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From: Chunyu Hu <chuhu.ncepu@gmail.com>
Date: Tue, 1 Aug 2017 21:02:12 +0800
Message-ID: <CABATaM6EY=Ea=uEC5=JQF7xXOJt9D6GvkCjfVu2GWuUyJUai+w@mail.gmail.com>
Subject: Re: [PATCH] tracing: Fix trace_pipe_raw read panic
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Chunyu Hu <chuhu@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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A gentle ping. Any comment on this? Maybe have better solution for this?

On 25 July 2017 at 04:24, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> 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@redhat.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.