Allow mutexes to take a lock ordering enum at construction.

This lets us specify whether and how mutexes of the same rank are allowed to be
acquired.  Currently, we only allow two polices (only a single mutex at a given
rank at a time, and mutexes acquired in ascending order), but we can plausibly
allow more (e.g. the "release uncontended mutexes before blocking").

include/jemalloc/internal/mutex_externs.h

@@ -11,7 +11,7 @@ extern bool isthreaded;
 #endif
 
 bool	malloc_mutex_init(malloc_mutex_t *mutex, const char *name,
-    witness_rank_t rank);
+    witness_rank_t rank, malloc_mutex_lock_order_t lock_order);
 void	malloc_mutex_prefork(tsdn_t *tsdn, malloc_mutex_t *mutex);
 void	malloc_mutex_postfork_parent(tsdn_t *tsdn, malloc_mutex_t *mutex);
 void	malloc_mutex_postfork_child(tsdn_t *tsdn, malloc_mutex_t *mutex);

include/jemalloc/internal/mutex_structs.h

@@ -40,12 +40,14 @@ struct malloc_mutex_s {
 		 * memory cost.
 		 */
 #if !defined(JEMALLOC_DEBUG)
-		witness_t		witness;
+		witness_t			witness;
+		malloc_mutex_lock_order_t	lock_order;
 #endif
 	};
 
 #if defined(JEMALLOC_DEBUG)
-	witness_t		witness;
+	witness_t			witness;
+	malloc_mutex_lock_order_t	lock_order;
 #endif
 };
 

include/jemalloc/internal/mutex_types.h

@@ -3,6 +3,16 @@
 
 typedef struct malloc_mutex_s malloc_mutex_t;
 
+typedef enum {
+	/* Can only acquire one mutex of a given witness rank at a time. */
+	malloc_mutex_rank_exclusive,
+	/*
+	 * Can acquire multiple mutexes of the same witness rank, but in
+	 * address-ascending order only.
+	 */
+	malloc_mutex_address_ordered
+} malloc_mutex_lock_order_t;
+
 /*
  * Based on benchmark results, a fixed spin with this amount of retries works
  * well for our critical sections.