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Subject[PATCH v3 14/68] fscache: Implement functions add/remove a cache
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Implement functions to allow the cache backend to add or remove a cache:

(1) Declare a cache to be live:

int fscache_add_cache(struct fscache_cache *cache,
const struct fscache_cache_ops *ops,
void *cache_priv);

Take a previously acquired cache cookie, set the operations table and
private data and mark the cache open for access.

(2) Withdraw a cache from service:

void fscache_withdraw_cache(struct fscache_cache *cache);

This marks the cache as withdrawn and thus prevents further
cache-level and volume-level accesses.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819596022.215744.8799712491432238827.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906896599.143852.17049208999019262884.stgit@warthog.procyon.org.uk/ # v2
---

fs/fscache/cache.c | 70 +++++++++++++++++++++++++++++++++++++++++
include/linux/fscache-cache.h | 13 ++++++++
2 files changed, 83 insertions(+)

diff --git a/fs/fscache/cache.c b/fs/fscache/cache.c
index e867cff53a70..bbd102be91c4 100644
--- a/fs/fscache/cache.c
+++ b/fs/fscache/cache.c
@@ -210,12 +210,55 @@ void fscache_relinquish_cache(struct fscache_cache *cache)
fscache_cache_put_prep_failed :
fscache_cache_put_relinquish;

+ cache->ops = NULL;
cache->cache_priv = NULL;
smp_store_release(&cache->state, FSCACHE_CACHE_IS_NOT_PRESENT);
fscache_put_cache(cache, where);
}
EXPORT_SYMBOL(fscache_relinquish_cache);

+/**
+ * fscache_add_cache - Declare a cache as being open for business
+ * @cache: The cache-level cookie representing the cache
+ * @ops: Table of cache operations to use
+ * @cache_priv: Private data for the cache record
+ *
+ * Add a cache to the system, making it available for netfs's to use.
+ *
+ * See Documentation/filesystems/caching/backend-api.rst for a complete
+ * description.
+ */
+int fscache_add_cache(struct fscache_cache *cache,
+ const struct fscache_cache_ops *ops,
+ void *cache_priv)
+{
+ int n_accesses;
+
+ _enter("{%s,%s}", ops->name, cache->name);
+
+ BUG_ON(fscache_cache_state(cache) != FSCACHE_CACHE_IS_PREPARING);
+
+ /* Get a ref on the cache cookie and keep its n_accesses counter raised
+ * by 1 to prevent wakeups from transitioning it to 0 until we're
+ * withdrawing caching services from it.
+ */
+ n_accesses = atomic_inc_return(&cache->n_accesses);
+ trace_fscache_access_cache(cache->debug_id, refcount_read(&cache->ref),
+ n_accesses, fscache_access_cache_pin);
+
+ down_write(&fscache_addremove_sem);
+
+ cache->ops = ops;
+ cache->cache_priv = cache_priv;
+ fscache_set_cache_state(cache, FSCACHE_CACHE_IS_ACTIVE);
+
+ up_write(&fscache_addremove_sem);
+ pr_notice("Cache \"%s\" added (type %s)\n", cache->name, ops->name);
+ _leave(" = 0 [%s]", cache->name);
+ return 0;
+}
+EXPORT_SYMBOL(fscache_add_cache);
+
/**
* fscache_begin_cache_access - Pin a cache so it can be accessed
* @cache: The cache-level cookie
@@ -278,6 +321,33 @@ void fscache_end_cache_access(struct fscache_cache *cache, enum fscache_access_t
wake_up_var(&cache->n_accesses);
}

+/**
+ * fscache_withdraw_cache - Withdraw a cache from the active service
+ * @cache: The cache cookie
+ *
+ * Begin the process of withdrawing a cache from service. This stops new
+ * cache-level and volume-level accesses from taking place and waits for
+ * currently ongoing cache-level accesses to end.
+ */
+void fscache_withdraw_cache(struct fscache_cache *cache)
+{
+ int n_accesses;
+
+ pr_notice("Withdrawing cache \"%s\" (%u objs)\n",
+ cache->name, atomic_read(&cache->object_count));
+
+ fscache_set_cache_state(cache, FSCACHE_CACHE_IS_WITHDRAWN);
+
+ /* Allow wakeups on dec-to-0 */
+ n_accesses = atomic_dec_return(&cache->n_accesses);
+ trace_fscache_access_cache(cache->debug_id, refcount_read(&cache->ref),
+ n_accesses, fscache_access_cache_unpin);
+
+ wait_var_event(&cache->n_accesses,
+ atomic_read(&cache->n_accesses) == 0);
+}
+EXPORT_SYMBOL(fscache_withdraw_cache);
+
#ifdef CONFIG_PROC_FS
static const char fscache_cache_states[NR__FSCACHE_CACHE_STATE] = "-PAEW";

diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 66624407ba84..f78add6e7823 100644
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -33,6 +33,7 @@ enum fscache_cache_state {
* Cache cookie.
*/
struct fscache_cache {
+ const struct fscache_cache_ops *ops;
struct list_head cache_link; /* Link in cache list */
void *cache_priv; /* Private cache data (or NULL) */
refcount_t ref;
@@ -44,6 +45,14 @@ struct fscache_cache {
char *name;
};

+/*
+ * cache operations
+ */
+struct fscache_cache_ops {
+ /* name of cache provider */
+ const char *name;
+};
+
extern struct workqueue_struct *fscache_wq;

/*
@@ -52,6 +61,10 @@ extern struct workqueue_struct *fscache_wq;
extern struct rw_semaphore fscache_addremove_sem;
extern struct fscache_cache *fscache_acquire_cache(const char *name);
extern void fscache_relinquish_cache(struct fscache_cache *cache);
+extern int fscache_add_cache(struct fscache_cache *cache,
+ const struct fscache_cache_ops *ops,
+ void *cache_priv);
+extern void fscache_withdraw_cache(struct fscache_cache *cache);

extern void fscache_end_volume_access(struct fscache_volume *volume,
struct fscache_cookie *cookie,

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