Patch "mbcache: add functions to delete entry if unused" has been added to the 4.19-stable tree

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This is a note to let you know that I've just added the patch titled

    mbcache: add functions to delete entry if unused

to the 4.19-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mbcache-add-functions-to-delete-entry-if-unused.patch
and it can be found in the queue-4.19 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 5597b9d1616c53e0617c3a9b97b933096ab162ff
Author: Jan Kara <jack@xxxxxxx>
Date:   Tue Jul 12 12:54:21 2022 +0200

    mbcache: add functions to delete entry if unused
    
    [ Upstream commit 3dc96bba65f53daa217f0a8f43edad145286a8f5 ]
    
    Add function mb_cache_entry_delete_or_get() to delete mbcache entry if
    it is unused and also add a function to wait for entry to become unused
    - mb_cache_entry_wait_unused(). We do not share code between the two
    deleting function as one of them will go away soon.
    
    CC: stable@xxxxxxxxxxxxxxx
    Fixes: 82939d7999df ("ext4: convert to mbcache2")
    Signed-off-by: Jan Kara <jack@xxxxxxx>
    Link: https://lore.kernel.org/r/20220712105436.32204-2-jack@xxxxxxx
    Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
    Stable-dep-of: a44e84a9b776 ("ext4: fix deadlock due to mbcache entry corruption")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/mbcache.c b/fs/mbcache.c
index 12203e5a91e7..c76030e20d92 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -10,7 +10,7 @@
 /*
  * Mbcache is a simple key-value store. Keys need not be unique, however
  * key-value pairs are expected to be unique (we use this fact in
- * mb_cache_entry_delete()).
+ * mb_cache_entry_delete_or_get()).
  *
  * Ext2 and ext4 use this cache for deduplication of extended attribute blocks.
  * Ext4 also uses it for deduplication of xattr values stored in inodes.
@@ -124,6 +124,19 @@ void __mb_cache_entry_free(struct mb_cache_entry *entry)
 }
 EXPORT_SYMBOL(__mb_cache_entry_free);
 
+/*
+ * mb_cache_entry_wait_unused - wait to be the last user of the entry
+ *
+ * @entry - entry to work on
+ *
+ * Wait to be the last user of the entry.
+ */
+void mb_cache_entry_wait_unused(struct mb_cache_entry *entry)
+{
+	wait_var_event(&entry->e_refcnt, atomic_read(&entry->e_refcnt) <= 3);
+}
+EXPORT_SYMBOL(mb_cache_entry_wait_unused);
+
 static struct mb_cache_entry *__entry_find(struct mb_cache *cache,
 					   struct mb_cache_entry *entry,
 					   u32 key)
@@ -216,7 +229,7 @@ struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 }
 EXPORT_SYMBOL(mb_cache_entry_get);
 
-/* mb_cache_entry_delete - remove a cache entry
+/* mb_cache_entry_delete - try to remove a cache entry
  * @cache - cache we work with
  * @key - key
  * @value - value
@@ -253,6 +266,55 @@ void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value)
 }
 EXPORT_SYMBOL(mb_cache_entry_delete);
 
+/* mb_cache_entry_delete_or_get - remove a cache entry if it has no users
+ * @cache - cache we work with
+ * @key - key
+ * @value - value
+ *
+ * Remove entry from cache @cache with key @key and value @value. The removal
+ * happens only if the entry is unused. The function returns NULL in case the
+ * entry was successfully removed or there's no entry in cache. Otherwise the
+ * function grabs reference of the entry that we failed to delete because it
+ * still has users and return it.
+ */
+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);
+
+	return NULL;
+}
+EXPORT_SYMBOL(mb_cache_entry_delete_or_get);
+
 /* mb_cache_entry_touch - cache entry got used
  * @cache - cache the entry belongs to
  * @entry - entry that got used
diff --git a/include/linux/mbcache.h b/include/linux/mbcache.h
index 20f1e3ff6013..8eca7f25c432 100644
--- a/include/linux/mbcache.h
+++ b/include/linux/mbcache.h
@@ -30,15 +30,23 @@ 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_wait_unused(struct mb_cache_entry *entry);
 static inline int mb_cache_entry_put(struct mb_cache *cache,
 				     struct mb_cache_entry *entry)
 {
-	if (!atomic_dec_and_test(&entry->e_refcnt))
+	unsigned int cnt = atomic_dec_return(&entry->e_refcnt);
+
+	if (cnt > 0) {
+		if (cnt <= 3)
+			wake_up_var(&entry->e_refcnt);
 		return 0;
+	}
 	__mb_cache_entry_free(entry);
 	return 1;
 }
 
+struct mb_cache_entry *mb_cache_entry_delete_or_get(struct mb_cache *cache,
+						    u32 key, u64 value);
 void mb_cache_entry_delete(struct mb_cache *cache, u32 key, u64 value);
 struct mb_cache_entry *mb_cache_entry_get(struct mb_cache *cache, u32 key,
 					  u64 value);



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