fscache: Implement simple cookie state machine

Implement a very simple cookie state machine to handle lookup,
invalidation, withdrawal, relinquishment and, to be added later, commit on
LRU discard.

Three cache methods are provided: ->lookup_cookie() to look up and, if
necessary, create a data storage object; ->withdraw_cookie() to free the
resources associated with that object and potentially delete it; and
->prepare_to_write(), to do prepare for changes to the cached data to be
modified locally.

Changes
=======
ver #3:
 - Fix a race between LRU discard and relinquishment whereby the former
   would override the latter and thus the latter would never happen[1].

ver #2:
 - Don't hold n_accesses elevated whilst cache is bound to a cookie, but
   rather add a flag that prevents the state machine from being queued when
   n_accesses reaches 0.
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/599331.1639410068@warthog.procyon.org.uk/ [1]
Link: https://lore.kernel.org/r/163819599657.215744.15799615296912341745.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906903925.143852.1805855338154353867.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967105456.1823006.14730395299835841776.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021510706.640689.7961423370243272583.stgit@warthog.procyon.org.uk/ # v4

include/linux/fscache-cache.h

@@ -57,6 +57,15 @@ struct fscache_cache_ops {
 
 	/* Free the cache's data attached to a volume */
 	void (*free_volume)(struct fscache_volume *volume);
+
+	/* Look up a cookie in the cache */
+	bool (*lookup_cookie)(struct fscache_cookie *cookie);
+
+	/* Withdraw an object without any cookie access counts held */
+	void (*withdraw_cookie)(struct fscache_cookie *cookie);
+
+	/* Prepare to write to a live cache object */
+	void (*prepare_to_write)(struct fscache_cookie *cookie);
 };
 
 extern struct workqueue_struct *fscache_wq;
@@ -72,6 +81,7 @@ extern int fscache_add_cache(struct fscache_cache *cache,
 			     void *cache_priv);
 extern void fscache_withdraw_cache(struct fscache_cache *cache);
 extern void fscache_withdraw_volume(struct fscache_volume *volume);
+extern void fscache_withdraw_cookie(struct fscache_cookie *cookie);
 
 extern void fscache_io_error(struct fscache_cache *cache);
 
@@ -85,8 +95,21 @@ extern void fscache_put_cookie(struct fscache_cookie *cookie,
 			       enum fscache_cookie_trace where);
 extern void fscache_end_cookie_access(struct fscache_cookie *cookie,
 				      enum fscache_access_trace why);
-extern void fscache_set_cookie_state(struct fscache_cookie *cookie,
-				     enum fscache_cookie_state state);
+extern void fscache_cookie_lookup_negative(struct fscache_cookie *cookie);
+extern void fscache_caching_failed(struct fscache_cookie *cookie);
+
+/**
+ * fscache_cookie_state - Read the state of a cookie
+ * @cookie: The cookie to query
+ *
+ * Get the state of a cookie, imposing an ordering between the cookie contents
+ * and the state value.  Paired with fscache_set_cookie_state().
+ */
+static inline
+enum fscache_cookie_state fscache_cookie_state(struct fscache_cookie *cookie)
+{
+	return smp_load_acquire(&cookie->state);
+}
 
 /**
  * fscache_get_key - Get a pointer to the cookie key

include/trace/events/fscache.h

@@ -68,6 +68,8 @@ enum fscache_access_trace {
 	fscache_access_acquire_volume_end,
 	fscache_access_cache_pin,
 	fscache_access_cache_unpin,
+	fscache_access_lookup_cookie_end,
+	fscache_access_lookup_cookie_end_failed,
 	fscache_access_relinquish_volume,
 	fscache_access_relinquish_volume_end,
 	fscache_access_unlive,
@@ -124,6 +126,8 @@ enum fscache_access_trace {
 	EM(fscache_access_acquire_volume_end,	"END   acq_vol")	\
 	EM(fscache_access_cache_pin,		"PIN   cache  ")	\
 	EM(fscache_access_cache_unpin,		"UNPIN cache  ")	\
+	EM(fscache_access_lookup_cookie_end,	"END   lookup ")	\
+	EM(fscache_access_lookup_cookie_end_failed,"END   lookupf")	\
 	EM(fscache_access_relinquish_volume,	"BEGIN rlq_vol")	\
 	EM(fscache_access_relinquish_volume_end,"END   rlq_vol")	\
 	E_(fscache_access_unlive,		"END   unlive ")