FS-Cache: Provide the ability to enable/disable cookies

Provide the ability to enable and disable fscache cookies.  A disabled cookie
will reject or ignore further requests to:

	Acquire a child cookie
	Invalidate and update backing objects
	Check the consistency of a backing object
	Allocate storage for backing page
	Read backing pages
	Write to backing pages

but still allows:

	Checks/waits on the completion of already in-progress objects
	Uncaching of pages
	Relinquishment of cookies

Two new operations are provided:

 (1) Disable a cookie:

	void fscache_disable_cookie(struct fscache_cookie *cookie,
				    bool invalidate);

     If the cookie is not already disabled, this locks the cookie against other
     dis/enablement ops, marks the cookie as being disabled, discards or
     invalidates any backing objects and waits for cessation of activity on any
     associated object.

     This is a wrapper around a chunk split out of fscache_relinquish_cookie(),
     but it reinitialises the cookie such that it can be reenabled.

     All possible failures are handled internally.  The caller should consider
     calling fscache_uncache_all_inode_pages() afterwards to make sure all page
     markings are cleared up.

 (2) Enable a cookie:

	void fscache_enable_cookie(struct fscache_cookie *cookie,
				   bool (*can_enable)(void *data),
				   void *data)

     If the cookie is not already enabled, this locks the cookie against other
     dis/enablement ops, invokes can_enable() and, if the cookie is not an
     index cookie, will begin the procedure of acquiring backing objects.

     The optional can_enable() function is passed the data argument and returns
     a ruling as to whether or not enablement should actually be permitted to
     begin.

     All possible failures are handled internally.  The cookie will only be
     marked as enabled if provisional backing objects are allocated.

A later patch will introduce these to NFS.  Cookie enablement during nfs_open()
is then contingent on i_writecount <= 0.  can_enable() checks for a race
between open(O_RDONLY) and open(O_WRONLY/O_RDWR).  This simplifies NFS's cookie
handling and allows us to get rid of open(O_RDONLY) accidentally introducing
caching to an inode that's open for writing already.

One operation has its API modified:

 (3) Acquire a cookie.

	struct fscache_cookie *fscache_acquire_cookie(
		struct fscache_cookie *parent,
		const struct fscache_cookie_def *def,
		void *netfs_data,
		bool enable);

     This now has an additional argument that indicates whether the requested
     cookie should be enabled by default.  It doesn't need the can_enable()
     function because the caller must prevent multiple calls for the same netfs
     object and it doesn't need to take the enablement lock because no one else
     can get at the cookie before this returns.

Signed-off-by: David Howells <dhowells@redhat.com

17 files changed, 334 insertions, 132 deletions

diff --git a/Documentation/filesystems/caching/netfs-api.txt b/Documentation/filesystems/caching/netfs-api.txt
index 11a0a40ce44..aed6b94160b 100644
--- a/Documentation/filesystems/caching/netfs-api.txt
+++ b/Documentation/filesystems/caching/netfs-api.txt
@@ -29,15 +29,16 @@ This document contains the following sections:
 	 (6) Index registration
 	 (7) Data file registration
 	 (8) Miscellaneous object registration
-	 (9) Setting the data file size
+ 	 (9) Setting the data file size
 	(10) Page alloc/read/write
 	(11) Page uncaching
 	(12) Index and data file consistency
-	(13) Miscellaneous cookie operations
-	(14) Cookie unregistration
-	(15) Index invalidation
-	(16) Data file invalidation
-	(17) FS-Cache specific page flags.
+	(13) Cookie enablement
+	(14) Miscellaneous cookie operations
+	(15) Cookie unregistration
+	(16) Index invalidation
+	(17) Data file invalidation
+	(18) FS-Cache specific page flags.
 
 
 =============================
@@ -334,7 +335,8 @@ the path to the file:
 	struct fscache_cookie *
 	fscache_acquire_cookie(struct fscache_cookie *parent,
 			       const struct fscache_object_def *def,
-			       void *netfs_data);
+			       void *netfs_data,
+			       bool enable);
 
 This function creates an index entry in the index represented by parent,
 filling in the index entry by calling the operations pointed to by def.
@@ -350,6 +352,10 @@ object needs to be created somewhere down the hierarchy.  Furthermore, an index
 may be created in several different caches independently at different times.
 This is all handled transparently, and the netfs doesn't see any of it.
 
+A cookie will be created in the disabled state if enabled is false.  A cookie
+must be enabled to do anything with it.  A disabled cookie can be enabled by
+calling fscache_enable_cookie() (see below).
+
 For example, with AFS, a cell would be added to the primary index.  This index
 entry would have a dependent inode containing a volume location index for the
 volume mappings within this cell:
@@ -357,7 +363,7 @@ volume mappings within this cell:
 	cell->cache =
 		fscache_acquire_cookie(afs_cache_netfs.primary_index,
 				       &afs_cell_cache_index_def,
-				       cell);
+				       cell, true);
 
 Then when a volume location was accessed, it would be entered into the cell's
 index and an inode would be allocated that acts as a volume type and hash chain
@@ -366,7 +372,7 @@ combination:
 	vlocation->cache =
 		fscache_acquire_cookie(cell->cache,
 				       &afs_vlocation_cache_index_def,
-				       vlocation);
+				       vlocation, true);
 
 And then a particular flavour of volume (R/O for example) could be added to
 that index, creating another index for vnodes (AFS inode equivalents):
@@ -374,7 +380,7 @@ that index, creating another index for vnodes (AFS inode equivalents):
 	volume->cache =
 		fscache_acquire_cookie(vlocation->cache,
 				       &afs_volume_cache_index_def,
-				       volume);
+				       volume, true);
 
 
 ======================
@@ -388,7 +394,7 @@ the object definition should be something other than index type.
 	vnode->cache =
 		fscache_acquire_cookie(volume->cache,
 				       &afs_vnode_cache_object_def,
-				       vnode);
+				       vnode, true);
 
 
 =================================
@@ -404,7 +410,7 @@ it would be some other type of object such as a data file.
 	xattr->cache =
 		fscache_acquire_cookie(vnode->cache,
 				       &afs_xattr_cache_object_def,
-				       xattr);
+				       xattr, true);
 
 Miscellaneous objects might be used to store extended attributes or directory
 entries for example.
@@ -733,6 +739,47 @@ Note that partial updates may happen automatically at other times, such as when
 data blocks are added to a data file object.
 
