[PATCH] docs: ftrace: clarify when tracing is disabled by the trace file

From: Peter Wu
Date: Thu Aug 22 2019 - 19:48:35 EST

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@xxxxxxxxxxxxxx
Signed-off-by: Peter Wu <peter@xxxxxxxxxxxxx>
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.


@@ -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.


@@ -3153,7 +3155,10 @@ different. The trace is live.

Note, reading the trace_pipe file will block until more input is
+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
+not have this limitation.

trace entries