SLOW_WORK: Allow the owner of a work item to determine if it is queued or not

Add a function (slow_work_is_queued()) to permit the owner of a work item to
determine if the item is queued or not.

The work item is counted as being queued if it is actually on the queue, not
just if it is pending.  If it is executing and pending, then it is not on the
queue, but will rather be put back on the queue when execution finishes.

This permits a caller to quickly work out if it may be able to put another,
dependent work item on the queue behind it, or whether it will have to wait
till that is finished.

This can be used by CacheFiles to work out whether the creation a new object
can be immediately deferred when it has to wait for an old object to be
deleted, or whether a wait must take place.  If a wait is necessary, then the
slow-work thread can otherwise get blocked, preventing the deletion from
taking place.

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

Documentation/slow-work.txt

@@ -144,6 +144,21 @@ from being taken away before it completes.  module should almost certainly be
 THIS_MODULE.
 
 
+================
+HELPER FUNCTIONS
+================
+
+The slow-work facility provides a function by which it can be determined
+whether or not an item is queued for later execution:
+
+	bool queued = slow_work_is_queued(struct slow_work *work);
+
+If it returns false, then the item is not on the queue (it may be executing
+with a requeue pending).  This can be used to work out whether an item on which
+another depends is on the queue, thus allowing a dependent item to be queued
+after it.
+
+
 ===============
 ITEM OPERATIONS
 ===============

include/linux/slow-work.h

@@ -120,6 +120,25 @@ static inline void vslow_work_init(struct slow_work *work,
 	INIT_LIST_HEAD(&work->link);
 }
 
+/**
+ * slow_work_is_queued - Determine if a slow work item is on the work queue
+ * work: The work item to test
+ *
+ * Determine if the specified slow-work item is on the work queue.  This
+ * returns true if it is actually on the queue.
+ *
+ * If the item is executing and has been marked for requeue when execution
+ * finishes, then false will be returned.
+ *
+ * Anyone wishing to wait for completion of execution can wait on the
+ * SLOW_WORK_EXECUTING bit.
+ */
+static inline bool slow_work_is_queued(struct slow_work *work)
+{
+	unsigned long flags = work->flags;
+	return flags & SLOW_WORK_PENDING && !(flags & SLOW_WORK_EXECUTING);
+}
+
 extern int slow_work_enqueue(struct slow_work *work);
 extern void slow_work_cancel(struct slow_work *work);
 extern int slow_work_register_user(struct module *owner);