Refactor rtree to be lock-free.

Recent huge allocation refactoring associates huge allocations with
arenas, but it remains necessary to quickly look up huge allocation
metadata during reallocation/deallocation.  A global radix tree remains
a good solution to this problem, but locking would have become the
primary bottleneck after (upcoming) migration of chunk management from
global to per arena data structures.

This lock-free implementation uses double-checked reads to traverse the
tree, so that in the steady state, each read or write requires only a
single atomic operation.

This implementation also assures that no more than two tree levels
actually exist, through a combination of careful virtual memory
allocation which makes large sparse nodes cheap, and skipping the root
node on x64 (possible because the top 16 bits are all 0 in practice).

include/jemalloc/internal/chunk.h

@@ -35,7 +35,7 @@ extern malloc_mutex_t	chunks_mtx;
 /* Chunk statistics. */
 extern chunk_stats_t	stats_chunks;
 
-extern rtree_t		*chunks_rtree;
+extern rtree_t		chunks_rtree;
 
 extern size_t		chunksize;
 extern size_t		chunksize_mask; /* (chunksize - 1). */

include/jemalloc/internal/jemalloc_internal.h.in

@@ -955,7 +955,7 @@ ivsalloc(const void *ptr, bool demote)
 {
 
 	/* Return 0 if ptr is not within a chunk managed by jemalloc. */
-	if (rtree_get(chunks_rtree, (uintptr_t)CHUNK_ADDR2BASE(ptr)) == 0)
+	if (rtree_get(&chunks_rtree, (uintptr_t)CHUNK_ADDR2BASE(ptr)) == 0)
 		return (0);
 
 	return (isalloc(ptr, demote));

include/jemalloc/internal/private_symbols.txt

@@ -369,14 +369,21 @@ quarantine_alloc_hook
 quarantine_alloc_hook_work
 quarantine_cleanup
 register_zone
+rtree_child_read
+rtree_child_read_hard
+rtree_child_tryread
 rtree_delete
 rtree_get
-rtree_get_locked
 rtree_new
-rtree_postfork_child
-rtree_postfork_parent
-rtree_prefork
+rtree_node_valid
 rtree_set
+rtree_start_level
+rtree_subkey
+rtree_subtree_read
+rtree_subtree_read_hard
+rtree_subtree_tryread
+rtree_val_read
+rtree_val_write
 s2u
 s2u_compute
 s2u_lookup

include/jemalloc/internal/rtree.h

@@ -1,170 +1,270 @@
 /*
  * This radix tree implementation is tailored to the singular purpose of
- * tracking which chunks are currently owned by jemalloc.  This functionality
- * is mandatory for OS X, where jemalloc must be able to respond to object
- * ownership queries.
+ * associating metadata with chunks that are currently owned by jemalloc.
  *
  *******************************************************************************
  */
 #ifdef JEMALLOC_H_TYPES
 
+typedef struct rtree_node_elm_s rtree_node_elm_t;
+typedef struct rtree_level_s rtree_level_t;
 typedef struct rtree_s rtree_t;
 
 /*
- * Size of each radix tree node (must be a power of 2).  This impacts tree
- * depth.
+ * RTREE_BITS_PER_LEVEL must be a power of two that is no larger than the
+ * machine address width.
  */
-#define	RTREE_NODESIZE (1U << 16)
+#define	LG_RTREE_BITS_PER_LEVEL	4
+#define	RTREE_BITS_PER_LEVEL	(ZU(1) << LG_RTREE_BITS_PER_LEVEL)
+#define	RTREE_HEIGHT_MAX						\
+    ((ZU(1) << (LG_SIZEOF_PTR+3)) / RTREE_BITS_PER_LEVEL)
 
-typedef void *(rtree_alloc_t)(size_t);
-typedef void (rtree_dalloc_t)(void *);
+/* Used for two-stage lock-free node initialization. */
+#define	RTREE_NODE_INITIALIZING	((rtree_node_elm_t *)0x1)
+
+/*
+ * The node allocation callback function's argument is the number of contiguous
+ * rtree_node_elm_t structures to allocate, and the resulting memory must be
+ * zeroed.
+ */
+typedef rtree_node_elm_t *(rtree_node_alloc_t)(size_t);
+typedef void (rtree_node_dalloc_t)(rtree_node_elm_t *);
 
 #endif /* JEMALLOC_H_TYPES */
 /******************************************************************************/
 #ifdef JEMALLOC_H_STRUCTS
 
