[PATCH v2] docs: ftrace: clarify when tracing is disabled by the trace file

[Peter Wu]

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>
---
v2: fix typo s/trace_file/trace_pipe/ (spotted by Steven)
---
 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..e3060eedb22d 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_pipe file does
+not have this limitation.
 
 trace entries
 -------------
-- 
2.22.0