[PATCH] writeback: fix pin_sb_for_writeback

[Christoph Hellwig <hch@lst.de>]

We need to check for s_instances to make sure we don't bother working
against a filesystem that is beeing unmounted, and we need to call
put_super to make sure a superblock is freed when we race against
umount.  Also no need to keep sb_lock after we got a reference on it.

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

---

	Note: this is against Jens' tree with the writeback changes applied.
The logic in mainline is the same, but the context is slightl different.

Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c	2010-06-09 13:15:29.152004893 +0200
+++ linux-2.6/fs/fs-writeback.c	2010-06-09 13:15:29.946253462 +0200
@@ -534,19 +534,21 @@ select_queue:
 static bool pin_sb_for_writeback(struct super_block *sb)
 {
 	spin_lock(&sb_lock);
+	if (list_empty(&sb->s_instances)) {
+		spin_unlock(&sb_lock);
+		return false;
+	}
+
 	sb->s_count++;
+	spin_unlock(&sb_lock);
+
 	if (down_read_trylock(&sb->s_umount)) {
-		if (sb->s_root) {
-			spin_unlock(&sb_lock);
+		if (sb->s_root)
 			return true;
-		}
-		/*
-		 * umounted, drop rwsem again and fall through to failure
-		 */
 		up_read(&sb->s_umount);
 	}
-	sb->s_count--;
-	spin_unlock(&sb_lock);
+
+	put_super(sb);
 	return false;
 }