 
+=================
+COOKIE ENABLEMENT
+=================
+
+Cookies exist in one of two states: enabled and disabled.  If a cookie is
+disabled, it ignores all attempts to acquire child cookies; check, update or
+invalidate its state; allocate, read or write backing pages - though it is
+still possible to uncache pages and relinquish the cookie.
+
+The initial enablement state is set by fscache_acquire_cookie(), but the cookie
+can be enabled or disabled later.  To disable a cookie, call:
+    
+	void fscache_disable_cookie(struct fscache_cookie *cookie,
+    				    bool invalidate);
+    
+If the cookie is not already disabled, this locks the cookie against other
+enable and disable ops, marks the cookie as being disabled, discards or
+invalidates any backing objects and waits for cessation of activity on any
+associated object before unlocking the cookie.
+
+All possible failures are handled internally.  The caller should consider
+calling fscache_uncache_all_inode_pages() afterwards to make sure all page
+markings are cleared up.
+    
+Cookies can be enabled or reenabled with:
+    
+    	void fscache_enable_cookie(struct fscache_cookie *cookie,
+    				   bool (*can_enable)(void *data),
+    				   void *data)
+    
+If the cookie is not already enabled, this locks the cookie against other
+enable and disable ops, invokes can_enable() and, if the cookie is not an index
+cookie, will begin the procedure of acquiring backing objects.
+
+The optional can_enable() function is passed the data argument and returns a
+ruling as to whether or not enablement should actually be permitted to begin.
+
+All possible failures are handled internally.  The cookie will only be marked
+as enabled if provisional backing objects are allocated.
+
+
 ===============================
 MISCELLANEOUS COOKIE OPERATIONS
 ===============================
@@ -778,7 +825,7 @@ COOKIE UNREGISTRATION
 To get rid of a cookie, this function should be called.
 
 	void fscache_relinquish_cookie(struct fscache_cookie *cookie,
-				       int retire);
+				       bool retire);
 
 If retire is non-zero, then the object will be marked for recycling, and all
 copies of it will be removed from all active caches in which it is present.
diff --git a/fs/9p/cache.c b/fs/9p/cache.c
index a9ea73d6dcf..2b7a032c37b 100644
--- a/fs/9p/cache.c
+++ b/fs/9p/cache.c
@@ -90,7 +90,7 @@ void v9fs_cache_session_get_cookie(struct v9fs_session_info *v9ses)
 
 	v9ses->fscache = fscache_acquire_cookie(v9fs_cache_netfs.primary_index,
 						&v9fs_cache_session_index_def,
-						v9ses);
+						v9ses, true);
 	p9_debug(P9_DEBUG_FSC, "session %p get cookie %p\n",
 		 v9ses, v9ses->fscache);
 }
@@ -204,7 +204,7 @@ void v9fs_cache_inode_get_cookie(struct inode *inode)
 	v9ses = v9fs_inode2v9ses(inode);
 	v9inode->fscache = fscache_acquire_cookie(v9ses->fscache,
 						  &v9fs_cache_inode_index_def,
-						  v9inode);
+						  v9inode, true);
 
 	p9_debug(P9_DEBUG_FSC, "inode %p get cookie %p\n",
 		 inode, v9inode->fscache);
@@ -271,7 +271,7 @@ void v9fs_cache_inode_reset_cookie(struct inode *inode)
 	v9ses = v9fs_inode2v9ses(inode);
 	v9inode->fscache = fscache_acquire_cookie(v9ses->fscache,
 						  &v9fs_cache_inode_index_def,
-						  v9inode);
+						  v9inode, true);
 	p9_debug(P9_DEBUG_FSC, "inode %p revalidating cookie old %p new %p\n",
 		 inode, old, v9inode->fscache);
 
diff --git a/fs/afs/cell.c b/fs/afs/cell.c
index 3c090b7555e..ca0a3cf9379 100644
--- a/fs/afs/cell.c
+++ b/fs/afs/cell.c
@@ -179,7 +179,7 @@ struct afs_cell *afs_cell_create(const char *name, unsigned namesz,
 	/* put it up for caching (this never returns an error) */
 	cell->cache = fscache_acquire_cookie(afs_cache_netfs.primary_index,
 					     &afs_cell_cache_index_def,
-					     cell);
+					     cell, true);
 #endif
 
 	/* add to the cell lists */
diff --git a/fs/afs/inode.c b/fs/afs/inode.c
index 789bc253b5f..ce25d755b7a 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -259,7 +259,7 @@ struct inode *afs_iget(struct super_block *sb, struct key *key,
 #ifdef CONFIG_AFS_FSCACHE
 	vnode->cache = fscache_acquire_cookie(vnode->volume->cache,
 					      &afs_vnode_cache_index_def,
-					      vnode);
+					      vnode, true);
 #endif
 
 	ret = afs_inode_map_status(vnode, key);
diff --git a/fs/afs/vlocation.c b/fs/afs/vlocation.c
index 57bcb159653..b6df2e83809 100644
--- a/fs/afs/vlocation.c
+++ b/fs/afs/vlocation.c
@@ -308,7 +308,8 @@ static int afs_vlocation_fill_in_record(struct afs_vlocation *vl,
 	/* see if we have an in-cache copy (will set vl->valid if there is) */
 #ifdef CONFIG_AFS_FSCACHE
 	vl->cache = fscache_acquire_cookie(vl->cell->cache,
-					   &afs_vlocation_cache_index_def, vl);
+					   &afs_vlocation_cache_index_def, vl,
+					   true);
 #endif
 
 	if (vl->valid) {
diff --git a/fs/afs/volume.c b/fs/afs/volume.c
index 401eeb21869..2b607257820 100644
--- a/fs/afs/volume.c
+++ b/fs/afs/volume.c
@@ -131,7 +131,7 @@ struct afs_volume *afs_volume_lookup(struct afs_mount_params *params)
 #ifdef CONFIG_AFS_FSCACHE
 	volume->cache = fscache_acquire_cookie(vlocation->cache,
 					       &afs_volume_cache_index_def,
-					       volume);
+					       volume, true);
 #endif
 	afs_get_vlocation(vlocation);
 	volume->vlocation = vlocation;
diff --git a/fs/cachefiles/interface.c b/fs/cachefiles/interface.c
index 43eb5592cde..00baf141998 100644
--- a/fs/cachefiles/interface.c
+++ b/fs/cachefiles/interface.c
@@ -270,7 +270,7 @@ static void cachefiles_drop_object(struct fscache_object *_object)
 #endif
 
