linux-kernel - Re: [PATCH 10/13] tracing/uprobes: Fetch args before reserving a ring buffer

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Date:	Thu, 22 Aug 2013 21:08:30 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Hyeoncheol Lee <cheol.lee@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH 10/13] tracing/uprobes: Fetch args before reserving a
 ring buffer

On Fri, 23 Aug 2013 07:57:15 +0800
"zhangwei(Jovi)" <jovi.zhangwei@...wei.com> wrote:


> > 
> > What about creating a per cpu buffer when uprobes are registered, and
> > delete them when they are finished? Basically what trace_printk() does
> > if it detects that there are users of trace_printk() in the kernel.
> > Note, it does not deallocate them when finished, as it is never
> > finished until reboot ;-)
> > 
> > -- Steve
> >
> I also thought out this approach, but the issue is we cannot fetch user
> memory into per-cpu buffer, because use per-cpu buffer should under
> preempt disabled, and fetching user memory could sleep.

Actually, we could create a per_cpu mutex to match the per_cpu buffers.
This is not unlike what we do in -rt.

	int cpu;
	struct mutex *mutex;
	void *buf;


	/*
	 * Use per cpu buffers for fastest access, but we might migrate
	 * So the mutex makes sure we have sole access to it.
	 */

	cpu = raw_smp_processor_id();
	mutex = per_cpu(uprobe_cpu_mutex, cpu);
	buf = per_cpu(uprobe_cpu_buffer, cpu);

	mutex_lock(mutex);
	store_trace_args(..., buf,...);
	mutex_unlock(mutex);

-- Steve
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