[XFS] extend completions to provide XFS object flush requirements

XFS object flushing doesn't quite match existing completion semantics.  It
mixed exclusive access with completion.  That is, we need to mark an object as
being flushed before flushing it to disk, and then block any other attempt to
flush it until the completion occurs.  We do this but adding an extra count to
the completion before we start using them.  However, we still need to
determine if there is a completion in progress, and allow no-blocking attempts
fo completions to decrement the count.

To do this we introduce:

int try_wait_for_completion(struct completion *x)
	returns a failure status if done == 0, otherwise decrements done
	to zero and returns a "started" status. This is provided
	to allow counted completions to begin safely while holding
	object locks in inverted order.

int completion_done(struct completion *x)
	returns 1 if there is no waiter, 0 if there is a waiter
	(i.e. a completion in progress).

This replaces the use of semaphores for providing this exclusion
and completion mechanism.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31816a
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>

include/linux/completion.h

@@ -55,4 +55,49 @@ extern void complete_all(struct completion *);
 
 #define INIT_COMPLETION(x)	((x).done = 0)
 
+
+/**
+ *	try_wait_for_completion - try to decrement a completion without blocking
+ *	@x:	completion structure
+ *
+ *	Returns: 0 if a decrement cannot be done without blocking
+ *		 1 if a decrement succeeded.
+ *
+ *	If a completion is being used as a counting completion,
+ *	attempt to decrement the counter without blocking. This
+ *	enables us to avoid waiting if the resource the completion
+ *	is protecting is not available.
+ */
+static inline bool try_wait_for_completion(struct completion *x)
+{
+	int ret = 1;
+
+	spin_lock_irq(&x->wait.lock);
+	if (!x->done)
+		ret = 0;
+	else
+		x->done--;
+	spin_unlock_irq(&x->wait.lock);
+	return ret;
+}
+
+/**
+ *	completion_done - Test to see if a completion has any waiters
+ *	@x:	completion structure
+ *
+ *	Returns: 0 if there are waiters (wait_for_completion() in progress)
+ *		 1 if there are no waiters.
+ *
+ */
+static inline bool completion_done(struct completion *x)
+{
+	int ret = 1;
+
+	spin_lock_irq(&x->wait.lock);
+	if (!x->done)
+		ret = 0;
+	spin_unlock_irq(&x->wait.lock);
+	return ret;
+}
+
 #endif