linux-kernel - Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump patching without stop

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Date:	Mon, 18 Jan 2010 13:33:09 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, rostedt@...dmis.org,
	Jason Baron <jbaron@...hat.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, tglx@...utronix.de, andi@...stfloor.org,
	roland@...hat.com, rth@...hat.com
Subject: Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump
	patching without stop_machine

* Masami Hiramatsu (mhiramat@...hat.com) wrote:
> Mathieu Desnoyers wrote:
> > * Arjan van de Ven (arjan@...radead.org) wrote:
> >> On Mon, 18 Jan 2010 10:59:30 -0500
> >> Masami Hiramatsu <mhiramat@...hat.com> wrote:
> >>
> >>> Yeah, so in the latest patch, I updated it to use int3 even if
> >>> len == 1. :-)
> >>>
> >>
> >>
> >> int3 is not making a difference for your case; there is no guarantee
> >> that the other processor even sees the "int3 inbetween state" at all;
> >> if it's not safe without int3 then it won't be safe with int3 either.
> > 
> > What Masami means is that he updated his patch to use the int3+IPI
> > broadcast scheme.
> 
> Right.
> 
> > 
> > Therefore, the CPUs not seeing the int3 inbetween state will be forced
> > to issue a serializing instruction while the int3 is in place anyway.
> 
> By the way, in kprobes, we just use a text_poke() to put int3.
> I assume that we'd better send IPI afterward, wouldn't it?

Only if you need to ensure that you reached a state where all CPUs are
seeing the int3.

Note that kprobes already issues synchronize_sched() after breakpoint
removal, which should have a similar effect. However, AFAIK, it does not
synchronize after inserting the int3.

Thanks,

Mathieu

> 
> Thank you,
> 
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America), Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@...hat.com
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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