 	/* delete retired objects */
-	if (test_bit(FSCACHE_COOKIE_RETIRED, &object->fscache.cookie->flags) &&
+	if (test_bit(FSCACHE_OBJECT_RETIRED, &object->fscache.flags) &&
 	    _object != cache->cache.fsdef
 	    ) {
 		_debug("- retire object OBJ%x", object->fscache.debug_id);
diff --git a/fs/ceph/cache.c b/fs/ceph/cache.c
index 6bfe65e0b03..7db2e6ca4b8 100644
--- a/fs/ceph/cache.c
+++ b/fs/ceph/cache.c
@@ -68,7 +68,7 @@ int ceph_fscache_register_fs(struct ceph_fs_client* fsc)
 {
 	fsc->fscache = fscache_acquire_cookie(ceph_cache_netfs.primary_index,
 					      &ceph_fscache_fsid_object_def,
-					      fsc);
+					      fsc, true);
 
 	if (fsc->fscache == NULL) {
 		pr_err("Unable to resgister fsid: %p fscache cookie", fsc);
@@ -204,7 +204,7 @@ void ceph_fscache_register_inode_cookie(struct ceph_fs_client* fsc,
 
 	ci->fscache = fscache_acquire_cookie(fsc->fscache,
 					     &ceph_fscache_inode_object_def,
-					     ci);
+					     ci, true);
 done:
 	mutex_unlock(&inode->i_mutex);
 
diff --git a/fs/cifs/fscache.c b/fs/cifs/fscache.c
index 2f4bc5a5805..fe2492d2a8f 100644
--- a/fs/cifs/fscache.c
+++ b/fs/cifs/fscache.c
@@ -27,7 +27,7 @@ void cifs_fscache_get_client_cookie(struct TCP_Server_Info *server)
 {
 	server->fscache =
 		fscache_acquire_cookie(cifs_fscache_netfs.primary_index,
-				&cifs_fscache_server_index_def, server);
+				&cifs_fscache_server_index_def, server, true);
 	cifs_dbg(FYI, "%s: (0x%p/0x%p)\n",
 		 __func__, server, server->fscache);
 }
@@ -46,7 +46,7 @@ void cifs_fscache_get_super_cookie(struct cifs_tcon *tcon)
 
 	tcon->fscache =
 		fscache_acquire_cookie(server->fscache,
-				&cifs_fscache_super_index_def, tcon);
+				&cifs_fscache_super_index_def, tcon, true);
 	cifs_dbg(FYI, "%s: (0x%p/0x%p)\n",
 		 __func__, server->fscache, tcon->fscache);
 }
@@ -69,7 +69,7 @@ static void cifs_fscache_enable_inode_cookie(struct inode *inode)
 
 	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_FSCACHE) {
 		cifsi->fscache = fscache_acquire_cookie(tcon->fscache,
-				&cifs_fscache_inode_object_def, cifsi);
+				&cifs_fscache_inode_object_def, cifsi, true);
 		cifs_dbg(FYI, "%s: got FH cookie (0x%p/0x%p)\n",
 			 __func__, tcon->fscache, cifsi->fscache);
 	}
@@ -119,7 +119,7 @@ void cifs_fscache_reset_inode_cookie(struct inode *inode)
 		cifsi->fscache = fscache_acquire_cookie(
 					cifs_sb_master_tcon(cifs_sb)->fscache,
 					&cifs_fscache_inode_object_def,
-					cifsi);
+					cifsi, true);
 		cifs_dbg(FYI, "%s: new cookie 0x%p oldcookie 0x%p\n",
 			 __func__, cifsi->fscache, old);
 	}
diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index d851aa555d2..29d7feb62cf 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -58,15 +58,16 @@ void fscache_cookie_init_once(void *_cookie)
 struct fscache_cookie *__fscache_acquire_cookie(
 	struct fscache_cookie *parent,
 	const struct fscache_cookie_def *def,
-	void *netfs_data)
+	void *netfs_data,
+	bool enable)
 {
 	struct fscache_cookie *cookie;
 
 	BUG_ON(!def);
 
-	_enter("{%s},{%s},%p",
+	_enter("{%s},{%s},%p,%u",
 	       parent ? (char *) parent->def->name : "<no-parent>",
-	       def->name, netfs_data);
+	       def->name, netfs_data, enable);
 
 	fscache_stat(&fscache_n_acquires);
 
@@ -106,7 +107,7 @@ struct fscache_cookie *__fscache_acquire_cookie(
 	cookie->def		= def;
 	cookie->parent		= parent;
 	cookie->netfs_data	= netfs_data;
-	cookie->flags		= 0;
+	cookie->flags		= (1 << FSCACHE_COOKIE_NO_DATA_YET);
 
 	/* radix tree insertion won't use the preallocation pool unless it's
 	 * told it may not wait */
@@ -124,16 +125,22 @@ struct fscache_cookie *__fscache_acquire_cookie(
 		break;
 	}
 
-	/* if the object is an index then we need do nothing more here - we
-	 * create indices on disk when we need them as an index may exist in
-	 * multiple caches */
-	if (cookie->def->type != FSCACHE_COOKIE_TYPE_INDEX) {
-		if (fscache_acquire_non_index_cookie(cookie) < 0) {
-			atomic_dec(&parent->n_children);
-			__fscache_cookie_put(cookie);
-			fscache_stat(&fscache_n_acquires_nobufs);
-			_leave(" = NULL");
-			return NULL;
+	if (enable) {
+		/* if the object is an index then we need do nothing more here
+		 * - we create indices on disk when we need them as an index
+		 * may exist in multiple caches */
+		if (cookie->def->type != FSCACHE_COOKIE_TYPE_INDEX) {
+			if (fscache_acquire_non_index_cookie(cookie) == 0) {
+				set_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags);
+			} else {
+				atomic_dec(&parent->n_children);
+				__fscache_cookie_put(cookie);
+				fscache_stat(&fscache_n_acquires_nobufs);
+				_leave(" = NULL");
+				return NULL;
+			}
+		} else {
+			set_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags);
 		}
 	}
 
