Re: [PATCH 14/17] btrfs: introduce BTRFS_NESTING_NEW_ROOT for adding new roots

[David Sterba <dsterba@suse.cz>]

On Mon, Aug 10, 2020 at 11:42:39AM -0400, Josef Bacik wrote:
> --- a/fs/btrfs/locking.h
> +++ b/fs/btrfs/locking.h
> @@ -16,6 +16,11 @@
>  #define BTRFS_WRITE_LOCK_BLOCKING 3
>  #define BTRFS_READ_LOCK_BLOCKING 4
>  
> +/*
> + * We are limited in number of subclasses by MAX_LOCKDEP_SUBCLASSES, which at
> + * the time of this patch is 8, which is how many we're using here.  Keep this
> + * in mind if you decide you want to add another subclass.
> + */
>  enum btrfs_lock_nesting {
>  	BTRFS_NESTING_NORMAL = 0,
>  
> @@ -55,6 +60,15 @@ enum btrfs_lock_nesting {
>  	 * handle this case where we need to allocate a new split block.
>  	 */
>  	BTRFS_NESTING_SPLIT,
> +
> +	/*
> +	 * When promoting a new block to a root we need to have a special
> +	 * subclass so we don't confuse lockdep, as it will appear that we are
> +	 * locking a higher level node before a lower level one.  Copying also
> +	 * has this problem as it appears we're locking the same block again
> +	 * when we make a snapshot of an existing root.
> +	 */
> +	BTRFS_NESTING_NEW_ROOT,
>  };

To prevent accidental addition (and not reading the comment about the
maximum count), I suggest to use the static_assert with one extra
definition like

--- a/fs/btrfs/locking.h
+++ b/fs/btrfs/locking.h
@@ -14,11 +14,6 @@
 #define BTRFS_WRITE_LOCK 1
 #define BTRFS_READ_LOCK 2
 
-/*
- * We are limited in number of subclasses by MAX_LOCKDEP_SUBCLASSES, which at
- * the time of this patch is 8, which is how many we're using here.  Keep this
- * in mind if you decide you want to add another subclass.
- */
 enum btrfs_lock_nesting {
        BTRFS_NESTING_NORMAL = 0,
 
@@ -67,8 +62,19 @@ enum btrfs_lock_nesting {
         * when we make a snapshot of an existing root.
         */
        BTRFS_NESTING_NEW_ROOT,
+
+       /*
+        * We are limited in number of subclasses by MAX_LOCKDEP_SUBCLASSES,
+        * which at the time of this patch is 8, which is how many we're using
+        * here.  Keep this in mind if you decide you want to add another
+        * subclass.
+        */
+       BTRFS_NESTING_MAX
 };
 
+static_assert(BTRFS_NESTING_MAX <= MAX_LOCKDEP_SUBCLASSES,
+             "too many lock subclasses defined");
+
 struct btrfs_path;
 
 void __btrfs_tree_lock(struct extent_buffer *eb, enum btrfs_lock_nesting nest);
---