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, 20 insertions, 2 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 1a850402937d..c3b15f8dca00 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1362,6 +1362,17 @@ struct btrfs_fs_info {
         wait_queue_head_t transaction_blocked_wait;
         wait_queue_head_t async_submit_wait;
 
+        /*
+         * Used to protect the incompat_flags, compat_flags, compat_ro_flags
+         * when they are updated.
+         *
+         * Because we do not clear the flags for ever, so we needn't use
+         * the lock on the read side.
+         *
+         * We also needn't use the lock when we mount the fs, because
+         * there is no other task which will update the flag.
+         */
+        spinlock_t super_lock;
         struct btrfs_super_block *super_copy;
         struct btrfs_super_block *super_for_commit;
         struct block_device *__bdev;
@@ -3688,8 +3699,15 @@ static inline void __btrfs_set_fs_incompat(struct btrfs_fs_info *fs_info,
         disk_super = fs_info->super_copy;
         features = btrfs_super_incompat_flags(disk_super);
         if (!(features & flag)) {
-                features |= flag;
-                btrfs_set_super_incompat_flags(disk_super, features);
+                spin_lock(&fs_info->super_lock);
+                features = btrfs_super_incompat_flags(disk_super);
+                if (!(features & flag)) {
+                        features |= flag;
+                        btrfs_set_super_incompat_flags(disk_super, features);
+                        printk(KERN_INFO "btrfs: setting %llu feature flag\n",
+                                         flag);
+                }
+                spin_unlock(&fs_info->super_lock);
         }
 }