@@ -144,6 +151,39 @@ struct fscache_cookie *__fscache_acquire_cookie(
 EXPORT_SYMBOL(__fscache_acquire_cookie);
 
 /*
+ * Enable a cookie to permit it to accept new operations.
+ */
+void __fscache_enable_cookie(struct fscache_cookie *cookie,
+			     bool (*can_enable)(void *data),
+			     void *data)
+{
+	_enter("%p", cookie);
+
+	wait_on_bit_lock(&cookie->flags, FSCACHE_COOKIE_ENABLEMENT_LOCK,
+			 fscache_wait_bit, TASK_UNINTERRUPTIBLE);
+
+	if (test_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags))
+		goto out_unlock;
+
+	if (can_enable && !can_enable(data)) {
+		/* The netfs decided it didn't want to enable after all */
+	} else if (cookie->def->type != FSCACHE_COOKIE_TYPE_INDEX) {
+		/* Wait for outstanding disablement to complete */
+		__fscache_wait_on_invalidate(cookie);
+
+		if (fscache_acquire_non_index_cookie(cookie) == 0)
+			set_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags);
+	} else {
+		set_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags);
+	}
+
+out_unlock:
+	clear_bit_unlock(FSCACHE_COOKIE_ENABLEMENT_LOCK, &cookie->flags);
+	wake_up_bit(&cookie->flags, FSCACHE_COOKIE_ENABLEMENT_LOCK);
+}
+EXPORT_SYMBOL(__fscache_enable_cookie);
+
+/*
  * acquire a non-index cookie
  * - this must make sure the index chain is instantiated and instantiate the
  *   object representation too
@@ -157,7 +197,7 @@ static int fscache_acquire_non_index_cookie(struct fscache_cookie *cookie)
 
 	_enter("");
 
-	cookie->flags = 1 << FSCACHE_COOKIE_UNAVAILABLE;
+	set_bit(FSCACHE_COOKIE_UNAVAILABLE, &cookie->flags);
 
 	/* now we need to see whether the backing objects for this cookie yet
 	 * exist, if not there'll be nothing to search */
@@ -180,9 +220,7 @@ static int fscache_acquire_non_index_cookie(struct fscache_cookie *cookie)
 
 	_debug("cache %s", cache->tag->name);
 
-	cookie->flags =
-		(1 << FSCACHE_COOKIE_LOOKING_UP) |
-		(1 << FSCACHE_COOKIE_NO_DATA_YET);
+	set_bit(FSCACHE_COOKIE_LOOKING_UP, &cookie->flags);
 
 	/* ask the cache to allocate objects for this cookie and its parent
 	 * chain */
@@ -398,7 +436,8 @@ void __fscache_invalidate(struct fscache_cookie *cookie)
 	if (!hlist_empty(&cookie->backing_objects)) {
 		spin_lock(&cookie->lock);
 
-		if (!hlist_empty(&cookie->backing_objects) &&
+		if (fscache_cookie_enabled(cookie) &&
+		    !hlist_empty(&cookie->backing_objects) &&
 		    !test_and_set_bit(FSCACHE_COOKIE_INVALIDATING,
 				      &cookie->flags)) {
 			object = hlist_entry(cookie->backing_objects.first,
@@ -452,10 +491,14 @@ void __fscache_update_cookie(struct fscache_cookie *cookie)
 
 	spin_lock(&cookie->lock);
 
-	/* update the index entry on disk in each cache backing this cookie */
-	hlist_for_each_entry(object,
-			     &cookie->backing_objects, cookie_link) {
-		fscache_raise_event(object, FSCACHE_OBJECT_EV_UPDATE);
+	if (fscache_cookie_enabled(cookie)) {
+		/* update the index entry on disk in each cache backing this
+		 * cookie.
+		 */
+		hlist_for_each_entry(object,
+				     &cookie->backing_objects, cookie_link) {
+			fscache_raise_event(object, FSCACHE_OBJECT_EV_UPDATE);
+		}
 	}
 
 	spin_unlock(&cookie->lock);
@@ -464,28 +507,14 @@ void __fscache_update_cookie(struct fscache_cookie *cookie)
 EXPORT_SYMBOL(__fscache_update_cookie);
 
 /*
- * release a cookie back to the cache
- * - the object will be marked as recyclable on disk if retire is true
- * - all dependents of this cookie must have already been unregistered
- *   (indices/files/pages)
+ * Disable a cookie to stop it from accepting new requests from the netfs.
  */
-void __fscache_relinquish_cookie(struct fscache_cookie *cookie, int retire)
+void __fscache_disable_cookie(struct fscache_cookie *cookie, bool invalidate)
 {
 	struct fscache_object *object;
+	bool awaken = false;
 
-	fscache_stat(&fscache_n_relinquishes);
-	if (retire)
-		fscache_stat(&fscache_n_relinquishes_retire);
-
-	if (!cookie) {
-		fscache_stat(&fscache_n_relinquishes_null);
-		_leave(" [no cookie]");
-		return;
-	}
-
-	_enter("%p{%s,%p,%d},%d",
-	       cookie, cookie->def->name, cookie->netfs_data,
-	       atomic_read(&cookie->n_active), retire);
+	_enter("%p,%u", cookie, invalidate);
 
 	ASSERTCMP(atomic_read(&cookie->n_active), >, 0);
 
@@ -495,24 +524,82 @@ void __fscache_relinquish_cookie(struct fscache_cookie *cookie, int retire)
 		BUG();
 	}
 
