linux-kernel - Re: [PATCH] tracing: Fix trace_pipe

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Date:   Mon, 24 Jul 2017 16:24:10 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Chunyu Hu <chuhu@...hat.com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Fix trace_pipe_raw read panic


Hi Chunyu,

Thanks for the patch. I'm currently traveling, and will have little
time to test it. Hopefully I can get to it sometime this week.

-- Steve


On Mon, 24 Jul 2017 15:21:06 +0800
Chunyu Hu <chuhu@...hat.com> wrote:

> per_cpu trace directories and files are created for all possible cpus,
> but only the cpus which have ever been on-lined have their own per cpu
> ring buffer (allocated by cpuhp threads). While trace_buffers_open, the
> open handler for trace file 'trace_pipe_raw' is always trying to access
> field of ring_buffer_per_cpu, and would panic with the NULL pointer.
> 
> Align the behavior of trace_pipe_raw with trace_pipe, that returns -NODEV
> when openning it if that cpu does not have trace ring buffer.
> 
> Reproduce:
> cat /sys/kernel/debug/tracing/per_cpu/cpu31/trace_pipe_raw
> (cpu31 is never on-lined, this is a 16 cores x86_64 box)
> 
> Tested with:
> 1) boot with maxcpus=14, read trace_pipe_raw of cpu15.
>    Got -NODEV.
> 2) oneline cpu15, read trace_pipe_raw of cpu15.
>    Get the raw trace data.