[PATCH 3/6] writeback: enforce s_umount locking in writeback_inodes_sb

[Christoph Hellwig <hch@lst.de>]

Make sure that not only sync_filesystem but all callers of writeback_inodes_sb
have the superblock protected against remount.  As-is this disables all
functionality for these callers, but the next patch relies on this locking to
fix writeback_inodes_sb for sync_filesystem.

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

Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c	2010-06-08 16:25:35.000000000 +0200
+++ linux-2.6/fs/fs-writeback.c	2010-06-08 16:27:47.846255651 +0200
@@ -1180,6 +1180,8 @@ void writeback_inodes_sb(struct super_bl
 		.sync_mode	= WB_SYNC_NONE,
 	};
 
+	WARN_ON(!rwsem_is_locked(&sb->s_umount));
+
 	args.nr_pages = nr_dirty + nr_unstable +
 			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
 
@@ -1197,7 +1199,9 @@ EXPORT_SYMBOL(writeback_inodes_sb);
 int writeback_inodes_sb_if_idle(struct super_block *sb)
 {
 	if (!writeback_in_progress(sb->s_bdi)) {
+		down_read(&sb->s_umount);
 		writeback_inodes_sb(sb);
+		up_read(&sb->s_umount);
 		return 1;
 	} else
 		return 0;
@@ -1220,6 +1224,8 @@ void sync_inodes_sb(struct super_block *
 		.range_cyclic	= 0,
 	};
 
+	WARN_ON(!rwsem_is_locked(&sb->s_umount));
+
 	bdi_queue_work_onstack(&args);
 	wait_sb_inodes(sb);
 }
Index: linux-2.6/fs/ubifs/budget.c
===================================================================
--- linux-2.6.orig/fs/ubifs/budget.c	2010-06-08 16:22:16.000000000 +0200
+++ linux-2.6/fs/ubifs/budget.c	2010-06-08 16:26:50.464004778 +0200
@@ -62,7 +62,9 @@
  */
 static void shrink_liability(struct ubifs_info *c, int nr_to_write)
 {
+	down_read(&c->vfs_sb->s_umount);
 	writeback_inodes_sb(c->vfs_sb);
+	up_read(&c->vfs_sb->s_umount);
 }
 
 /**