-	/* No further netfs-accessing operations on this cookie permitted */
-	set_bit(FSCACHE_COOKIE_RELINQUISHED, &cookie->flags);
-	if (retire)
-		set_bit(FSCACHE_COOKIE_RETIRED, &cookie->flags);
+	wait_on_bit_lock(&cookie->flags, FSCACHE_COOKIE_ENABLEMENT_LOCK,
+			 fscache_wait_bit, TASK_UNINTERRUPTIBLE);
+	if (!test_and_clear_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags))
+		goto out_unlock_enable;
+
+	/* If the cookie is being invalidated, wait for that to complete first
+	 * so that we can reuse the flag.
+	 */
+	__fscache_wait_on_invalidate(cookie);
+
+	/* Dispose of the backing objects */
+	set_bit(FSCACHE_COOKIE_INVALIDATING, &cookie->flags);
 
 	spin_lock(&cookie->lock);
-	hlist_for_each_entry(object, &cookie->backing_objects, cookie_link) {
-		fscache_raise_event(object, FSCACHE_OBJECT_EV_KILL);
+	if (!hlist_empty(&cookie->backing_objects)) {
+		hlist_for_each_entry(object, &cookie->backing_objects, cookie_link) {
+			if (invalidate)
+				set_bit(FSCACHE_OBJECT_RETIRED, &object->flags);
+			fscache_raise_event(object, FSCACHE_OBJECT_EV_KILL);
+		}
+	} else {
+		if (test_and_clear_bit(FSCACHE_COOKIE_INVALIDATING, &cookie->flags))
+			awaken = true;
 	}
 	spin_unlock(&cookie->lock);
+	if (awaken)
+		wake_up_bit(&cookie->flags, FSCACHE_COOKIE_INVALIDATING);
 
 	/* Wait for cessation of activity requiring access to the netfs (when
-	 * n_active reaches 0).
+	 * n_active reaches 0).  This makes sure outstanding reads and writes
+	 * have completed.
 	 */
 	if (!atomic_dec_and_test(&cookie->n_active))
 		wait_on_atomic_t(&cookie->n_active, fscache_wait_atomic_t,
 				 TASK_UNINTERRUPTIBLE);
 
+	/* Reset the cookie state if it wasn't relinquished */
+	if (!test_bit(FSCACHE_COOKIE_RELINQUISHED, &cookie->flags)) {
+		atomic_inc(&cookie->n_active);
+		set_bit(FSCACHE_COOKIE_NO_DATA_YET, &cookie->flags);
+	}
+
+out_unlock_enable:
+	clear_bit_unlock(FSCACHE_COOKIE_ENABLEMENT_LOCK, &cookie->flags);
+	wake_up_bit(&cookie->flags, FSCACHE_COOKIE_ENABLEMENT_LOCK);
+	_leave("");
+}
+EXPORT_SYMBOL(__fscache_disable_cookie);
+
+/*
+ * release a cookie back to the cache
+ * - the object will be marked as recyclable on disk if retire is true
+ * - all dependents of this cookie must have already been unregistered
+ *   (indices/files/pages)
+ */
+void __fscache_relinquish_cookie(struct fscache_cookie *cookie, bool retire)
+{
+	fscache_stat(&fscache_n_relinquishes);
+	if (retire)
+		fscache_stat(&fscache_n_relinquishes_retire);
+
+	if (!cookie) {
+		fscache_stat(&fscache_n_relinquishes_null);
+		_leave(" [no cookie]");
+		return;
+	}
+
+	_enter("%p{%s,%p,%d},%d",
+	       cookie, cookie->def->name, cookie->netfs_data,
+	       atomic_read(&cookie->n_active), retire);
+
+	/* No further netfs-accessing operations on this cookie permitted */
+	set_bit(FSCACHE_COOKIE_RELINQUISHED, &cookie->flags);
+
+	__fscache_disable_cookie(cookie, retire);
+
 	/* Clear pointers back to the netfs */
 	cookie->netfs_data	= NULL;
 	cookie->def		= NULL;
@@ -592,7 +679,8 @@ int __fscache_check_consistency(struct fscache_cookie *cookie)
 
 	spin_lock(&cookie->lock);
 
-	if (hlist_empty(&cookie->backing_objects))
+	if (!fscache_cookie_enabled(cookie) ||
+	    hlist_empty(&cookie->backing_objects))
 		goto inconsistent;
 	object = hlist_entry(cookie->backing_objects.first,
 			     struct fscache_object, cookie_link);
diff --git a/fs/fscache/fsdef.c b/fs/fscache/fsdef.c
index 10a2ade0bdf..5a117df2a9e 100644
--- a/fs/fscache/fsdef.c
+++ b/fs/fscache/fsdef.c
@@ -59,6 +59,7 @@ struct fscache_cookie fscache_fsdef_index = {
 	.lock		= __SPIN_LOCK_UNLOCKED(fscache_fsdef_index.lock),
 	.backing_objects = HLIST_HEAD_INIT,
 	.def		= &fscache_fsdef_index_def,
+	.flags		= 1 << FSCACHE_COOKIE_ENABLED,
 };
 EXPORT_SYMBOL(fscache_fsdef_index);
 
diff --git a/fs/fscache/netfs.c b/fs/fscache/netfs.c
index b1bb6117473..989f3940154 100644
--- a/fs/fscache/netfs.c
+++ b/fs/fscache/netfs.c
@@ -45,6 +45,7 @@ int __fscache_register_netfs(struct fscache_netfs *netfs)
 	netfs->primary_index->def		= &fscache_fsdef_netfs_def;
 	netfs->primary_index->parent		= &fscache_fsdef_index;
 	netfs->primary_index->netfs_data	= netfs;
+	netfs->primary_index->flags		= 1 << FSCACHE_COOKIE_ENABLED;
 
 	atomic_inc(&netfs->primary_index->parent->usage);
 	atomic_inc(&netfs->primary_index->parent->n_children);
diff --git a/fs/fscache/object.c b/fs/fscache/object.c
index 86d75a60b20..dcb82161777 100644
--- a/fs/fscache/object.c
+++ b/fs/fscache/object.c
@@ -495,6 +495,7 @@ void fscache_object_lookup_negative(struct fscache_object *object)
 		 * returning ENODATA.
 		 */
 		set_bit(FSCACHE_COOKIE_NO_DATA_YET, &cookie->flags);
+		clear_bit(FSCACHE_COOKIE_UNAVAILABLE, &cookie->flags);
 
