mbcache: automatically delete entries from cache on freeing

Use the fact that entries with elevated refcount are not removed from
the hash and just move removal of the entry from the hash to the entry
freeing time. When doing this we also change the generic code to hold
one reference to the cache entry, not two of them, which makes code
somewhat more obvious.
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220712105436.32204-10-jack@suse.czSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/mbcache.c

@@ -90,7 +90,7 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
 		return -ENOMEM;
 
 	INIT_LIST_HEAD(&entry->e_list);
-	/* One ref for hash, one ref returned */
+	/* Initial hash reference */
 	atomic_set(&entry->e_refcnt, 1);
 	entry->e_key = key;
 	entry->e_value = value;
@@ -106,21 +106,28 @@ int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
 		}
 	}
 	hlist_bl_add_head(&entry->e_hash_list, head);
-	hlist_bl_unlock(head);
-
+	/*
+	 * Add entry to LRU list before it can be found by
+	 * mb_cache_entry_delete() to avoid races
+	 */
 	spin_lock(&cache->c_list_lock);
 	list_add_tail(&entry->e_list, &cache->c_list);
-	/* Grab ref for LRU list */
-	atomic_inc(&entry->e_refcnt);
 	cache->c_entry_count++;
 	spin_unlock(&cache->c_list_lock);
+	hlist_bl_unlock(head);
 
 	return 0;
 }
 EXPORT_SYMBOL(mb_cache_entry_create);
 
-void __mb_cache_entry_free(struct mb_cache_entry *entry)
+void __mb_cache_entry_free(struct mb_cache *cache, struct mb_cache_entry *entry)
 {
+	struct hlist_bl_head *head;
+
+	head = mb_cache_entry_head(cache, entry->e_key);
+	hlist_bl_lock(head);
+	hlist_bl_del(&entry->e_hash_list);
+	hlist_bl_unlock(head);
 	kmem_cache_free(mb_entry_cache, entry);
 }
 EXPORT_SYMBOL(__mb_cache_entry_free);
@@ -134,7 +141,7 @@ EXPORT_SYMBOL(__mb_cache_entry_free);
  */
 void mb_cache_entry_wait_unused(struct mb_cache_entry *entry)
 {
-	wait_var_event(&entry->e_refcnt, atomic_read(&entry->e_refcnt) <= 3);
+	wait_var_event(&entry->e_refcnt, atomic_read(&entry->e_refcnt) <= 2);
 }
 EXPORT_SYMBOL(mb_cache_entry_wait_unused);
 
@@ -155,10 +162,9 @@ static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 	while (node) {
 		entry = hlist_bl_entry(node, struct mb_cache_entry,
 				       e_hash_list);
-		if (entry->e_key == key && entry->e_reusable) {
-			atomic_inc(&entry->e_refcnt);
+		if (entry->e_key == key && entry->e_reusable &&
+		    atomic_inc_not_zero(&entry->e_refcnt))
 			goto out;
-		}
 		node = node->next;
 	}
 	entry = NULL;
@@ -218,10 +224,9 @@ struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 	head = mb_cache_entry_head(cache, key);
 	hlist_bl_lock(head);
 	hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
-		if (entry->e_key == key && entry->e_value == value) {
-			atomic_inc(&entry->e_refcnt);
+		if (entry->e_key == key && entry->e_value == value &&
+		    atomic_inc_not_zero(&entry->e_refcnt))
 			goto out;
-		}
 	}
 	entry = NULL;
 out:
@@ -244,37 +249,25 @@ EXPORT_SYMBOL(mb_cache_entry_get);
 struct mb_cache_entry *mb_cache_entry_delete_or_get(struct mb_cache *cache,
 						    u32 key, u64 value)
 {
-	struct hlist_bl_node *node;
-	struct hlist_bl_head *head;
 	struct mb_cache_entry *entry;
 
-	head = mb_cache_entry_head(cache, key);
-	hlist_bl_lock(head);
-	hlist_bl_for_each_entry(entry, node, head, e_hash_list) {
-		if (entry->e_key == key && entry->e_value == value) {
-			if (atomic_read(&entry->e_refcnt) > 2) {
-				atomic_inc(&entry->e_refcnt);
-				hlist_bl_unlock(head);
-				return entry;
-			}
-			/* We keep hash list reference to keep entry alive */
-			hlist_bl_del_init(&entry->e_hash_list);
-			hlist_bl_unlock(head);
-			spin_lock(&cache->c_list_lock);
-			if (!list_empty(&entry->e_list)) {
-				list_del_init(&entry->e_list);
-				if (!WARN_ONCE(cache->c_entry_count == 0,
-		"mbcache: attempt to decrement c_entry_count past zero"))
-					cache->c_entry_count--;
-				atomic_dec(&entry->e_refcnt);
-			}
-			spin_unlock(&cache->c_list_lock);
-			mb_cache_entry_put(cache, entry);
-			return NULL;
-		}
-	}
-	hlist_bl_unlock(head);
+	entry = mb_cache_entry_get(cache, key, value);
+	if (!entry)
+		return NULL;
+
+	/*
+	 * Drop the ref we got from mb_cache_entry_get() and the initial hash
+	 * ref if we are the last user
+	 */
+	if (atomic_cmpxchg(&entry->e_refcnt, 2, 0) != 2)
+		return entry;
 
