[PATCH] perf_events: fix errors path in perf_output_begin()

[Stephane Eranian <eranian@google.com>]

  In case the sampling buffer has no "payload" pages, nr_pages is 0.
  The problem is that the error path in perf_output_begin() skips to
  a label which assumes perf_output_lock() has been issued which is
  not the case. That triggers a WARN_ON() is perf_output_unlock().

  This patch fixes the problem by adding a new label and skipping
  perf_task_unlock() in case data->nr_pages is 0. 

  Signed-off-by: Stephane Eranian <eranian@google.com>

--
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index a4fa381..95137b6 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -3035,8 +3035,10 @@ int perf_output_begin(struct perf_output_handle *handle,
 	handle->nmi	= nmi;
 	handle->sample	= sample;
 
-	if (!data->nr_pages)
-		goto fail;
+	if (!data->nr_pages) {
+		atomic_inc(&data->lost);
+		goto out;
+	}
 
 	have_lost = atomic_read(&data->lost);
 	if (have_lost)