tracing: Move pipe reference to trace array instead of current_tracer

If a process has the trace_pipe open on a trace_array, the current tracer
for that trace array should not be changed. This was original enforced by a
global lock, but when instances were introduced, it was moved to the
current_trace. But this structure is shared by all instances, and a
trace_pipe is for a single instance. There's no reason that a process that
has trace_pipe open on one instance should prevent another instance from
changing its current tracer. Move the reference counter to the trace_array
instead.

This is marked as "Fixes" but is more of a clean up than a true fix.
Backport if you want, but its not critical.

Fixes: cf6ab6d9 ("tracing: Add ref count to tracer for when they are being read by pipe")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -5891,7 +5891,7 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
 	}
 
 	/* If trace pipe files are being read, we can't change the tracer */
-	if (tr->current_trace->ref) {
+	if (tr->trace_ref) {
 		ret = -EBUSY;
 		goto out;
 	}
@@ -6107,7 +6107,7 @@ static int tracing_open_pipe(struct inode *inode, struct file *filp)
 
 	nonseekable_open(inode, filp);
 
-	tr->current_trace->ref++;
+	tr->trace_ref++;
 out:
 	mutex_unlock(&trace_types_lock);
 	return ret;
@@ -6126,7 +6126,7 @@ static int tracing_release_pipe(struct inode *inode, struct file *file)
 
 	mutex_lock(&trace_types_lock);
 
-	tr->current_trace->ref--;
+	tr->trace_ref--;
 
 	if (iter->trace->pipe_close)
 		iter->trace->pipe_close(iter);
@@ -7428,7 +7428,7 @@ static int tracing_buffers_open(struct inode *inode, struct file *filp)
 
 	filp->private_data = info;
 
-	tr->current_trace->ref++;
+	tr->trace_ref++;
 
 	mutex_unlock(&trace_types_lock);
 
@@ -7529,7 +7529,7 @@ static int tracing_buffers_release(struct inode *inode, struct file *file)
 
 	mutex_lock(&trace_types_lock);
 
-	iter->tr->current_trace->ref--;
+	iter->tr->trace_ref--;
 
 	__trace_array_put(iter->tr);
 
@@ -8737,7 +8737,7 @@ static int __remove_instance(struct trace_array *tr)
 	int i;
 
 	/* Reference counter for a newly created trace array = 1. */
-	if (tr->ref > 1 || (tr->current_trace && tr->current_trace->ref))
+	if (tr->ref > 1 || (tr->current_trace && tr->trace_ref))
 		return -EBUSY;
 
 	list_del(&tr->list);

kernel/trace/trace.h

@@ -356,6 +356,7 @@ struct trace_array {
 	struct trace_event_file *trace_marker_file;
 	cpumask_var_t		tracing_cpumask; /* only trace on set CPUs */
 	int			ref;
+	int			trace_ref;
 #ifdef CONFIG_FUNCTION_TRACER
 	struct ftrace_ops	*ops;
 	struct trace_pid_list	__rcu *function_pids;
@@ -547,7 +548,6 @@ struct tracer {
 	struct tracer		*next;
 	struct tracer_flags	*flags;
 	int			enabled;
-	int			ref;
 	bool			print_max;
 	bool			allow_instances;
 #ifdef CONFIG_TRACER_MAX_TRACE