linux-kernel - Re: [RFC] [PATCH 7/7] Ftrace plugin for Uprobes

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Date:	Thu, 14 Jan 2010 12:43:01 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	utrace-devel <utrace-devel@...hat.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Mark Wielaard <mjw@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 7/7] Ftrace plugin for Uprobes

On Thu, 2010-01-14 at 12:35 +0100, Frederic Weisbecker wrote:
> On Thu, Jan 14, 2010 at 12:23:11PM +0100, Peter Zijlstra wrote:
> > On Mon, 2010-01-11 at 17:56 +0530, Srikar Dronamraju wrote:
> > > This patch implements ftrace plugin for uprobes.
> > 
> > Right, like others have said, trace events is a much saner interface.
> > 
> > So the easiest way I can see that working is to register uprobes against
> > a file (not a pid). Then on creation it uses rmap to find all current
> > maps of that file and install the probe if there is a consumer for that
> > map.
> > 
> > Then for each new mmap() of that file, we also need to check if there's
> > a consumer ready and install the probe.
> 
> 
> 
> That looks racy.
> 
> Say you first create a probe on /bin/ls:
> 
> perf probe p addr_in_ls /bin/ls
> 
> then something else launches /bin/ls behind you, probe
> is set on it
> 
> then you launch:
> 
> perf record -e "probe:...." /bin/ls
> 
> Then it goes recording the previous instance.

Uhm, why? Only the perf /bin/ls instance has a consumer and will thus
have a probe installed.

(Or if you want to use ftrace you need to always have all instances
probed anyway)

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