Btrfs: use a lock to protect incompat/compat flag of the super block

The following case will make the incompat/compat flag of the super block
be recovered.
 Task1					|Task2
 flags = btrfs_super_incompat_flags();	|
					|flags = btrfs_super_incompat_flags();
 flags |= new_flag1;			|
					|flags |= new_flag2;
 btrfs_set_super_incompat_flags(flags);	|
					|btrfs_set_super_incompat_flags(flags);
the new_flag1 is recovered.

In order to avoid this problem, we introduce a lock named super_lock into
the btrfs_fs_info structure. If we want to update incompat/compat flags
of the super block, we must hold it.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 9f83e5b870d2..8a7a366267ec 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2086,6 +2086,7 @@ int open_ctree(struct super_block *sb,
         spin_lock_init(&fs_info->defrag_inodes_lock);
         spin_lock_init(&fs_info->free_chunk_lock);
         spin_lock_init(&fs_info->tree_mod_seq_lock);
+        spin_lock_init(&fs_info->super_lock);
         rwlock_init(&fs_info->tree_mod_log_lock);
         mutex_init(&fs_info->reloc_mutex);
         seqlock_init(&fs_info->profiles_lock);
@@ -2349,6 +2350,10 @@ int open_ctree(struct super_block *sb,
                 goto fail_alloc;
         }
 
+        /*
+         * Needn't use the lock because there is no other task which will
+         * update the flag.
+         */
         btrfs_set_super_incompat_flags(disk_super, features);
 
         features = btrfs_super_compat_ro_flags(disk_super) &