linux-kernel - Re: [PATCH] tracing: consolidate protection of reader access to the ring buffer

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Date:	Wed, 06 Jan 2010 10:28:07 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: consolidate protection of reader access to
 the ring buffer

On Wed, 2010-01-06 at 20:08 +0800, Lai Jiangshan wrote:
> At the beginning, access to the ring buffer are fully serialized
> by trace_types_lock. Patch d7350c3f4569 gives more freedom to readers,
> and patch b04cc6b1f6 adds code to protect trace_pipe and cpu#/trace_pipe.
> 
> But actually it is not enough, ring buffer readers are not always
> read-only, they may consume data.
> 
> This patch makes accesses to trace, trace_pipe, trace_pipe_raw
> cpu#/trace, cpu#/trace_pipe and cpu#/trace_pipe_raw are serialized.
> And removes tracing_reader_cpumask which is used to protect trace_pipe.
> 
> detail:
> 
> ring buffer serializes readers, but it is low level protection.
> The validity of the events (which returns by ring_buffer_peek() ..etc)
> are not protected by ring buffer.
> 
> The content of events may become garbage if we allow other process consumes
> these events concurrently:
>   A) the page of the consumed events may become a normal page
>      (not reader page) in ring buffer, and this page will be rewrited
>      by events producer.
>   B) The page of the consumed events may become a page for splice_read,
>      and this page will be returned to system.
> 
> This patch adds trace_access_lock() and trace_access_unlock() primitives.
> 
> These primitives allow multi process access to different cpu ring buffer
> concurrently.
> 
> These primitives don't distinguish read-only and read-consume access.
> Multi read-only access are also serialized.
> 
> And we don't use these primitives when we open files,
> we only use them when we read files.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>


I queue this up with your other pending patches for .34

Thanks!

-- Steve


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