 		_debug("wake up lookup %p", &cookie->flags);
 		clear_bit_unlock(FSCACHE_COOKIE_LOOKING_UP, &cookie->flags);
@@ -527,6 +528,7 @@ void fscache_obtained_object(struct fscache_object *object)
 
 		/* We do (presumably) have data */
 		clear_bit_unlock(FSCACHE_COOKIE_NO_DATA_YET, &cookie->flags);
+		clear_bit(FSCACHE_COOKIE_UNAVAILABLE, &cookie->flags);
 
 		/* Allow write requests to begin stacking up and read requests
 		 * to begin shovelling data.
@@ -679,7 +681,8 @@ static const struct fscache_state *fscache_drop_object(struct fscache_object *ob
 	 */
 	spin_lock(&cookie->lock);
 	hlist_del_init(&object->cookie_link);
-	if (test_and_clear_bit(FSCACHE_COOKIE_INVALIDATING, &cookie->flags))
+	if (hlist_empty(&cookie->backing_objects) &&
+	    test_and_clear_bit(FSCACHE_COOKIE_INVALIDATING, &cookie->flags))
 		awaken = true;
 	spin_unlock(&cookie->lock);
 
@@ -927,7 +930,7 @@ static const struct fscache_state *_fscache_invalidate_object(struct fscache_obj
 	 */
 	if (!fscache_use_cookie(object)) {
 		ASSERT(object->cookie->stores.rnode == NULL);
-		set_bit(FSCACHE_COOKIE_RETIRED, &cookie->flags);
+		set_bit(FSCACHE_OBJECT_RETIRED, &object->flags);
 		_leave(" [no cookie]");
 		return transit_to(KILL_OBJECT);
 	}
diff --git a/fs/fscache/page.c b/fs/fscache/page.c
index 0fe42a6d0e9..7f5c658af75 100644
--- a/fs/fscache/page.c
+++ b/fs/fscache/page.c
@@ -204,7 +204,8 @@ int __fscache_attr_changed(struct fscache_cookie *cookie)
 
 	spin_lock(&cookie->lock);
 
-	if (hlist_empty(&cookie->backing_objects))
+	if (!fscache_cookie_enabled(cookie) ||
+	    hlist_empty(&cookie->backing_objects))
 		goto nobufs;
 	object = hlist_entry(cookie->backing_objects.first,
 			     struct fscache_object, cookie_link);
@@ -410,7 +411,7 @@ int __fscache_read_or_alloc_page(struct fscache_cookie *cookie,
 		return -ERESTARTSYS;
 
 	op = fscache_alloc_retrieval(cookie, page->mapping,
-				     end_io_func,context);
+				     end_io_func, context);
 	if (!op) {
 		_leave(" = -ENOMEM");
 		return -ENOMEM;
@@ -419,7 +420,8 @@ int __fscache_read_or_alloc_page(struct fscache_cookie *cookie,
 
 	spin_lock(&cookie->lock);
 
-	if (hlist_empty(&cookie->backing_objects))
+	if (!fscache_cookie_enabled(cookie) ||
+	    hlist_empty(&cookie->backing_objects))
 		goto nobufs_unlock;
 	object = hlist_entry(cookie->backing_objects.first,
 			     struct fscache_object, cookie_link);
@@ -551,7 +553,8 @@ int __fscache_read_or_alloc_pages(struct fscache_cookie *cookie,
 
 	spin_lock(&cookie->lock);
 
-	if (hlist_empty(&cookie->backing_objects))
+	if (!fscache_cookie_enabled(cookie) ||
+	    hlist_empty(&cookie->backing_objects))
 		goto nobufs_unlock;
 	object = hlist_entry(cookie->backing_objects.first,
 			     struct fscache_object, cookie_link);
@@ -666,7 +669,8 @@ int __fscache_alloc_page(struct fscache_cookie *cookie,
 
 	spin_lock(&cookie->lock);
 
-	if (hlist_empty(&cookie->backing_objects))
+	if (!fscache_cookie_enabled(cookie) ||
+	    hlist_empty(&cookie->backing_objects))
 		goto nobufs_unlock;
 	object = hlist_entry(cookie->backing_objects.first,
 			     struct fscache_object, cookie_link);
@@ -938,7 +942,8 @@ int __fscache_write_page(struct fscache_cookie *cookie,
 	ret = -ENOBUFS;
 	spin_lock(&cookie->lock);
 
-	if (hlist_empty(&cookie->backing_objects))
+	if (!fscache_cookie_enabled(cookie) ||
+	    hlist_empty(&cookie->backing_objects))
 		goto nobufs;
 	object = hlist_entry(cookie->backing_objects.first,
 			     struct fscache_object, cookie_link);
diff --git a/fs/nfs/fscache.c b/fs/nfs/fscache.c
index 24d1d1c5fca..cd6e7efd430 100644
--- a/fs/nfs/fscache.c
+++ b/fs/nfs/fscache.c
@@ -39,7 +39,7 @@ void nfs_fscache_get_client_cookie(struct nfs_client *clp)
 	/* create a cache index for looking up filehandles */
 	clp->fscache = fscache_acquire_cookie(nfs_fscache_netfs.primary_index,
 					      &nfs_fscache_server_index_def,
-					      clp);
+					      clp, true);
 	dfprintk(FSCACHE, "NFS: get client cookie (0x%p/0x%p)\n",
 		 clp, clp->fscache);
 }
@@ -139,7 +139,7 @@ void nfs_fscache_get_super_cookie(struct super_block *sb, const char *uniq, int
 	/* create a cache index for looking up filehandles */
 	nfss->fscache = fscache_acquire_cookie(nfss->nfs_client->fscache,
 					       &nfs_fscache_super_index_def,
-					       nfss);
+					       nfss, true);
 	dfprintk(FSCACHE, "NFS: get superblock cookie (0x%p/0x%p)\n",
 		 nfss, nfss->fscache);
 	return;
@@ -200,7 +200,7 @@ static void nfs_fscache_enable_inode_cookie(struct inode *inode)
 		nfsi->fscache = fscache_acquire_cookie(
 			NFS_SB(sb)->fscache,
 			&nfs_fscache_inode_object_def,
-			nfsi);
+			nfsi, true);
 
 		dfprintk(FSCACHE, "NFS: get FH cookie (0x%p/0x%p/0x%p)\n",
 			 sb, nfsi, nfsi->fscache);
@@ -327,7 +327,7 @@ void nfs_fscache_reset_inode_cookie(struct inode *inode)
 		nfsi->fscache = fscache_acquire_cookie(
 			nfss->nfs_client->fscache,
 			&nfs_fscache_inode_object_def,
-			nfsi);
+			nfsi, true);
 
 		dfprintk(FSCACHE,
 			 "NFS: revalidation new cookie (0x%p/0x%p/0x%p/0x%p)\n",
diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 96a2b66f596..771484993ca 100644
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -308,36 +308,6 @@ struct fscache_cache_ops {
 	void (*dissociate_pages)(struct fscache_cache *cache);
 };
 
