[PATCH 3.16 294/366] ring_buffer: tracing: Inherit the tracing setting to next ring buffer

[Ben Hutchings]

3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

commit 73c8d8945505acdcbae137c2e00a1232e0be709f upstream.

Maintain the tracing on/off setting of the ring_buffer when switching
to the trace buffer snapshot.

Taking a snapshot is done by swapping the backup ring buffer
(max_tr_buffer). But since the tracing on/off setting is defined
by the ring buffer, when swapping it, the tracing on/off setting
can also be changed. This causes a strange result like below:

  /sys/kernel/debug/tracing # cat tracing_on
  1
  /sys/kernel/debug/tracing # echo 0 > tracing_on
  /sys/kernel/debug/tracing # cat tracing_on
  0
  /sys/kernel/debug/tracing # echo 1 > snapshot
  /sys/kernel/debug/tracing # cat tracing_on
  1
  /sys/kernel/debug/tracing # echo 1 > snapshot
  /sys/kernel/debug/tracing # cat tracing_on
  0

We don't touch tracing_on, but snapshot changes tracing_on
setting each time. This is an anomaly, because user doesn't know
that each "ring_buffer" stores its own tracing-enable state and
the snapshot is done by swapping ring buffers.

Link: http://lkml.kernel.org/r/153149929558.11274.11730609978254724394.stgit@devbox

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
Cc: Hiraku Toyooka <hiraku.toyooka@xxxxxxxxxxxxxxxx>
Fixes: debdd57f5145 ("tracing: Make a snapshot feature available from userspace")
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
[ Updated commit log and comment in the code ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/linux/ring_buffer.h |  1 +
 kernel/trace/ring_buffer.c  | 16 ++++++++++++++++
 kernel/trace/trace.c        |  6 ++++++
 3 files changed, 23 insertions(+)

--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -162,6 +162,7 @@ void ring_buffer_record_enable(struct ri
 void ring_buffer_record_off(struct ring_buffer *buffer);
 void ring_buffer_record_on(struct ring_buffer *buffer);
 int ring_buffer_record_is_on(struct ring_buffer *buffer);
+int ring_buffer_record_is_set_on(struct ring_buffer *buffer);
 void ring_buffer_record_disable_cpu(struct ring_buffer *buffer, int cpu);
 void ring_buffer_record_enable_cpu(struct ring_buffer *buffer, int cpu);
 
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3165,6 +3165,22 @@ int ring_buffer_record_is_on(struct ring
 }
 
 /**
+ * ring_buffer_record_is_set_on - return true if the ring buffer is set writable
+ * @buffer: The ring buffer to see if write is set enabled
+ *
+ * Returns true if the ring buffer is set writable by ring_buffer_record_on().
+ * Note that this does NOT mean it is in a writable state.
+ *
+ * It may return true when the ring buffer has been disabled by
+ * ring_buffer_record_disable(), as that is a temporary disabling of
+ * the ring buffer.
+ */
+int ring_buffer_record_is_set_on(struct ring_buffer *buffer)
+{
+	return !(atomic_read(&buffer->record_disabled) & RB_BUFFER_OFF);
+}
+
+/**
  * ring_buffer_record_disable_cpu - stop all writes into the cpu_buffer
  * @buffer: The ring buffer to stop writes to.
  * @cpu: The CPU buffer to stop
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1046,6 +1046,12 @@ update_max_tr(struct trace_array *tr, st
 
 	arch_spin_lock(&tr->max_lock);
 
+	/* Inherit the recordable setting from trace_buffer */
+	if (ring_buffer_record_is_set_on(tr->trace_buffer.buffer))
+		ring_buffer_record_on(tr->max_buffer.buffer);
+	else
+		ring_buffer_record_off(tr->max_buffer.buffer);
+
 	buf = tr->trace_buffer.buffer;
 	tr->trace_buffer.buffer = tr->max_buffer.buffer;
 	tr->max_buffer.buffer = buf;