[PATCH 07/16] writeback: enforce s_umount locking in writeback_inodes_sb

[Stefan Bader]

From: Christoph Hellwig <hch at lst.de>

BugLink: http://bugs.launchpad.net/bugs/585092

commit cf37e972478ec58a8a54a6b4f951815f0ae28f78 upstream

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 at lst.de>
Signed-off-by: Jens Axboe <jaxboe at fusionio.com>
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 fs/fs-writeback.c |    6 ++++++
 fs/ubifs/budget.c |    2 ++
 2 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 3a1afb3..7f91e6f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1162,6 +1162,8 @@ void writeback_inodes_sb(struct super_block *sb)
 		.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);
 
@@ -1179,7 +1181,9 @@ 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;
@@ -1202,6 +1206,8 @@ void sync_inodes_sb(struct super_block *sb)
 		.range_cyclic	= 0,
 	};
 
+	WARN_ON(!rwsem_is_locked(&sb->s_umount));
+
 	bdi_queue_work_onstack(&args);
 	wait_sb_inodes(sb);
 }
diff --git a/fs/ubifs/budget.c b/fs/ubifs/budget.c
index 076ca50..c8ff0d1 100644
--- a/fs/ubifs/budget.c
+++ b/fs/ubifs/budget.c
@@ -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);
 }
 
 /**
-- 
1.7.0.4