[PATCH] tracing: Do not disable tracing when reading the trace file

[Steven Rostedt (VMware)]

When opening the "trace" file, it is no longer necessary to disable tracing.

Note, a new option is created called "pause-on-trace", when set, will cause
the trace file to emulate its original behavior.

Link: http://lkml.kernel.org/r/20200317213416.903351225@xxxxxxxxxxx

Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 Documentation/trace/ftrace.rst | 6 ++++++
 kernel/trace/trace.c           | 9 ++++++---
 kernel/trace/trace.h           | 1 +
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 99a0890e20ec..c33950a35d65 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -1125,6 +1125,12 @@ Here are the available options:
 	the trace displays additional information about the
 	latency, as described in "Latency trace format".
 
+  pause-on-trace
+	When set, opening the trace file for read, will pause
+	writing to the ring buffer (as if tracing_on was set to zero).
+	This simulates the original behavior of the trace file.
+	When the file is closed, tracing will be enabled again.
+
   record-cmd
 	When any event or tracer is enabled, a hook is enabled
 	in the sched_switch trace point to fill comm cache
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 47889123be7f..650fa81fffe8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4273,8 +4273,11 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
 	if (trace_clocks[tr->clock_id].in_ns)
 		iter->iter_flags |= TRACE_FILE_TIME_IN_NS;
 
-	/* stop the trace while dumping if we are not opening "snapshot" */
-	if (!iter->snapshot)
+	/*
+	 * If pause-on-trace is enabled, then stop the trace while
+	 * dumping, unless this is the "snapshot" file
+	 */
+	if (!iter->snapshot && (tr->trace_flags & TRACE_ITER_PAUSE_ON_TRACE))
 		tracing_stop_tr(tr);
 
 	if (iter->cpu_file == RING_BUFFER_ALL_CPUS) {
@@ -4371,7 +4374,7 @@ static int tracing_release(struct inode *inode, struct file *file)
 	if (iter->trace && iter->trace->close)
 		iter->trace->close(iter);
 
-	if (!iter->snapshot)
+	if (!iter->snapshot && tr->stop_count)
 		/* reenable tracing if it was previously enabled */
 		tracing_start_tr(tr);
 
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index c61e1b1c85a6..f37e05135986 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1302,6 +1302,7 @@ extern int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
 		C(IRQ_INFO,		"irq-info"),		\
 		C(MARKERS,		"markers"),		\
 		C(EVENT_FORK,		"event-fork"),		\
+		C(PAUSE_ON_TRACE,	"pause-on-trace"),	\
 		FUNCTION_FLAGS					\
 		FGRAPH_FLAGS					\
 		STACK_FLAGS					\
-- 
2.20.1