[PATCH v2 5/5] mm/slub: refactor deactivate_slab()

[Hyeonggon Yoo]

Simplify deactivate_slab() by unlocking n->list_lock and retrying
cmpxchg_double() when cmpxchg_double() fails, and perform
add_{partial,full} only when it succeed.

Releasing and taking n->list_lock again here is not harmful as SLUB
avoids deactivating slabs as much as possible.

[ vbabka@xxxxxxx: perform add_{partial,full} when cmpxchg_double()
  succeed. ]

Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
---
 mm/slub.c | 81 ++++++++++++++++++++++---------------------------------
 1 file changed, 32 insertions(+), 49 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index f9ae983a3dc6..c1a693ec5874 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2344,8 +2344,8 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
 {
 	enum slab_modes { M_NONE, M_PARTIAL, M_FULL, M_FREE };
 	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
-	int lock = 0, free_delta = 0;
-	enum slab_modes l = M_NONE, m = M_NONE;
+	int free_delta = 0;
+	enum slab_modes mode = M_NONE;
 	void *nextfree, *freelist_iter, *freelist_tail;
 	int tail = DEACTIVATE_TO_HEAD;
 	unsigned long flags = 0;
@@ -2387,14 +2387,10 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
 	 * Ensure that the slab is unfrozen while the list presence
 	 * reflects the actual number of objects during unfreeze.
 	 *
-	 * We setup the list membership and then perform a cmpxchg
-	 * with the count. If there is a mismatch then the slab
-	 * is not unfrozen but the slab is on the wrong list.
-	 *
-	 * Then we restart the process which may have to remove
-	 * the slab from the list that we just put it on again
-	 * because the number of objects in the slab may have
-	 * changed.
+	 * We first perform cmpxchg holding lock and insert to list
+	 * when it succeed. If there is mismatch then slub is not
+	 * unfrozen and number of objects in the slab may have changed.
+	 * Then release lock and retry cmpxchg again.
 	 */
 redo:
 
@@ -2414,57 +2410,44 @@ static void deactivate_slab(struct kmem_cache *s, struct slab *slab,
 	new.frozen = 0;
 
 	if (!new.inuse && n->nr_partial >= s->min_partial)
-		m = M_FREE;
+		mode = M_FREE;
 	else if (new.freelist) {
-		m = M_PARTIAL;
-		if (!lock) {
-			lock = 1;
-			/*
-			 * Taking the spinlock removes the possibility that
-			 * acquire_slab() will see a slab that is frozen
-			 */
-			spin_lock_irqsave(&n->list_lock, flags);
-		}
-	} else {
-		m = M_FULL;
-		if (kmem_cache_debug_flags(s, SLAB_STORE_USER) && !lock) {
-			lock = 1;
-			/*
-			 * This also ensures that the scanning of full
-			 * slabs from diagnostic functions will not see
-			 * any frozen slabs.
-			 */
-			spin_lock_irqsave(&n->list_lock, flags);
-		}
+		mode = M_PARTIAL;
+		/*
+		 * Taking the spinlock removes the possibility that
+		 * acquire_slab() will see a slab that is frozen
+		 */
+		spin_lock_irqsave(&n->list_lock, flags);
+	} else if (kmem_cache_debug_flags(s, SLAB_STORE_USER)) {
+		mode = M_FULL;
+		/*
+		 * This also ensures that the scanning of full
+		 * slabs from diagnostic functions will not see
+		 * any frozen slabs.
+		 */
+		spin_lock_irqsave(&n->list_lock, flags);
 	}
 
-	if (l != m) {
-		if (l == M_PARTIAL)
-			remove_partial(n, slab);
-		else if (l == M_FULL)
-			remove_full(s, n, slab);
-
-		if (m == M_PARTIAL)
-			add_partial(n, slab, tail);
-		else if (m == M_FULL)
-			add_full(s, n, slab);
-	}
 
-	l = m;
 	if (!cmpxchg_double_slab(s, slab,
 				old.freelist, old.counters,
 				new.freelist, new.counters,
-				"unfreezing slab"))
+				"unfreezing slab")) {
+		if (mode == M_PARTIAL || mode == M_FULL)
+			spin_unlock_irqrestore(&n->list_lock, flags);
 		goto redo;
+	}
 
-	if (lock)
-		spin_unlock_irqrestore(&n->list_lock, flags);
 
-	if (m == M_PARTIAL)
+	if (mode == M_PARTIAL) {
+		add_partial(n, slab, tail);
+		spin_unlock_irqrestore(&n->list_lock, flags);
 		stat(s, tail);
-	else if (m == M_FULL)
+	} else if (mode == M_FULL) {
+		add_full(s, n, slab);
+		spin_unlock_irqrestore(&n->list_lock, flags);
 		stat(s, DEACTIVATE_FULL);
-	else if (m == M_FREE) {
+	} else if (mode == M_FREE) {
 		stat(s, DEACTIVATE_EMPTY);
 		discard_slab(s, slab);
 		stat(s, FREE_SLAB);
-- 
2.33.1