FS-Cache: Add use/unuse/wake cookie wrappers

Add wrapper functions for dealing with cookie->n_active:

 (*) __fscache_use_cookie() to increment it.

 (*) __fscache_unuse_cookie() to decrement and test against zero.

 (*) __fscache_wake_unused_cookie() to wake up anyone waiting for it to reach
     zero.

The second and third are split so that the third can be done after cookie->lock
has been released in case the waiter wakes up whilst we're still holding it and
tries to get it.

We will need to wake-on-zero once the cookie disablement patch is applied
because it will then be possible to see n_active become zero without the cookie
being relinquished.

Also move the cookie usement out of fscache_attr_changed_op() and into
fscache_attr_changed() and the operation struct so that cookie disablement
will be able to track it.

Whilst we're at it, only increment n_active if we're about to do
fscache_submit_op() so that we don't have to deal with undoing it if anything
earlier fails.  Possibly this should be moved into fscache_submit_op() which
could look at FSCACHE_OP_UNUSE_COOKIE.

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

fs/fscache/cookie.c

@@ -568,6 +568,7 @@ int __fscache_check_consistency(struct fscache_cookie *cookie)
 {
 	struct fscache_operation *op;
 	struct fscache_object *object;
+	bool wake_cookie = false;
 	int ret;
 
 	_enter("%p,", cookie);
@@ -600,7 +601,7 @@ int __fscache_check_consistency(struct fscache_cookie *cookie)
 
 	op->debug_id = atomic_inc_return(&fscache_op_debug_id);
 
-	atomic_inc(&cookie->n_active);
+	__fscache_use_cookie(cookie);
 	if (fscache_submit_op(object, op) < 0)
 		goto submit_failed;
 
@@ -622,9 +623,11 @@ int __fscache_check_consistency(struct fscache_cookie *cookie)
 	return ret;
 
 submit_failed:
-	atomic_dec(&cookie->n_active);
+	wake_cookie = __fscache_unuse_cookie(cookie);
 inconsistent:
 	spin_unlock(&cookie->lock);
+	if (wake_cookie)
+		__fscache_wake_unused_cookie(cookie);
 	kfree(op);
 	_leave(" = -ESTALE");
 	return -ESTALE;