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

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Message-ID: <20131101152232.GA2703@redhat.com>
Date:	Fri, 1 Nov 2013 16:22:32 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hyeoncheol Lee <cheol.lee@....com>,
	Hemant Kumar <hkshaw@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH 10/13] tracing/uprobes: Fetch args before reserving a
	ring buffer

On 11/01, Oleg Nesterov wrote:
>
> Hi Namhyung,
>
> Sorry if this was already discussed. But I can't really understand
> the idea of this per-cpu buffer...
>
> On 10/29, Namhyung Kim wrote:
> >
> > Fetching from user space should be done in a non-atomic context.  So
> > use a per-cpu buffer and copy its content to the ring buffer
> > atomically.  Note that we can migrate during accessing user memory
> > thus use a per-cpu mutex to protect concurrent accesses.
>
> And if the task migrates or just sleeps in page fault, another task
> which hits another uprobe on the same CPU should wait.
>
> Why we can't simply add trace_uprobe->buffer instead? Only to save
> some memory? But every uprobe is very expensive in this sense anyway.

Ah, please ignore... handler_chain() is not self-serialized, so
tu->buffer needs locking/waiting too.

Oleg.

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