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Subject[PATCH v3 21/68] fscache: Count data storage objects in a cache
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Count the data storage objects that are currently allocated in a cache.
This is used to pin certain cache structures until cache withdrawal is
complete.

Three helpers are provided to manage and make use of the count:

(1) void fscache_count_object(struct fscache_cache *cache);

This should be called by the cache backend to note that an object has
been allocated and attached to the cache.

(2) void fscache_uncount_object(struct fscache_cache *cache);

This should be called by the backend to note that an object has been
destroyed. This sends a wakeup event that allows cache withdrawal to
proceed if it was waiting for that object.

(3) void fscache_wait_for_objects(struct fscache_cache *cache);

This can be used by the backend to wait for all outstanding cache
object to be destroyed.

Each cache's counter is displayed as part of /proc/fs/fscache/caches.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819608594.215744.1812706538117388252.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906911646.143852.168184059935530127.stgit@warthog.procyon.org.uk/ # v2
---

fs/fscache/cache.c | 2 ++
include/linux/fscache-cache.h | 39 +++++++++++++++++++++++++++++++++++++++
2 files changed, 41 insertions(+)

diff --git a/fs/fscache/cache.c b/fs/fscache/cache.c
index 25eac61f1c29..2749933852a9 100644
--- a/fs/fscache/cache.c
+++ b/fs/fscache/cache.c
@@ -13,6 +13,8 @@
static LIST_HEAD(fscache_caches);
DECLARE_RWSEM(fscache_addremove_sem);
EXPORT_SYMBOL(fscache_addremove_sem);
+DECLARE_WAIT_QUEUE_HEAD(fscache_clearance_waiters);
+EXPORT_SYMBOL(fscache_clearance_waiters);

static atomic_t fscache_cache_debug_id;

diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 566497cf5f13..337335d7a5e2 100644
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -76,6 +76,7 @@ struct fscache_cache_ops {
};

extern struct workqueue_struct *fscache_wq;
+extern wait_queue_head_t fscache_clearance_waiters;

/*
* out-of-line cache backend functions
@@ -140,4 +141,42 @@ static inline struct fscache_cookie *fscache_cres_cookie(struct netfs_cache_reso
return cres->cache_priv;
}

+/**
+ * fscache_count_object - Tell fscache that an object has been added
+ * @cache: The cache to account to
+ *
+ * Tell fscache that an object has been added to the cache. This prevents the
+ * cache from tearing down the cache structure until the object is uncounted.
+ */
+static inline void fscache_count_object(struct fscache_cache *cache)
+{
+ atomic_inc(&cache->object_count);
+}
+
+/**
+ * fscache_uncount_object - Tell fscache that an object has been removed
+ * @cache: The cache to account to
+ *
+ * Tell fscache that an object has been removed from the cache and will no
+ * longer be accessed. After this point, the cache cookie may be destroyed.
+ */
+static inline void fscache_uncount_object(struct fscache_cache *cache)
+{
+ if (atomic_dec_and_test(&cache->object_count))
+ wake_up_all(&fscache_clearance_waiters);
+}
+
+/**
+ * fscache_wait_for_objects - Wait for all objects to be withdrawn
+ * @cache: The cache to query
+ *
+ * Wait for all extant objects in a cache to finish being withdrawn
+ * and go away.
+ */
+static inline void fscache_wait_for_objects(struct fscache_cache *cache)
+{
+ wait_event(fscache_clearance_waiters,
+ atomic_read(&cache->object_count) == 0);
+}
+
#endif /* _LINUX_FSCACHE_CACHE_H */

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