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

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Date:   Sat, 12 Aug 2023 09:45:52 +0800
From:   Zheng Yejian <zhengyejian1@...wei.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     <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 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.

> 
> -- Steve
>