[PATCH] tracing: Reset last-boot buffers when reading out all cpu buffers

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Reset the last-boot ring buffers when read() reads out all cpu
buffers through trace_pipe/trace_pipe_raw. This prevents ftrace to
unwind ring buffer read pointer next boot.

Note that this resets only when all per-cpu buffers are empty, and
read via read(2) syscall. For example, if you read only one of the
per-cpu trace_pipe, it does not reset it. Also, reading buffer by
splice(2) syscall, it does not reset because there will be the
data in reader page.

Suggested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 kernel/trace/trace.c |   22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 2c1764ed87b0..433671d3aa43 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6677,6 +6677,22 @@ static int tracing_wait_pipe(struct file *filp)
 	return 1;
 }
 
+static bool update_last_data_if_empty(struct trace_array *tr)
+{
+	if (!(tr->flags & TRACE_ARRAY_FL_LAST_BOOT))
+		return false;
+
+	if (!ring_buffer_empty(tr->array_buffer.buffer))
+		return false;
+
+	/*
+	 * If the buffer contains the last boot data and all per-cpu
+	 * buffers are empty, reset it from the kernel side.
+	 */
+	update_last_data(tr);
+	return true;
+}
+
 /*
  * Consumer reader.
  */
@@ -6708,6 +6724,9 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
 	}
 
 waitagain:
+	if (update_last_data_if_empty(iter->tr))
+		return 0;
+
 	sret = tracing_wait_pipe(filp);
 	if (sret <= 0)
 		return sret;
@@ -8286,6 +8305,9 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
 
 	if (ret < 0) {
 		if (trace_empty(iter) && !iter->closed) {
+			if (update_last_data_if_empty(iter->tr))
+				return 0;
+
 			if ((filp->f_flags & O_NONBLOCK))
 				return -EAGAIN;