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, 7 insertions, 6 deletions

diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index 221953bfc859..06e8dfd538d6 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -72,6 +72,7 @@ static struct the_nilfs *alloc_nilfs(struct block_device *bdev)
         atomic_set(&nilfs->ns_writer_refcount, -1);
         atomic_set(&nilfs->ns_ndirtyblks, 0);
         init_rwsem(&nilfs->ns_sem);
+        init_rwsem(&nilfs->ns_super_sem);
         mutex_init(&nilfs->ns_writer_mutex);
         INIT_LIST_HEAD(&nilfs->ns_list);
         INIT_LIST_HEAD(&nilfs->ns_supers);
@@ -681,10 +682,10 @@ struct nilfs_sb_info *nilfs_find_sbinfo(struct the_nilfs *nilfs,
 {
         struct nilfs_sb_info *sbi;
 
-        down_read(&nilfs->ns_sem);
+        down_read(&nilfs->ns_super_sem);
         /*
          * The SNAPSHOT flag and sb->s_flags are supposed to be
-         * protected with nilfs->ns_sem.
+         * protected with nilfs->ns_super_sem.
          */
         sbi = nilfs->ns_current;
         if (rw_mount) {
@@ -705,12 +706,12 @@ struct nilfs_sb_info *nilfs_find_sbinfo(struct the_nilfs *nilfs,
                         goto found; /* snapshot mount */
         }
  out:
-        up_read(&nilfs->ns_sem);
+        up_read(&nilfs->ns_super_sem);
         return NULL;
 
  found:
         atomic_inc(&sbi->s_count);
-        up_read(&nilfs->ns_sem);
+        up_read(&nilfs->ns_super_sem);
         return sbi;
 }
 
@@ -720,7 +721,7 @@ int nilfs_checkpoint_is_mounted(struct the_nilfs *nilfs, __u64 cno,
         struct nilfs_sb_info *sbi;
         int ret = 0;
 
-        down_read(&nilfs->ns_sem);
+        down_read(&nilfs->ns_super_sem);
         if (cno == 0 || cno > nilfs->ns_cno)
                 goto out_unlock;
 
@@ -737,6 +738,6 @@ int nilfs_checkpoint_is_mounted(struct the_nilfs *nilfs, __u64 cno,
                 ret++;
 
  out_unlock:
-        up_read(&nilfs->ns_sem);
+        up_read(&nilfs->ns_super_sem);
         return ret;
 }