+struct rtree_node_elm_s {
+	union {
+		rtree_node_elm_t	*child;
+		void			*val;
+	};
+};
+
+struct rtree_level_s {
+	/*
+	 * A non-NULL subtree points to a subtree rooted along the hypothetical
+	 * path to the leaf node corresponding to key 0.  Depending on what keys
+	 * have been used to store to the tree, an arbitrary combination of
+	 * subtree pointers may remain NULL.
+	 *
+	 * Suppose keys comprise 48 bits, and LG_RTREE_BITS_PER_LEVEL is 4.
+	 * This results in a 3-level tree, and the leftmost leaf can be directly
+	 * accessed via subtrees[2], the subtree prefixed by 0x0000 (excluding
+	 * 0x00000000) can be accessed via subtrees[1], and the remainder of the
+	 * tree can be accessed via subtrees[0].
+	 *
+	 *   levels[0] : [<unused> | 0x0001******** | 0x0002******** | ...]
+	 *
+	 *   levels[1] : [<unused> | 0x00000001**** | 0x00000002**** | ... ]
+	 *
+	 *   levels[2] : [val(0x000000000000) | val(0x000000000001) | ...]
+	 *
+	 * This has practical implications on x64, which currently uses only the
+	 * lower 47 bits of virtual address space in userland, thus leaving
+	 * subtrees[0] unused and avoiding a level of tree traversal.
+	 */
+	rtree_node_elm_t	*subtree;
+	/* Number of key bits distinguished by this level. */
+	unsigned		bits;
+	/*
+	 * Cumulative number of key bits distinguished by traversing to
+	 * corresponding tree level.
+	 */
+	unsigned		cumbits;
+};
+
 struct rtree_s {
-	rtree_alloc_t	*alloc;
-	rtree_dalloc_t	*dalloc;
-	malloc_mutex_t	mutex;
-	void		**root;
-	unsigned	height;
-	unsigned	level2bits[1]; /* Dynamically sized. */
+	rtree_node_alloc_t	*alloc;
+	rtree_node_dalloc_t	*dalloc;
+	unsigned		height;
+	/*
+	 * Precomputed table used to convert from the number of leading 0 key
+	 * bits to which subtree level to start at.
+	 */
+	unsigned		start_level[RTREE_HEIGHT_MAX];
+	rtree_level_t		levels[RTREE_HEIGHT_MAX];
 };
 
 #endif /* JEMALLOC_H_STRUCTS */
 /******************************************************************************/
 #ifdef JEMALLOC_H_EXTERNS
 
-rtree_t	*rtree_new(unsigned bits, rtree_alloc_t *alloc, rtree_dalloc_t *dalloc);
+bool rtree_new(rtree_t *rtree, unsigned bits, rtree_node_alloc_t *alloc,
+    rtree_node_dalloc_t *dalloc);
 void	rtree_delete(rtree_t *rtree);
-void	rtree_prefork(rtree_t *rtree);
-void	rtree_postfork_parent(rtree_t *rtree);
-void	rtree_postfork_child(rtree_t *rtree);
+rtree_node_elm_t	*rtree_subtree_read_hard(rtree_t *rtree,
+    unsigned level);
+rtree_node_elm_t	*rtree_child_read_hard(rtree_t *rtree,
+    rtree_node_elm_t *elm, unsigned level);
 
 #endif /* JEMALLOC_H_EXTERNS */
 /******************************************************************************/
 #ifdef JEMALLOC_H_INLINES
 
 #ifndef JEMALLOC_ENABLE_INLINE
-#ifdef JEMALLOC_DEBUG
-uint8_t rtree_get_locked(rtree_t *rtree, uintptr_t key);
-#endif
-uint8_t	rtree_get(rtree_t *rtree, uintptr_t key);
-bool	rtree_set(rtree_t *rtree, uintptr_t key, uint8_t val);
+unsigned	rtree_start_level(rtree_t *rtree, uintptr_t key);
+uintptr_t	rtree_subkey(rtree_t *rtree, uintptr_t key, unsigned level);
+
+bool	rtree_node_valid(rtree_node_elm_t *node);
+rtree_node_elm_t	*rtree_child_tryread(rtree_node_elm_t *elm);
+rtree_node_elm_t	*rtree_child_read(rtree_t *rtree, rtree_node_elm_t *elm,
+    unsigned level);
+void	*rtree_val_read(rtree_t *rtree, rtree_node_elm_t *elm);
+void	rtree_val_write(rtree_t *rtree, rtree_node_elm_t *elm, void *val);
+rtree_node_elm_t	*rtree_subtree_tryread(rtree_t *rtree, unsigned level);
+rtree_node_elm_t	*rtree_subtree_read(rtree_t *rtree, unsigned level);
+
+void	*rtree_get(rtree_t *rtree, uintptr_t key);
+bool	rtree_set(rtree_t *rtree, uintptr_t key, void *val);
 #endif
 
