobes: allow ignoring of probe hits

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Date:	Sun, 21 Oct 2012 21:40:58 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rabin Vincent <rabin@....in>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/9] uprobes: allow ignoring of probe hits

On 10/21, Rabin Vincent wrote:
>
> On Wed, Oct 17, 2012 at 07:35:10PM +0200, Oleg Nesterov wrote:
> >
> > And, to clarify, I am not arguing. Just curious.
> >
> > So, is this like cmov on x86? And this patch allows to not report if
> > the condition is not true? Or there are other issues on arm?
>
> Yes, I guess this is like CMOV on x86.  In the ARM instruction set most
> instructions can be conditionally executed.
>
> In order to set the probe on a conditional instruction, we use an
> undefined instruction with the same condition as the instruction we
> replace.  However, it is implementation defined whether an undefined
> instruction with a failing condition code will trigger an undefined
> instruction exception or just be executed as a NOP.  So for those
> processor implementations where we do get the undefined instruction
> exception even for a failing condition code, we have to ignore it in
> order to provide consistent behaviour.

OK, I see, thanks for your explanation.

Oleg.

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