slub: Invert locking and avoid slab lock

Locking slabs is no longer necesary if the arch supports cmpxchg operations
and if no debuggin features are used on a slab. If the arch does not support
cmpxchg then we fallback to use the slab lock to do a cmpxchg like operation.

The patch also changes the lock order. Slab locks are subsumed to the node lock
now. With that approach slab_trylocking is no longer necessary.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -2,10 +2,11 @@
  * SLUB: A slab allocator that limits cache line use instead of queuing
  * objects in per cpu and per node lists.
  *
- * The allocator synchronizes using per slab locks and only
+ * The allocator synchronizes using per slab locks or atomic operatios
- * uses a centralized lock to manage a pool of partial slabs.
+ * and only uses a centralized lock to manage a pool of partial slabs.
  *
  * (C) 2007 SGI, Christoph Lameter
+ * (C) 2011 Linux Foundation, Christoph Lameter
  */
 
 #include <linux/mm.h>
@@ -32,15 +33,27 @@
 
 /*
  * Lock order:
- *   1. slab_lock(page)
+ *   1. slub_lock (Global Semaphore)
- *   2. slab->list_lock
+ *   2. node->list_lock
+ *   3. slab_lock(page) (Only on some arches and for debugging)
  *
- *   The slab_lock protects operations on the object of a particular
+ *   slub_lock
- *   slab and its metadata in the page struct. If the slab lock
+ *
- *   has been taken then no allocations nor frees can be performed
+ *   The role of the slub_lock is to protect the list of all the slabs
- *   on the objects in the slab nor can the slab be added or removed
+ *   and to synchronize major metadata changes to slab cache structures.
- *   from the partial or full lists since this would mean modifying
+ *
- *   the page_struct of the slab.
+ *   The slab_lock is only used for debugging and on arches that do not
+ *   have the ability to do a cmpxchg_double. It only protects the second
+ *   double word in the page struct. Meaning
+ *	A. page->freelist	-> List of object free in a page
+ *	B. page->counters	-> Counters of objects
+ *	C. page->frozen		-> frozen state
+ *
+ *   If a slab is frozen then it is exempt from list management. It is not
+ *   on any list. The processor that froze the slab is the one who can
+ *   perform list operations on the page. Other processors may put objects
+ *   onto the freelist but the processor that froze the slab is the only
+ *   one that can retrieve the objects from the page's freelist.
  *
  *   The list_lock protects the partial and full list on each node and
  *   the partial slab counter. If taken then no new slabs may be added or
@@ -53,20 +66,6 @@
  *   slabs, operations can continue without any centralized lock. F.e.
  *   allocating a long series of objects that fill up slabs does not require
  *   the list lock.
- *
- *   The lock order is sometimes inverted when we are trying to get a slab
- *   off a list. We take the list_lock and then look for a page on the list
- *   to use. While we do that objects in the slabs may be freed. We can
- *   only operate on the slab if we have also taken the slab_lock. So we use
- *   a slab_trylock() on the slab. If trylock was successful then no frees
- *   can occur anymore and we can use the slab for allocations etc. If the
- *   slab_trylock() does not succeed then frees are in progress in the slab and
- *   we must stay away from it for a while since we may cause a bouncing
- *   cacheline if we try to acquire the lock. So go onto the next slab.
- *   If all pages are busy then we may allocate a new slab instead of reusing
- *   a partial slab. A new slab has no one operating on it and thus there is
- *   no danger of cacheline contention.
- *
  *   Interrupts are disabled during allocation and deallocation in order to
  *   make the slab allocator safe to use in the context of an irq. In addition
  *   interrupts are disabled to ensure that the processor does not change
@@ -342,6 +341,19 @@ static inline int oo_objects(struct kmem_cache_order_objects x)
 	return x.x & OO_MASK;
 }
 
+/*
+ * Per slab locking using the pagelock
+ */
+static __always_inline void slab_lock(struct page *page)
+{
+	bit_spin_lock(PG_locked, &page->flags);
+}
+
+static __always_inline void slab_unlock(struct page *page)
+{
+	__bit_spin_unlock(PG_locked, &page->flags);
+}
+
 static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
 		void *freelist_old, unsigned long counters_old,
 		void *freelist_new, unsigned long counters_new,
@@ -356,11 +368,14 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
 	} else
 #endif
 	{
+		slab_lock(page);
 		if (page->freelist == freelist_old && page->counters == counters_old) {
 			page->freelist = freelist_new;
 			page->counters = counters_new;
+			slab_unlock(page);
 			return 1;
 		}
+		slab_unlock(page);
 	}
 
 	cpu_relax();
@@ -377,7 +392,7 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
 /*
  * Determine a map of object in use on a page.
  *
- * Slab lock or node listlock must be held to guarantee that the page does
+ * Node listlock must be held to guarantee that the page does
  * not vanish from under us.
  */
 static void get_map(struct kmem_cache *s, struct page *page, unsigned long *map)
@@ -808,10 +823,11 @@ static int check_slab(struct kmem_cache *s, struct page *page)
 static int on_freelist(struct kmem_cache *s, struct page *page, void *search)
 {
 	int nr = 0;
-	void *fp = page->freelist;
+	void *fp;
 	void *object = NULL;
 	unsigned long max_objects;
 
+	fp = page->freelist;
 	while (fp && nr <= page->objects) {
 		if (fp == search)
 			return 1;
@@ -1024,6 +1040,8 @@ bad:
 static noinline int free_debug_processing(struct kmem_cache *s,
 		 struct page *page, void *object, unsigned long addr)
 {
+	slab_lock(page);
+
 	if (!check_slab(s, page))
 		goto fail;
 