 #if (defined(JEMALLOC_ENABLE_INLINE) || defined(JEMALLOC_RTREE_C_))
-#define	RTREE_GET_GENERATE(f)						\
-/* The least significant bits of the key are ignored. */		\
-JEMALLOC_INLINE uint8_t							\
-f(rtree_t *rtree, uintptr_t key)					\
-{									\
-	uint8_t ret;							\
-	uintptr_t subkey;						\
-	unsigned i, lshift, height, bits;				\
-	void **node, **child;						\
-									\
-	RTREE_LOCK(&rtree->mutex);					\
-	for (i = lshift = 0, height = rtree->height, node = rtree->root;\
-	    i < height - 1;						\
-	    i++, lshift += bits, node = child) {			\
-		bits = rtree->level2bits[i];				\
-		subkey = (key << lshift) >> ((ZU(1) << (LG_SIZEOF_PTR +	\
-		    3)) - bits);					\
-		child = (void**)node[subkey];				\
-		if (child == NULL) {					\
-			RTREE_UNLOCK(&rtree->mutex);			\
-			return (0);					\
-		}							\
-	}								\
-									\
-	/*								\
-	 * node is a leaf, so it contains values rather than node	\
-	 * pointers.							\
-	 */								\
-	bits = rtree->level2bits[i];					\
-	subkey = (key << lshift) >> ((ZU(1) << (LG_SIZEOF_PTR+3)) -	\
-	    bits);							\
-	{								\
-		uint8_t *leaf = (uint8_t *)node;			\
-		ret = leaf[subkey];					\
-	}								\
-	RTREE_UNLOCK(&rtree->mutex);					\
-									\
-	RTREE_GET_VALIDATE						\
-	return (ret);							\
+JEMALLOC_INLINE unsigned
+rtree_start_level(rtree_t *rtree, uintptr_t key)
+{
+	unsigned start_level;
+
+	if (unlikely(key == 0))
+		return (rtree->height - 1);
+
+	start_level = rtree->start_level[lg_floor(key) >>
+	    LG_RTREE_BITS_PER_LEVEL];
+	assert(start_level < rtree->height);
+	return (start_level);
 }
 
-#ifdef JEMALLOC_DEBUG
-#  define RTREE_LOCK(l)		malloc_mutex_lock(l)
-#  define RTREE_UNLOCK(l)	malloc_mutex_unlock(l)
-#  define RTREE_GET_VALIDATE
-RTREE_GET_GENERATE(rtree_get_locked)
-#  undef RTREE_LOCK
-#  undef RTREE_UNLOCK
-#  undef RTREE_GET_VALIDATE
-#endif
+JEMALLOC_INLINE uintptr_t
+rtree_subkey(rtree_t *rtree, uintptr_t key, unsigned level)
+{
 
-#define	RTREE_LOCK(l)
-#define	RTREE_UNLOCK(l)
-#ifdef JEMALLOC_DEBUG
-   /*
-    * Suppose that it were possible for a jemalloc-allocated chunk to be
-    * munmap()ped, followed by a different allocator in another thread re-using
-    * overlapping virtual memory, all without invalidating the cached rtree
-    * value.  The result would be a false positive (the rtree would claim that
-    * jemalloc owns memory that it had actually discarded).  This scenario
-    * seems impossible, but the following assertion is a prudent sanity check.
-    */
-#  define RTREE_GET_VALIDATE						\
-	assert(rtree_get_locked(rtree, key) == ret);
-#else
-#  define RTREE_GET_VALIDATE
-#endif
-RTREE_GET_GENERATE(rtree_get)
-#undef RTREE_LOCK
-#undef RTREE_UNLOCK
-#undef RTREE_GET_VALIDATE
+	return ((key >> ((ZU(1) << (LG_SIZEOF_PTR+3)) -
+	    rtree->levels[level].cumbits)) & ((ZU(1) <<
+	    rtree->levels[level].bits) - 1));
+}
 