-/*
- * data file or index object cookie
- * - a file will only appear in one cache
- * - a request to cache a file may or may not be honoured, subject to
- *   constraints such as disk space
- * - indices are created on disk just-in-time
- */
-struct fscache_cookie {
-	atomic_t			usage;		/* number of users of this cookie */
-	atomic_t			n_children;	/* number of children of this cookie */
-	atomic_t			n_active;	/* number of active users of netfs ptrs */
-	spinlock_t			lock;
-	spinlock_t			stores_lock;	/* lock on page store tree */
-	struct hlist_head		backing_objects; /* object(s) backing this file/index */
-	const struct fscache_cookie_def	*def;		/* definition */
-	struct fscache_cookie		*parent;	/* parent of this entry */
-	void				*netfs_data;	/* back pointer to netfs */
-	struct radix_tree_root		stores;		/* pages to be stored on this cookie */
-#define FSCACHE_COOKIE_PENDING_TAG	0		/* pages tag: pending write to cache */
-#define FSCACHE_COOKIE_STORING_TAG	1		/* pages tag: writing to cache */
-
-	unsigned long			flags;
-#define FSCACHE_COOKIE_LOOKING_UP	0	/* T if non-index cookie being looked up still */
-#define FSCACHE_COOKIE_NO_DATA_YET	1	/* T if new object with no cached data yet */
-#define FSCACHE_COOKIE_UNAVAILABLE	2	/* T if cookie is unavailable (error, etc) */
-#define FSCACHE_COOKIE_INVALIDATING	3	/* T if cookie is being invalidated */
-#define FSCACHE_COOKIE_RELINQUISHED	4	/* T if cookie has been relinquished */
-#define FSCACHE_COOKIE_RETIRED		5	/* T if cookie was retired */
-};
-
 extern struct fscache_cookie fscache_fsdef_index;
 
 /*
@@ -400,6 +370,7 @@ struct fscache_object {
 #define FSCACHE_OBJECT_IS_LIVE		3	/* T if object is not withdrawn or relinquished */
 #define FSCACHE_OBJECT_IS_LOOKED_UP	4	/* T if object has been looked up */
 #define FSCACHE_OBJECT_IS_AVAILABLE	5	/* T if object has become active */
+#define FSCACHE_OBJECT_RETIRED		6	/* T if object was retired on relinquishment */
 
 	struct list_head	cache_link;	/* link in cache->object_list */
 	struct hlist_node	cookie_link;	/* link in cookie->backing_objects */
diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index 19b46458e4e..115bb81912c 100644
--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -167,6 +167,42 @@ struct fscache_netfs {
 };
 
 /*
+ * data file or index object cookie
+ * - a file will only appear in one cache
+ * - a request to cache a file may or may not be honoured, subject to
+ *   constraints such as disk space
+ * - indices are created on disk just-in-time
+ */
+struct fscache_cookie {
+	atomic_t			usage;		/* number of users of this cookie */
+	atomic_t			n_children;	/* number of children of this cookie */
+	atomic_t			n_active;	/* number of active users of netfs ptrs */
+	spinlock_t			lock;
+	spinlock_t			stores_lock;	/* lock on page store tree */
+	struct hlist_head		backing_objects; /* object(s) backing this file/index */
+	const struct fscache_cookie_def	*def;		/* definition */
+	struct fscache_cookie		*parent;	/* parent of this entry */
+	void				*netfs_data;	/* back pointer to netfs */
+	struct radix_tree_root		stores;		/* pages to be stored on this cookie */
+#define FSCACHE_COOKIE_PENDING_TAG	0		/* pages tag: pending write to cache */
+#define FSCACHE_COOKIE_STORING_TAG	1		/* pages tag: writing to cache */
+
+	unsigned long			flags;
+#define FSCACHE_COOKIE_LOOKING_UP	0	/* T if non-index cookie being looked up still */
+#define FSCACHE_COOKIE_NO_DATA_YET	1	/* T if new object with no cached data yet */
+#define FSCACHE_COOKIE_UNAVAILABLE	2	/* T if cookie is unavailable (error, etc) */
+#define FSCACHE_COOKIE_INVALIDATING	3	/* T if cookie is being invalidated */
+#define FSCACHE_COOKIE_RELINQUISHED	4	/* T if cookie has been relinquished */
+#define FSCACHE_COOKIE_ENABLED		5	/* T if cookie is enabled */
+#define FSCACHE_COOKIE_ENABLEMENT_LOCK	6	/* T if cookie is being en/disabled */
+};
+
+static inline bool fscache_cookie_enabled(struct fscache_cookie *cookie)
+{
+	return test_bit(FSCACHE_COOKIE_ENABLED, &cookie->flags);
+}
+
+/*
  * slow-path functions for when there is actually caching available, and the
  * netfs does actually have a valid token
  * - these are not to be called directly
@@ -181,8 +217,8 @@ extern void __fscache_release_cache_tag(struct fscache_cache_tag *);
 extern struct fscache_cookie *__fscache_acquire_cookie(
 	struct fscache_cookie *,
 	const struct fscache_cookie_def *,
-	void *);
-extern void __fscache_relinquish_cookie(struct fscache_cookie *, int);
+	void *, bool);
+extern void __fscache_relinquish_cookie(struct fscache_cookie *, bool);
 extern int __fscache_check_consistency(struct fscache_cookie *);
 extern void __fscache_update_cookie(struct fscache_cookie *);
 extern int __fscache_attr_changed(struct fscache_cookie *);
@@ -211,6 +247,9 @@ extern void __fscache_uncache_all_inode_pages(struct fscache_cookie *,
 					      struct inode *);
 extern void __fscache_readpages_cancel(struct fscache_cookie *cookie,
 				       struct list_head *pages);
+extern void __fscache_disable_cookie(struct fscache_cookie *, bool);
+extern void __fscache_enable_cookie(struct fscache_cookie *,
+				    bool (*)(void *), void *);
 
