Cycles: Fix possible race condition when generating Beckmann table

Two issues here:

- Checking table size to be non-zero is not a proper way to go here. This is
  because we first resize the table and then fill it in. So it was possible that
  non-initialized table was used.

  Trickery with using temporary memory and then doing table.swap() might work,
  but we can not guarantee that table size will be set after the data pointer.

- Mutex guard was useless, because every thread was using own mutex. Need to
  make mutex guard static so all threads are using same mutex.

1 files changed, 2 insertions, 1 deletions

diff --git a/intern/cycles/render/shader.h b/intern/cycles/render/shader.h
index 79a67d6756a..3fdcd3c0c5b 100644
--- a/intern/cycles/render/shader.h
+++ b/intern/cycles/render/shader.h
@@ -200,8 +200,9 @@ protected:
 	typedef unordered_map<ustring, uint, ustringHash> AttributeIDMap;
 	AttributeIDMap unique_attribute_id;
 
-	thread_mutex lookup_table_mutex;
+	static thread_mutex lookup_table_mutex;
 	static vector<float> beckmann_table;
+	static bool beckmann_table_ready;
 
 	size_t beckmann_table_offset;