writeback: enforce s_umount locking in writeback_inodes_sb

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>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 759666966c6d..2627f0dfcd9c 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1180,6 +1180,8 @@ 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);
 
@@ -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 *sb)
                 .range_cyclic   = 0,
         };
 
+        WARN_ON(!rwsem_is_locked(&sb->s_umount));
+
         bdi_queue_work_onstack(&args);
         wait_sb_inodes(sb);
 }