ANDROID: fix ABI-break in struct cgroup_root

Fix the ABI issues introduced by commit dd9542a ("cgroup: Make
operations on the cgroup root_list RCU safe"). Use _some_ of the bits
reserved in cgroup_root for the new root->rcu member. This new offset
restricts the usage of kfree_rcu() though, so this call has replaced
with call_rcu() instead. Update the stg file with this update:

type 'struct cgroup_root' changed
  member 'u8 android_backport_reserved1[28]' was removed
  member 'union { struct callback_head rcu; struct { u8 android_backport_reserved1[28]; }; union { }; }' was added

Bug: 379227997
Suggested-by: T.J. Mercier <[email protected]>
Change-Id: I3e76ca05ee6d68f7167bf487348f438cd2a00d5f
Signed-off-by: Carlos Llamas <[email protected]>

Author: Carlos Llamas

android/abi_gki_aarch64.stg

@@ -42307,6 +42307,10 @@ member {
   id: 0x2ef635de
   type_id: 0x34f6a768
 }
+member {
+  id: 0x2efe0273
+  type_id: 0x34d679de
+}
 member {
   id: 0x2f1819a7
   type_id: 0x334e168e
@@ -43924,6 +43928,11 @@ member {
   type_id: 0x73b9d605
   offset: 256
 }
+member {
+  id: 0x3f2a640d
+  type_id: 0x7387dd81
+  offset: 49472
+}
 member {
   id: 0x3f2df7ff
   type_id: 0x7399b0f3
@@ -49772,10 +49781,9 @@ member {
   type_id: 0x92233392
 }
 member {
-  id: 0xb2e8fb7e
+  id: 0xb2e8f417
   name: "android_backport_reserved1"
   type_id: 0x1159eb36
-  offset: 49472
 }
 member {
   id: 0x71aeb2cd
@@ -222059,6 +222067,14 @@ struct_union {
     member_id: 0x39ff19b6
   }
 }
+struct_union {
+  id: 0x34d679de
+  kind: STRUCT
+  definition {
+    bytesize: 32
+    member_id: 0xb2e8f417
+  }
+}
 struct_union {
   id: 0x34ed11d4
   kind: STRUCT
@@ -226230,6 +226246,16 @@ struct_union {
     member_id: 0x92478c56
   }
 }
+struct_union {
+  id: 0x7387dd81
+  kind: UNION
+  definition {
+    bytesize: 32
+    member_id: 0x95dac977
+    member_id: 0x2efe0273
+    member_id: 0x36752b74
+  }
+}
 struct_union {
   id: 0x7399b0f3
   kind: UNION
@@ -231813,7 +231839,7 @@ struct_union {
     member_id: 0x2d2d08b0
     member_id: 0xe8e1b772
     member_id: 0x0de6d95b
-    member_id: 0xb2e8fb7e
+    member_id: 0x3f2a640d
   }
 }
 struct_union {

android/abi_gki_aarch64.stg.allowed_breaks

@@ -82,3 +82,7 @@ type 'enum ftrace_dump_mode' changed
 
 1 variable symbol(s) removed
   'struct tracepoint __tracepoint_android_vh_suitable_migration_target_bypass'
+
+type 'struct cgroup_root' changed
+  member 'u8 android_backport_reserved1[28]' was removed
+  member 'union { struct callback_head rcu; struct { u8 android_backport_reserved1[28]; }; union { }; }' was added

include/linux/cgroup-defs.h

@@ -575,7 +575,6 @@ struct cgroup_root {
 
 	/* A list running through the active hierarchies */
 	struct list_head root_list;
-	struct rcu_head rcu;
 
 	/* Hierarchy-specific flags */
 	unsigned int flags;
@@ -586,7 +585,8 @@ struct cgroup_root {
 	/* The name for this hierarchy - may be empty */
 	char name[MAX_CGROUP_ROOT_NAMELEN];
 
-	ANDROID_BACKPORT_RESERVE_ARRAY(1, CGROUP_SUBSYS_COUNT * sizeof(atomic_t));
+	ANDROID_BACKPORT_USE_ARRAY(1, CGROUP_SUBSYS_COUNT * sizeof(atomic_t),
+				   struct rcu_head rcu);
 };
 
 /*

kernel/cgroup/cgroup.c

@@ -1315,9 +1315,14 @@ static void cgroup_exit_root_id(struct cgroup_root *root)
 	idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
 }
 
+static void __cgroup_free_root(struct rcu_head *rcu)
+{
+	kfree(container_of(rcu, struct cgroup_root, rcu));
+}
+
 void cgroup_free_root(struct cgroup_root *root)
 {
-	kfree_rcu(root, rcu);
+	call_rcu(&root->rcu, __cgroup_free_root);
 }
 
 static void cgroup_destroy_root(struct cgroup_root *root)