linux-kernel - Re: [PATCH] docs: ftrace: clarify when tracing is disabled by the trace file

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Date:   Fri, 30 Aug 2019 17:51:08 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Wu <peter@...ensteyn.nl>
Cc:     Ingo Molnar <mingo@...hat.com>, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH] docs: ftrace: clarify when tracing is disabled by the
 trace file

On Fri, 23 Aug 2019 00:48:23 +0100
Peter Wu <peter@...ensteyn.nl> wrote:

> The current text could mislead the user into believing that only read()
> disables tracing. Clarify that any open() call that requests read access
> disables tracing.
> 
> Link: https://lkml.kernel.org/r/CAADnVQ+hU6QOC_dPmpjnuv=9g4SQEeaMEMqXOS2WpMj=q=LdiQ@mail.gmail.com
> Signed-off-by: Peter Wu <peter@...ensteyn.nl>
> ---
>  Documentation/trace/ftrace.rst | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> index f60079259669..965be5c9afb3 100644
> --- a/Documentation/trace/ftrace.rst
> +++ b/Documentation/trace/ftrace.rst
> @@ -125,7 +125,8 @@ of ftrace. Here is a list of some of the key files:
>  
>  	This file holds the output of the trace in a human
>  	readable format (described below). Note, tracing is temporarily
> -	disabled while this file is being read (opened).
> +	disabled when the file is open for reading. Once all readers
> +	are closed, tracing is re-enabled.
>  
>    trace_pipe:
>  
> @@ -139,8 +140,9 @@ of ftrace. Here is a list of some of the key files:
>  	will not be read again with a sequential read. The
>  	"trace" file is static, and if the tracer is not
>  	adding more data, it will display the same
> -	information every time it is read. This file will not
> -	disable tracing while being read.
> +	information every time it is read. Unlike the
> +	"trace" file, opening this file for reading will not
> +	temporarily disable tracing.
>  
>    trace_options:
>  
> @@ -3153,7 +3155,10 @@ different. The trace is live.
>  
>  
>  Note, reading the trace_pipe file will block until more input is
> -added.
> +added. This is contrary to the trace file. If any process opened
> +the trace file for reading, it will actually disable tracing and
> +prevent new entries from being added. The trace_file file does

I was just about to ack this, and then I saw the above.

  s/trace_file/trace_pipe/

Other than that, it looks good!

-- Steve

> +not have this limitation.
>  
>  trace entries
>  -------------