nilfs2: correct exclusion control in nilfs_remount function

nilfs_remount() changes mount state of a superblock instance.  Even
though nilfs accesses other superblock instances during mount or
remount, the mount state was not properly protected in
nilfs_remount().

Moreover, nilfs_remount() has a lock order reversal problem;
nilfs_get_sb() holds:

  1. bdev->bd_mount_sem
  2. sb->s_umount  (sget acquires)

and nilfs_remount() holds:

  1. sb->s_umount  (locked by the caller in vfs)
  2. bdev->bd_mount_sem

To avoid these problems, this patch divides a semaphore protecting
super block instances from nilfs->ns_sem, and applies it to the mount
state protection in nilfs_remount().

With this change, bd_mount_sem use is removed from nilfs_remount() and
the lock order reversal will be resolved.  And the new rw-semaphore,
nilfs->ns_super_sem will properly protect the mount state except the
modification from nilfs_error function.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 6 insertions, 1 deletions

diff --git a/fs/nilfs2/the_nilfs.h b/fs/nilfs2/the_nilfs.h
index be4c040fd629..d0cf4fb7c9ce 100644
--- a/fs/nilfs2/the_nilfs.h
+++ b/fs/nilfs2/the_nilfs.h
@@ -48,6 +48,7 @@ enum {
  * @ns_bdi: backing dev info
  * @ns_writer: back pointer to writable nilfs_sb_info
  * @ns_sem: semaphore for shared states
+ * @ns_super_sem: semaphore for global operations across super block instances
  * @ns_writer_mutex: mutex protecting ns_writer attach/detach
  * @ns_writer_refcount: number of referrers on ns_writer
  * @ns_current: back pointer to current mount
@@ -96,10 +97,15 @@ struct the_nilfs {
         struct backing_dev_info *ns_bdi;
         struct nilfs_sb_info   *ns_writer;
         struct rw_semaphore     ns_sem;
+        struct rw_semaphore     ns_super_sem;
         struct mutex            ns_writer_mutex;
         atomic_t                ns_writer_refcount;
 
+        /*
+         * components protected by ns_super_sem
+         */
         struct nilfs_sb_info   *ns_current;
+        struct list_head        ns_supers;
 
         /*
          * used for
@@ -113,7 +119,6 @@ struct the_nilfs {
         time_t                  ns_sbwtime[2];
         unsigned                ns_sbsize;
         unsigned                ns_mount_state;
-        struct list_head        ns_supers;
 
         /*
          * Following fields are dedicated to a writable FS-instance.