 JEMALLOC_INLINE bool
-rtree_set(rtree_t *rtree, uintptr_t key, uint8_t val)
+rtree_node_valid(rtree_node_elm_t *node)
+{
+
+	return ((uintptr_t)node > (uintptr_t)RTREE_NODE_INITIALIZING);
+}
+
+JEMALLOC_INLINE rtree_node_elm_t *
+rtree_child_tryread(rtree_node_elm_t *elm)
+{
+	rtree_node_elm_t *child;
+
+	/* Double-checked read (first read may be stale. */
+	child = elm->child;
+	if (!rtree_node_valid(child))
+		child = atomic_read_p((void **)&elm->child);
+	return (child);
+}
+
+JEMALLOC_INLINE rtree_node_elm_t *
+rtree_child_read(rtree_t *rtree, rtree_node_elm_t *elm, unsigned level)
+{
+	rtree_node_elm_t *child;
+
+	child = rtree_child_tryread(elm);
+	if (unlikely(!rtree_node_valid(child)))
+		child = rtree_child_read_hard(rtree, elm, level);
+	return (child);
+}
+
+JEMALLOC_INLINE void *
+rtree_val_read(rtree_t *rtree, rtree_node_elm_t *elm)
+{
+
+	return (atomic_read_p(&elm->val));
+}
+
+JEMALLOC_INLINE void
+rtree_val_write(rtree_t *rtree, rtree_node_elm_t *elm, void *val)
+{
+
+	atomic_write_p(&elm->val, val);
+}
+
+JEMALLOC_INLINE rtree_node_elm_t *
+rtree_subtree_tryread(rtree_t *rtree, unsigned level)
+{
+	rtree_node_elm_t *subtree;
+
+	/* Double-checked read (first read may be stale. */
+	subtree = rtree->levels[level].subtree;
+	if (!rtree_node_valid(subtree))
+		subtree = atomic_read_p((void **)&rtree->levels[level].subtree);
+	return (subtree);
+}
+
+JEMALLOC_INLINE rtree_node_elm_t *
+rtree_subtree_read(rtree_t *rtree, unsigned level)
+{
+	rtree_node_elm_t *subtree;
+
+	subtree = rtree_subtree_tryread(rtree, level);
+	if (unlikely(!rtree_node_valid(subtree)))
+		subtree = rtree_subtree_read_hard(rtree, level);
+	return (subtree);
+}
+
+JEMALLOC_INLINE void *
+rtree_get(rtree_t *rtree, uintptr_t key)
 {
 	uintptr_t subkey;
-	unsigned i, lshift, height, bits;
-	void **node, **child;
+	unsigned i, start_level;
+	rtree_node_elm_t *node, *child;
 
-	malloc_mutex_lock(&rtree->mutex);
-	for (i = lshift = 0, height = rtree->height, node = rtree->root;
-	    i < height - 1;
-	    i++, lshift += bits, node = child) {
-		bits = rtree->level2bits[i];
-		subkey = (key << lshift) >> ((ZU(1) << (LG_SIZEOF_PTR+3)) -
-		    bits);
-		child = (void**)node[subkey];
-		if (child == NULL) {
-			size_t size = ((i + 1 < height - 1) ? sizeof(void *)
-			    : (sizeof(uint8_t))) << rtree->level2bits[i+1];
-			child = (void**)rtree->alloc(size);
-			if (child == NULL) {
-				malloc_mutex_unlock(&rtree->mutex);
-				return (true);
-			}
-			memset(child, 0, size);
-			node[subkey] = child;
+	start_level = rtree_start_level(rtree, key);
+
+	for (i = start_level, node = rtree_subtree_tryread(rtree, start_level);
+	    /**/; i++, node = child) {
+		if (unlikely(!rtree_node_valid(node)))
+			return (NULL);
+		subkey = rtree_subkey(rtree, key, i);
+		if (i == rtree->height - 1) {
+			/*
+			 * node is a leaf, so it contains values rather than
+			 * child pointers.
+			 */
+			return (rtree_val_read(rtree, &node[subkey]));
 		}
+		assert(i < rtree->height - 1);
+		child = rtree_child_tryread(&node[subkey]);
 	}
+	not_reached();
+}
 
-	/* node is a leaf, so it contains values rather than node pointers. */
-	bits = rtree->level2bits[i];
-	subkey = (key << lshift) >> ((ZU(1) << (LG_SIZEOF_PTR+3)) - bits);
-	{
-		uint8_t *leaf = (uint8_t *)node;
-		leaf[subkey] = val;
+JEMALLOC_INLINE bool
+rtree_set(rtree_t *rtree, uintptr_t key, void *val)
+{
+	uintptr_t subkey;
+	unsigned i, start_level;
+	rtree_node_elm_t *node, *child;
+
+	start_level = rtree_start_level(rtree, key);
+
+	node = rtree_subtree_read(rtree, start_level);
+	if (node == NULL)
+		return (true);
+	for (i = start_level; /**/; i++, node = child) {
+		subkey = rtree_subkey(rtree, key, i);
+		if (i == rtree->height - 1) {
+			/*
+			 * node is a leaf, so it contains values rather than
+			 * child pointers.
+			 */
+			rtree_val_write(rtree, &node[subkey], val);
+			return (false);
+		}
+		assert(i < rtree->height - 1);
+		child = rtree_child_read(rtree, &node[subkey], i);
+		if (child == NULL)
+			return (true);
 	}
-	malloc_mutex_unlock(&rtree->mutex);
-
-	return (false);
+	not_reached();
 }
 #endif