ftrace: user run time file reading

This patch creates a file called trace_pipe in the tracing
debug directory. This file is a consumer of the trace buffers.
This means that reads of this file consumes the entries from
the trace buffers so that they will not be read a second time,
as contrast to the static buffers latency_trace and trace.

Reading from the trace_pipe will remove the entries from trace
and latency_trace too.

The advantage that trace_pipe has is that it can record live
traces. It will block when there is nothing in the buffer,
and read the entries as they are entered.  An EOF happens when
tracing is disabled (tracing_enabled = 0).
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/trace/trace.h

@@ -55,6 +55,7 @@ struct trace_entry {
 struct trace_array_cpu {
 	struct list_head	trace_pages;
 	atomic_t		disabled;
+	spinlock_t		lock;
 	cycle_t			time_offset;
 
 	/* these fields get copied into max-trace: */
@@ -88,6 +89,7 @@ struct trace_array {
 	long			ctrl;
 	int			cpu;
 	cycle_t			time_start;
+	struct task_struct	*waiter;
 	struct trace_array_cpu	*data[NR_CPUS];
 };