@@ -1059,10 +1077,12 @@ static noinline int free_debug_processing(struct kmem_cache *s,
 		set_track(s, object, TRACK_FREE, addr);
 	trace(s, page, object, 0);
 	init_object(s, object, SLUB_RED_INACTIVE);
+	slab_unlock(page);
 	return 1;
 
 fail:
 	slab_fix(s, "Object at 0x%p not freed", object);
+	slab_unlock(page);
 	return 0;
 }
 
@@ -1393,27 +1413,6 @@ static void discard_slab(struct kmem_cache *s, struct page *page)
 	free_slab(s, page);
 }
 
-/*
- * Per slab locking using the pagelock
- */
-static __always_inline void slab_lock(struct page *page)
-{
-	bit_spin_lock(PG_locked, &page->flags);
-}
-
-static __always_inline void slab_unlock(struct page *page)
-{
-	__bit_spin_unlock(PG_locked, &page->flags);
-}
-
-static __always_inline int slab_trylock(struct page *page)
-{
-	int rc = 1;
-
-	rc = bit_spin_trylock(PG_locked, &page->flags);
-	return rc;
-}
-
 /*
  * Management of partially allocated slabs.
  *
@@ -1445,17 +1444,13 @@ static inline void remove_partial(struct kmem_cache_node *n,
  *
  * Must hold list_lock.
  */
-static inline int lock_and_freeze_slab(struct kmem_cache *s,
+static inline int acquire_slab(struct kmem_cache *s,
 		struct kmem_cache_node *n, struct page *page)
 {
 	void *freelist;
 	unsigned long counters;
 	struct page new;
 
-
-	if (!slab_trylock(page))
-		return 0;
-
 	/*
 	 * Zap the freelist and set the frozen bit.
 	 * The old freelist is the list of objects for the
@@ -1491,7 +1486,6 @@ static inline int lock_and_freeze_slab(struct kmem_cache *s,
 		 */
 		printk(KERN_ERR "SLUB: %s : Page without available objects on"
 			" partial list\n", s->name);
-		slab_unlock(page);
 		return 0;
 	}
 }
@@ -1515,7 +1509,7 @@ static struct page *get_partial_node(struct kmem_cache *s,
 
 	spin_lock(&n->list_lock);
 	list_for_each_entry(page, &n->partial, lru)
-		if (lock_and_freeze_slab(s, n, page))
+		if (acquire_slab(s, n, page))
 			goto out;
 	page = NULL;
 out:
@@ -1804,8 +1798,6 @@ redo:
 				"unfreezing slab"))
 		goto redo;
 
-	slab_unlock(page);
-
 	if (lock)
 		spin_unlock(&n->list_lock);
 
@@ -1819,7 +1811,6 @@ redo:
 static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c)
 {
 	stat(s, CPUSLAB_FLUSH);
-	slab_lock(c->page);
 	deactivate_slab(s, c);
 }
 
@@ -1968,7 +1959,6 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 	if (!page)
 		goto new_slab;
 
-	slab_lock(page);
 	if (unlikely(!node_match(c, node)))
 		goto another_slab;
 
@@ -1994,8 +1984,6 @@ load_freelist:
 
 	stat(s, ALLOC_REFILL);
 
-	slab_unlock(page);
-
 	c->freelist = get_freepointer(s, object);
 	c->tid = next_tid(c->tid);
 	local_irq_restore(flags);
@@ -2031,7 +2019,6 @@ new_slab:
 		page->inuse = page->objects;
 
 		stat(s, ALLOC_SLAB);
-		slab_lock(page);
 		c->node = page_to_nid(page);
 		c->page = page;
 		goto load_freelist;
@@ -2205,7 +2192,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 	unsigned long uninitialized_var(flags);
 
 	local_irq_save(flags);
-	slab_lock(page);
 	stat(s, FREE_SLOWPATH);
 
 	if (kmem_cache_debug(s) && !free_debug_processing(s, page, x, addr))
@@ -2271,7 +2257,6 @@ static void __slab_free(struct kmem_cache *s, struct page *page,
 	spin_unlock(&n->list_lock);
 
 out_unlock:
-	slab_unlock(page);
 	local_irq_restore(flags);
 	return;
 
@@ -2285,7 +2270,6 @@ slab_empty:
 	}
 
 	spin_unlock(&n->list_lock);
-	slab_unlock(page);
 	local_irq_restore(flags);
 	stat(s, FREE_SLAB);
 	discard_slab(s, page);
@@ -3202,14 +3186,8 @@ int kmem_cache_shrink(struct kmem_cache *s)
 		 * list_lock. page->inuse here is the upper limit.
 		 */
 		list_for_each_entry_safe(page, t, &n->partial, lru) {
-			if (!page->inuse && slab_trylock(page)) {
+			if (!page->inuse) {
-				/*
-				 * Must hold slab lock here because slab_free
-				 * may have freed the last object and be
-				 * waiting to release the slab.
-				 */
 				remove_partial(n, page);
-				slab_unlock(page);
 				discard_slab(s, page);
 			} else {
 				list_move(&page->lru,
@@ -3797,12 +3775,9 @@ static int validate_slab(struct kmem_cache *s, struct page *page,
 static void validate_slab_slab(struct kmem_cache *s, struct page *page,
 						unsigned long *map)
 {
-	if (slab_trylock(page)) {
+	slab_lock(page);
-		validate_slab(s, page, map);
+	validate_slab(s, page, map);
-		slab_unlock(page);
+	slab_unlock(page);
-	} else
-		printk(KERN_INFO "SLUB %s: Skipped busy slab 0x%p\n",
-			s->name, page);
 }
 
 static int validate_slab_node(struct kmem_cache *s,