tracing: Fix read blocking on trace_pipe_raw

If the ring buffer is empty, a read to trace_pipe_raw wont block.
The tracing code has the infrastructure to wake up waiting readers,
but the trace_pipe_raw doesn't take advantage of that.

When a read is done to trace_pipe_raw without the O_NONBLOCK flag
set, have the read block until there's data in the requested buffer.

Signed-off-by: Steven Rostedt <[e-mail]>

kernel/trace/trace.c

···
 
 	info->iter.tr		= tr;
 	info->iter.cpu_file	= tc->cpu;
+	info->iter.trace	= tr->current_trace;
 	info->spare		= NULL;
 	/* Force reading ring buffer for first read */
 	info->read		= (unsigned int)-1;
···
 	if (info->read < PAGE_SIZE)
 		goto read;
 
+ again:
 	trace_access_lock(iter->cpu_file);
 	ret = ring_buffer_read_page(iter->tr->buffer,
 				    &info->spare,
 				    count,
 				    iter->cpu_file, 0);
 	trace_access_unlock(iter->cpu_file);
-	if (ret < 0)
+
+	if (ret < 0) {
+		if (trace_empty(iter)) {
+			if ((filp->f_flags & O_NONBLOCK))
+				return -EAGAIN;
+			iter->trace->wait_pipe(iter);
+			if (signal_pending(current))
+				return -EINTR;
+			goto again;
+		}
 		return 0;
+	}
 
 	info->read = 0;
 
-read:
+ read:
 	size = PAGE_SIZE - info->read;
 	if (size > count)
 		size = count;
···
 			ret = -EAGAIN;
 			goto out;
 		}
-		default_wait_pipe(iter);
+		iter->trace->wait_pipe(iter);
 		if (signal_pending(current)) {
 			ret = -EINTR;
 			goto out;