i_alloc_sem is a rather special rw_semaphore.  It's the last one that may
be released by a non-owner, and it's write side is always mirrored by
real exclusion.  It's intended use it to wait for all pending direct I/O
requests to finish before starting a truncate.

Replace it with a hand-grown construct:

 - exclusion for truncates is already guaranteed by i_mutex, so it can
   simply fall way
 - the reader side is replaced by an i_dio_count member in struct inode
   that counts the number of pending direct I/O requests.  Truncate can't
   proceed as long as it's non-zero
 - when i_dio_count reaches non-zero we wake up a pending truncate using
   wake_up_bit on a new bit in i_flags
 - new references to i_dio_count can't appear while we are waiting for
   it to read zero because the direct I/O count always needs i_mutex
   (or an equivalent like XFS's i_iolock) for starting a new operation.

This scheme is much simpler, and saves the space of a spinlock_t and a
struct list_head in struct inode (typically 160 bytes on a non-debug 64-bit
system).

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: linux-2.6/fs/direct-io.c
===================================================================

--- linux-2.6.orig/fs/direct-io.c	2011-06-20 14:55:31.000000000 +0200
+++ linux-2.6/fs/direct-io.c	2011-06-20 14:55:34.602490284 +0200

@@ -136,6 +136,27 @@ struct dio {
 };
 
 /*
+ * Wait for outstanding DIO requests to finish.  Must be locked against
+ * increments of i_dio_count by i_mutex.
+ */
+void inode_dio_wait(struct inode *inode)
+{
+	might_sleep();
+	while (atomic_read(&inode->i_dio_count)) {
+		wait_on_bit(&inode->i_state, __I_DIO_WAKEUP, inode_wait,
+			    TASK_UNINTERRUPTIBLE);
+	}
+}
+EXPORT_SYMBOL_GPL(inode_dio_wait);
+
+void inode_dio_wake(struct inode *inode)
+{
+	if (atomic_dec_and_test(&inode->i_dio_count))
+		wake_up_bit(&inode->i_state, __I_DIO_WAKEUP);
+}
+EXPORT_SYMBOL_GPL(inode_dio_wake);