Merge tag 'trace-v4.11-rc5-5' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

Pull two more ftrace fixes from Steven Rostedt:
 "While continuing my development, I uncovered two more small bugs.

  One is a race condition when enabling the snapshot function probe
  trigger. It enables the probe before allocating the snapshot, and if
  the probe triggers first, it stops tracing with a warning that the
  snapshot buffer was not allocated.

  The seconds is that the snapshot file should show how to use it when
  it is empty. But a bug fix from long ago broke the "is empty" test and
  the snapshot file no longer displays the help message"

* tag 'trace-v4.11-rc5-5' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
  ring-buffer: Have ring_buffer_iter_empty() return true when empty
  tracing: Allocate the snapshot buffer before enabling probe

Author: torvalds

kernel/trace/ring_buffer.c

@@ -3405,11 +3405,23 @@ EXPORT_SYMBOL_GPL(ring_buffer_iter_reset);
 int ring_buffer_iter_empty(struct ring_buffer_iter *iter)
 {
 	struct ring_buffer_per_cpu *cpu_buffer;
+	struct buffer_page *reader;
+	struct buffer_page *head_page;
+	struct buffer_page *commit_page;
+	unsigned commit;
 
 	cpu_buffer = iter->cpu_buffer;
 
-	return iter->head_page == cpu_buffer->commit_page &&
-		iter->head == rb_commit_index(cpu_buffer);
+	/* Remember, trace recording is off when iterator is in use */
+	reader = cpu_buffer->reader_page;
+	head_page = cpu_buffer->head_page;
+	commit_page = cpu_buffer->commit_page;
+	commit = rb_page_commit(commit_page);
+
+	return ((iter->head_page == commit_page && iter->head == commit) ||
+		(iter->head_page == reader && commit_page == head_page &&
+		 head_page->read == commit &&
+		 iter->head == rb_page_commit(cpu_buffer->reader_page)));
 }
 EXPORT_SYMBOL_GPL(ring_buffer_iter_empty);
 

kernel/trace/trace.c

@@ -6733,11 +6733,13 @@ ftrace_trace_snapshot_callback(struct ftrace_hash *hash,
 		return ret;
 
  out_reg:
-	ret = register_ftrace_function_probe(glob, ops, count);
+	ret = alloc_snapshot(&global_trace);
+	if (ret < 0)
+		goto out;
 
-	if (ret >= 0)
-		alloc_snapshot(&global_trace);
+	ret = register_ftrace_function_probe(glob, ops, count);
 
+ out:
 	return ret < 0 ? ret : 0;
 }