tracing: Add ref count to tracer for when they are being read by pipe

When one of the trace pipe files are being read (by either the trace_pipe
or trace_pipe_raw), do not allow the current_trace to change. By adding
a ref count that is incremented when the pipe files are opened, will
prevent the current_trace from being changed.

This will allow for the removal of the global trace_types_lock from
reading the pipe buffers (which is currently a bottle neck).
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -4140,6 +4140,12 @@ static int tracing_set_tracer(struct trace_array *tr, const char *buf)
 		goto out;
 	}
 
+	/* If trace pipe files are being read, we can't change the tracer */
+	if (tr->current_trace->ref) {
+		ret = -EBUSY;
+		goto out;
+	}
+
 	trace_branch_disable();
 
 	tr->current_trace->enabled--;
@@ -4363,6 +4369,8 @@ static int tracing_open_pipe(struct inode *inode, struct file *filp)
 		iter->trace->pipe_open(iter);
 
 	nonseekable_open(inode, filp);
+
+	tr->current_trace->ref++;
 out:
 	mutex_unlock(&trace_types_lock);
 	return ret;
@@ -4382,6 +4390,8 @@ static int tracing_release_pipe(struct inode *inode, struct file *file)
 
 	mutex_lock(&trace_types_lock);
 
+	tr->current_trace->ref--;
+
 	if (iter->trace->pipe_close)
 		iter->trace->pipe_close(iter);
 
@@ -5331,6 +5341,8 @@ static int tracing_buffers_open(struct inode *inode, struct file *filp)
 
 	filp->private_data = info;
 
+	tr->current_trace->ref++;
+
 	mutex_unlock(&trace_types_lock);
 
 	ret = nonseekable_open(inode, filp);
@@ -5437,6 +5449,8 @@ static int tracing_buffers_release(struct inode *inode, struct file *file)
 
 	mutex_lock(&trace_types_lock);
 
+	iter->tr->current_trace->ref--;
+
 	__trace_array_put(iter->tr);
 
 	if (info->spare)
@@ -6416,7 +6430,7 @@ static int instance_delete(const char *name)
 		goto out_unlock;
 
 	ret = -EBUSY;
-	if (tr->ref)
+	if (tr->ref || (tr->current_trace && tr->current_trace->ref))
 		goto out_unlock;
 
 	list_del(&tr->list);

kernel/trace/trace.h

@@ -388,6 +388,7 @@ struct tracer {
 	struct tracer		*next;
 	struct tracer_flags	*flags;
 	int			enabled;
+	int			ref;
 	bool			print_max;
 	bool			allow_instances;
 #ifdef CONFIG_TRACER_MAX_TRACE