linux-kernel - Re: [PATCH] tracing: Fix race when concurrently splice_read trace

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Message-Id: <20230813054702.22ce16d9191a1f6b84942a1e@kernel.org>
Date:   Sun, 13 Aug 2023 05:47:02 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Zheng Yejian <zhengyejian1@...wei.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, <mhiramat@...nel.org>,
        <laijs@...fujitsu.com>, <linux-kernel@...r.kernel.org>,
        <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Fix race when concurrently splice_read
 trace_pipe

On Sat, 12 Aug 2023 09:45:52 +0800
Zheng Yejian <zhengyejian1@...wei.com> wrote:

> On 2023/8/12 03:25, Steven Rostedt wrote:
> > On Thu, 10 Aug 2023 20:39:05 +0800
> > Zheng Yejian <zhengyejian1@...wei.com> wrote:
> > 
> >> When concurrently splice_read file trace_pipe and per_cpu/cpu*/trace_pipe,
> >> there are more data being read out than expected.
> 
> Sorry, I didn't make clear here. It not just read more but also lost
> some data. My case is that, for example:
>    1) Inject 3 events into ring_buffer: event1, event2, event3;
>    2) Concurrently splice_read through trace_pipes;
>    3) Then actually read out: event1, event3, event3. No event2, but 2 
> event3.
> 
> > 
> > Honestly the real fix is to prevent that use case. We should probably have
> > access to trace_pipe lock all the per_cpu trace_pipes too.
> 
> Yes, we could do that, but would it seem not that effective?
> because per_cpu trace_pipe only read its own ring_buffer and not race
> with ring_buffers in other cpus.

I think Steve said that only one of below is usable.

- Read trace_pipe

or

- Read per_cpu/cpu*/trace_pipe concurrently

And I think this makes sence, especially if you use splice (this *moves*
the page from the ring_buffer to other pipe).

Thank you,


> 
> > 
> > -- Steve
> > 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>