vfs: Avoid ABI change for dentry union changes

Commit 946e51f2bf37f1656916eb75bd0742ba33983c28 ("move d_rcu from
overlapping d_child to overlapping d_alias") looks disruptive and
it is an API change since the union is named.  However, it doesn't
actually move anything that modules need, so it is not an ABI
change and we can safely hide it from genksysms.

Verify this by adding an unused function with some BUILD_BUG_ONs
to assert the size and alignment of fields remain the same.

Gbp-Pq: Topic debian
Gbp-Pq: Name vfs-avoid-abi-change-for-dentry-union-changes.patch

diff --git a/fs/dcache.c b/fs/dcache.c
index e2dc0874a1f0f48f895b98bddd64d70ed7bb4624..66f9823cf2b6fe7b309dae141b1e8955026ddd21 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -255,6 +255,24 @@ static void dentry_free(struct dentry *dentry)
                call_rcu(&dentry->d_u.d_rcu, __d_free);
 }
 
+/*
+ * bwh: Assert that dentry union changes didn't change the structure
+ * layout other than to move d_rcu.
+ */
+static void __always_unused dcache_abi_check(void)
+{
+       struct dentry dentry;
+       union {
+               struct list_head d_child;
+               struct rcu_head d_rcu;
+       } old_d_u;
+       BUILD_BUG_ON(sizeof(dentry.d_child) != sizeof(old_d_u) ||
+                    __alignof__(dentry.d_child) != __alignof__(old_d_u));
+       BUILD_BUG_ON(sizeof(dentry.d_u.d_alias) != sizeof(dentry.d_u) ||
+                    __alignof__(dentry.d_u.d_alias) !=
+                    __alignof__(dentry.d_u));
+}
+
 /**
  * dentry_rcuwalk_barrier - invalidate in-progress rcu-walk lookups
  * @dentry: the target dentry

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 6cddc8708a9582b4997b590a5bbec0e3b4846c39..d6aa25772e09f472387acf273641c12e75212510 100644 (file)
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -124,15 +124,31 @@ struct dentry {
        void *d_fsdata;                 /* fs-specific data */
 
        struct list_head d_lru;         /* LRU list */
+#ifdef __GENKSYMS__
+       /*
+        * bwh: The union changes here don't move anything other than
+        * d_rcu (which modules definitely should not touch).  This is
+        * checked by dcache_abi_check().
+        */
+       union {
+#endif
        struct list_head d_child;       /* child of parent list */
+#ifdef __GENKSYMS__
+               struct rcu_head d_rcu;
+       } d_u;
+#endif
        struct list_head d_subdirs;     /* our children */
        /*
         * d_alias and d_rcu can share memory
         */
+#ifndef __GENKSYMS__
        union {
+#endif
                struct hlist_node d_alias;      /* inode alias list */
+#ifndef __GENKSYMS__
                struct rcu_head d_rcu;
        } d_u;
+#endif
 };
 
 /*