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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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