[PATCH 2/3] [BUGFIX] ring_buffer: tracing: Inherit the tracing setting to next ring buffer

[Masami Hiramatsu <mhiramat@xxxxxxxxxx>]

Inherit the tracing on/off setting on ring_buffer to next
trace buffer when taking a snapshot.

Taking a snapshot is done by swapping with backup ring buffer
(max_tr_buffer). But since the tracing on/off setting is set
in the ring buffer, when swapping it, 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 # 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 must be a bug, because user never know
that each "ring_buffer" stores tracing-enable state and
snapshot is done by swapping ring buffers.

This patch fixes above strange behavior.

Fixes: commit debdd57f5145 ("tracing: Make a snapshot feature available from userspace")
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Hiraku Toyooka <hiraku.toyooka@xxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 include/linux/ring_buffer.h |    1 +
 kernel/trace/ring_buffer.c  |   12 ++++++++++++
 kernel/trace/trace.c        |    6 ++++++
 3 files changed, 19 insertions(+)

diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
index b72ebdff0b77..003d09ab308d 100644
--- a/include/linux/ring_buffer.h
+++ b/include/linux/ring_buffer.h
@@ -165,6 +165,7 @@ void ring_buffer_record_enable(struct ring_buffer *buffer);
 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);
 
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 6a46af21765c..4038ed74ab95 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -3227,6 +3227,18 @@ int ring_buffer_record_is_on(struct ring_buffer *buffer)
 }
 
 /**
+ * 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.
+ */
+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
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 2556d8c097d2..bbd5a94a7ef1 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1378,6 +1378,12 @@ update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
 
 	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);
+
 	swap(tr->trace_buffer.buffer, tr->max_buffer.buffer);
 
 	__update_max_tr(tr, tsk, cpu);