+	spin_lock(&cache->c_list_lock);
+	if (!list_empty(&entry->e_list))
+		list_del_init(&entry->e_list);
+	cache->c_entry_count--;
+	spin_unlock(&cache->c_list_lock);
+	__mb_cache_entry_free(cache, entry);
 	return NULL;
 }
 EXPORT_SYMBOL(mb_cache_entry_delete_or_get);
@@ -306,42 +299,24 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
 				     unsigned long nr_to_scan)
 {
 	struct mb_cache_entry *entry;
-	struct hlist_bl_head *head;
 	unsigned long shrunk = 0;
 
 	spin_lock(&cache->c_list_lock);
 	while (nr_to_scan-- && !list_empty(&cache->c_list)) {
 		entry = list_first_entry(&cache->c_list,
 					 struct mb_cache_entry, e_list);
-		if (entry->e_referenced || atomic_read(&entry->e_refcnt) > 2) {
+		/* Drop initial hash reference if there is no user */
+		if (entry->e_referenced ||
+		    atomic_cmpxchg(&entry->e_refcnt, 1, 0) != 1) {
 			entry->e_referenced = 0;
 			list_move_tail(&entry->e_list, &cache->c_list);
 			continue;
 		}
 		list_del_init(&entry->e_list);
 		cache->c_entry_count--;
-		/*
-		 * We keep LRU list reference so that entry doesn't go away
-		 * from under us.
-		 */
 		spin_unlock(&cache->c_list_lock);
-		head = mb_cache_entry_head(cache, entry->e_key);
-		hlist_bl_lock(head);
-		/* Now a reliable check if the entry didn't get used... */
-		if (atomic_read(&entry->e_refcnt) > 2) {
-			hlist_bl_unlock(head);
-			spin_lock(&cache->c_list_lock);
-			list_add_tail(&entry->e_list, &cache->c_list);
-			cache->c_entry_count++;
-			continue;
-		}
-		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
-			hlist_bl_del_init(&entry->e_hash_list);
-			atomic_dec(&entry->e_refcnt);
-		}
-		hlist_bl_unlock(head);
-		if (mb_cache_entry_put(cache, entry))
-			shrunk++;
+		__mb_cache_entry_free(cache, entry);
+		shrunk++;
 		cond_resched();
 		spin_lock(&cache->c_list_lock);
 	}
@@ -433,11 +408,6 @@ void mb_cache_destroy(struct mb_cache *cache)
 	 * point.
 	 */
 	list_for_each_entry_safe(entry, next, &cache->c_list, e_list) {
-		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
-			hlist_bl_del_init(&entry->e_hash_list);
-			atomic_dec(&entry->e_refcnt);
-		} else
-			WARN_ON(1);
 		list_del(&entry->e_list);
 		WARN_ON(atomic_read(&entry->e_refcnt) != 1);
 		mb_cache_entry_put(cache, entry);

include/linux/mbcache.h

@@ -13,8 +13,16 @@ struct mb_cache;
 struct mb_cache_entry {
 	/* List of entries in cache - protected by cache->c_list_lock */
 	struct list_head	e_list;
-	/* Hash table list - protected by hash chain bitlock */
+	/*
+	 * Hash table list - protected by hash chain bitlock. The entry is
+	 * guaranteed to be hashed while e_refcnt > 0.
+	 */
 	struct hlist_bl_node	e_hash_list;
+	/*
+	 * Entry refcount. Once it reaches zero, entry is unhashed and freed.
+	 * While refcount > 0, the entry is guaranteed to stay in the hash and
+	 * e.g. mb_cache_entry_try_delete() will fail.
+	 */
 	atomic_t		e_refcnt;
 	/* Key in hash - stable during lifetime of the entry */
 	u32			e_key;
@@ -29,20 +37,20 @@ void mb_cache_destroy(struct mb_cache *cache);
 
 int mb_cache_entry_create(struct mb_cache *cache, gfp_t mask, u32 key,
 			  u64 value, bool reusable);
-void __mb_cache_entry_free(struct mb_cache_entry *entry);
+void __mb_cache_entry_free(struct mb_cache *cache,
+			   struct mb_cache_entry *entry);
 void mb_cache_entry_wait_unused(struct mb_cache_entry *entry);
-static inline int mb_cache_entry_put(struct mb_cache *cache,
-				     struct mb_cache_entry *entry)
+static inline void mb_cache_entry_put(struct mb_cache *cache,
+				      struct mb_cache_entry *entry)
 {
 	unsigned int cnt = atomic_dec_return(&entry->e_refcnt);
 
 	if (cnt > 0) {
-		if (cnt <= 3)
+		if (cnt <= 2)
 			wake_up_var(&entry->e_refcnt);
-		return 0;
+		return;
 	}
-	__mb_cache_entry_free(entry);
-	return 1;
+	__mb_cache_entry_free(cache, entry);
 }
 
 struct mb_cache_entry *mb_cache_entry_delete_or_get(struct mb_cache *cache,