fscache: Implement cache-level access helpers

Add a pair of functions to pin/unpin a cache that we're wanting to do a
high-level access to (such as creating or removing a volume):

	bool fscache_begin_cache_access(struct fscache_cache *cache,
					enum fscache_access_trace why);
	void fscache_end_cache_access(struct fscache_cache *cache,
				      enum fscache_access_trace why);

The way the access gate works/will work is:

 (1) If the cache tests as not live (state is not FSCACHE_CACHE_IS_ACTIVE),
     then we return false to indicate access was not permitted.

 (2) If the cache tests as live, then we increment the n_accesses count and
     then recheck the liveness, ending the access if it ceased to be live.

 (3) When we end the access, we decrement n_accesses and wake up the any
     waiters if it reaches 0.

 (4) Whilst the cache is caching, n_accesses is kept artificially
     incremented to prevent wakeups from happening.

 (5) When the cache is taken offline, the state is changed to prevent new
     accesses, n_accesses is decremented and we wait for n_accesses to
     become 0.

Note that some of this is implemented in a later patch.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819593239.215744.7537428720603638088.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906893368.143852.14164004598465617981.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967093977.1823006.6967886507023056409.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021499995.640689.18286203753480287850.stgit@warthog.procyon.org.uk/ # v4

fs/fscache/cache.c

@@ -216,6 +216,68 @@ void fscache_relinquish_cache(struct fscache_cache *cache)
 }
 EXPORT_SYMBOL(fscache_relinquish_cache);
 
+/**
+ * fscache_begin_cache_access - Pin a cache so it can be accessed
+ * @cache: The cache-level cookie
+ * @why: An indication of the circumstances of the access for tracing
+ *
+ * Attempt to pin the cache to prevent it from going away whilst we're
+ * accessing it and returns true if successful.  This works as follows:
+ *
+ *  (1) If the cache tests as not live (state is not FSCACHE_CACHE_IS_ACTIVE),
+ *      then we return false to indicate access was not permitted.
+ *
+ *  (2) If the cache tests as live, then we increment the n_accesses count and
+ *      then recheck the liveness, ending the access if it ceased to be live.
+ *
+ *  (3) When we end the access, we decrement n_accesses and wake up the any
+ *      waiters if it reaches 0.
+ *
+ *  (4) Whilst the cache is caching, n_accesses is kept artificially
+ *      incremented to prevent wakeups from happening.
+ *
+ *  (5) When the cache is taken offline, the state is changed to prevent new
+ *      accesses, n_accesses is decremented and we wait for n_accesses to
+ *      become 0.
+ */
+bool fscache_begin_cache_access(struct fscache_cache *cache, enum fscache_access_trace why)
+{
+	int n_accesses;
+
+	if (!fscache_cache_is_live(cache))
+		return false;
+
+	n_accesses = atomic_inc_return(&cache->n_accesses);
+	smp_mb__after_atomic(); /* Reread live flag after n_accesses */
+	trace_fscache_access_cache(cache->debug_id, refcount_read(&cache->ref),
+				   n_accesses, why);
+	if (!fscache_cache_is_live(cache)) {
+		fscache_end_cache_access(cache, fscache_access_unlive);
+		return false;
+	}
+	return true;
+}
+
+/**
+ * fscache_end_cache_access - Unpin a cache at the end of an access.
+ * @cache: The cache-level cookie
+ * @why: An indication of the circumstances of the access for tracing
+ *
+ * Unpin a cache after we've accessed it.  The @why indicator is merely
+ * provided for tracing purposes.
+ */
+void fscache_end_cache_access(struct fscache_cache *cache, enum fscache_access_trace why)
+{
+	int n_accesses;
+
+	smp_mb__before_atomic();
+	n_accesses = atomic_dec_return(&cache->n_accesses);
+	trace_fscache_access_cache(cache->debug_id, refcount_read(&cache->ref),
+				   n_accesses, why);
+	if (n_accesses == 0)
+		wake_up_var(&cache->n_accesses);
+}
+
 #ifdef CONFIG_PROC_FS
 static const char fscache_cache_states[NR__FSCACHE_CACHE_STATE] = "-PAEW";
 

fs/fscache/internal.h

@@ -23,6 +23,8 @@
 #ifdef CONFIG_PROC_FS
 extern const struct seq_operations fscache_caches_seq_ops;
 #endif
+bool fscache_begin_cache_access(struct fscache_cache *cache, enum fscache_access_trace why);
+void fscache_end_cache_access(struct fscache_cache *cache, enum fscache_access_trace why);
 struct fscache_cache *fscache_lookup_cache(const char *name, bool is_cache);
 void fscache_put_cache(struct fscache_cache *cache, enum fscache_cache_trace where);
 

fs/fscache/main.c

@@ -21,6 +21,8 @@ module_param_named(debug, fscache_debug, uint,
 MODULE_PARM_DESC(fscache_debug,
 		 "FS-Cache debugging mask");
 
+EXPORT_TRACEPOINT_SYMBOL(fscache_access_cache);
+
 struct workqueue_struct *fscache_wq;
 EXPORT_SYMBOL(fscache_wq);