 /**
  * fscache_register_netfs - Register a filesystem as desiring caching services
@@ -289,6 +328,7 @@ void fscache_release_cache_tag(struct fscache_cache_tag *tag)
  * @def: A description of the cache object, including callback operations
  * @netfs_data: An arbitrary piece of data to be kept in the cookie to
  * represent the cache object to the netfs
+ * @enable: Whether or not to enable a data cookie immediately
  *
  * This function is used to inform FS-Cache about part of an index hierarchy
  * that can be used to locate files.  This is done by requesting a cookie for
@@ -301,10 +341,12 @@ static inline
 struct fscache_cookie *fscache_acquire_cookie(
 	struct fscache_cookie *parent,
 	const struct fscache_cookie_def *def,
-	void *netfs_data)
+	void *netfs_data,
+	bool enable)
 {
-	if (fscache_cookie_valid(parent))
-		return __fscache_acquire_cookie(parent, def, netfs_data);
+	if (fscache_cookie_valid(parent) && fscache_cookie_enabled(parent))
+		return __fscache_acquire_cookie(parent, def, netfs_data,
+						enable);
 	else
 		return NULL;
 }
@@ -322,7 +364,7 @@ struct fscache_cookie *fscache_acquire_cookie(
  * description.
  */
 static inline
-void fscache_relinquish_cookie(struct fscache_cookie *cookie, int retire)
+void fscache_relinquish_cookie(struct fscache_cookie *cookie, bool retire)
 {
 	if (fscache_cookie_valid(cookie))
 		__fscache_relinquish_cookie(cookie, retire);
@@ -341,7 +383,7 @@ void fscache_relinquish_cookie(struct fscache_cookie *cookie, int retire)
 static inline
 int fscache_check_consistency(struct fscache_cookie *cookie)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_check_consistency(cookie);
 	else
 		return 0;
@@ -360,7 +402,7 @@ int fscache_check_consistency(struct fscache_cookie *cookie)
 static inline
 void fscache_update_cookie(struct fscache_cookie *cookie)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		__fscache_update_cookie(cookie);
 }
 
@@ -407,7 +449,7 @@ void fscache_unpin_cookie(struct fscache_cookie *cookie)
 static inline
 int fscache_attr_changed(struct fscache_cookie *cookie)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_attr_changed(cookie);
 	else
 		return -ENOBUFS;
@@ -429,7 +471,7 @@ int fscache_attr_changed(struct fscache_cookie *cookie)
 static inline
 void fscache_invalidate(struct fscache_cookie *cookie)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		__fscache_invalidate(cookie);
 }
 
@@ -503,7 +545,7 @@ int fscache_read_or_alloc_page(struct fscache_cookie *cookie,
 			       void *context,
 			       gfp_t gfp)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_read_or_alloc_page(cookie, page, end_io_func,
 						    context, gfp);
 	else
@@ -554,7 +596,7 @@ int fscache_read_or_alloc_pages(struct fscache_cookie *cookie,
 				void *context,
 				gfp_t gfp)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_read_or_alloc_pages(cookie, mapping, pages,
 						     nr_pages, end_io_func,
 						     context, gfp);
@@ -585,7 +627,7 @@ int fscache_alloc_page(struct fscache_cookie *cookie,
 		       struct page *page,
 		       gfp_t gfp)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_alloc_page(cookie, page, gfp);
 	else
 		return -ENOBUFS;
@@ -634,7 +676,7 @@ int fscache_write_page(struct fscache_cookie *cookie,
 		       struct page *page,
 		       gfp_t gfp)
 {
-	if (fscache_cookie_valid(cookie))
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
 		return __fscache_write_page(cookie, page, gfp);
 	else
 		return -ENOBUFS;
@@ -744,4 +786,47 @@ void fscache_uncache_all_inode_pages(struct fscache_cookie *cookie,
 		__fscache_uncache_all_inode_pages(cookie, inode);
 }
 
+/**
+ * fscache_disable_cookie - Disable a cookie
+ * @cookie: The cookie representing the cache object
+ * @invalidate: Invalidate the backing object
+ *
+ * Disable a cookie from accepting further alloc, read, write, invalidate,
+ * update or acquire operations.  Outstanding operations can still be waited
+ * upon and pages can still be uncached and the cookie relinquished.
+ *
+ * This will not return until all outstanding operations have completed.
+ *
+ * If @invalidate is set, then the backing object will be invalidated and
+ * detached, otherwise it will just be detached.
+ */
+static inline
+void fscache_disable_cookie(struct fscache_cookie *cookie, bool invalidate)
+{
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
+		__fscache_disable_cookie(cookie, invalidate);
+}
+
+/**
+ * fscache_enable_cookie - Reenable a cookie
+ * @cookie: The cookie representing the cache object
+ * @can_enable: A function to permit enablement once lock is held
+ * @data: Data for can_enable()
+ *
+ * Reenable a previously disabled cookie, allowing it to accept further alloc,
+ * read, write, invalidate, update or acquire operations.  An attempt will be
+ * made to immediately reattach the cookie to a backing object.
+ *
+ * The can_enable() function is called (if not NULL) once the enablement lock
+ * is held to rule on whether enablement is still permitted to go ahead.
+ */
+static inline
+void fscache_enable_cookie(struct fscache_cookie *cookie,
+			   bool (*can_enable)(void *data),
+			   void *data)
+{
+	if (fscache_cookie_valid(cookie) && !fscache_cookie_enabled(cookie))
+		__fscache_enable_cookie(cookie, can_enable, data);
+}
+
 #endif /* _LINUX_FSCACHE_H */