Add an asynchronous buffer read-ahead facility. Nobody

uses it for now, but I needed it for some tuning tests,
and it is potentially useful for others.

fs/buffer.c

@@ -1447,6 +1447,28 @@ __getblk(struct block_device *bdev, sector_t block, int size)
 }
 EXPORT_SYMBOL(__getblk);
 
+/*
+ * Do async read-ahead on a buffer..
+ */
+void
+__breadahead(struct block_device *bdev, sector_t block, int size)
+{
+	struct buffer_head *bh = __getblk(bdev, block, size);
+	if (!test_set_buffer_locked(bh)) {
+		if (!buffer_uptodate(bh)) {
+			/*
+			 * This eats the bh count from __getblk() and
+			 * unlocks when the read is done.
+			 */
+			bh->b_end_io = end_buffer_io_sync;
+			submit_bh(READ, bh);
+			return;
+		}
+		unlock_buffer(bh);
+	}
+	brelse(bh);
+}
+
 /**
  *  __bread() - reads a specified block and returns the bh
  *  @block: number of block

include/linux/buffer_head.h

@@ -167,6 +167,7 @@ struct buffer_head *__find_get_block(struct block_device *, sector_t, int);
 struct buffer_head * __getblk(struct block_device *, sector_t, int);
 void __brelse(struct buffer_head *);
 void __bforget(struct buffer_head *);
+void __breadahead(struct block_device *, sector_t block, int size);
 struct buffer_head *__bread(struct block_device *, sector_t block, int size);
 struct buffer_head *alloc_buffer_head(int gfp_flags);
 void free_buffer_head(struct buffer_head * bh);
@@ -241,6 +242,12 @@ sb_bread(struct super_block *sb, sector_t block)
 	return __bread(sb->s_bdev, block, sb->s_blocksize);
 }
 
+static inline void
+sb_breadahead(struct super_block *sb, sector_t block)
+{
+	__breadahead(sb->s_bdev, block, sb->s_blocksize);
+}
+
 static inline struct buffer_head *
 sb_getblk(struct super_block *sb, sector_t block)
 {