[PATCH 1/3] ext4: introduce macro sbi_array_rcu_deref() to access rcu protected fields

[Suraj Jitindar Singh <surajjs@amazon.com>]

The s_group_desc field in the super block info (sbi) is protected by rcu to
prevent access to an invalid pointer during online resize operations.
There are 2 other arrays in sbi, s_group_info and s_flex_groups, which
require similar rcu protection which is introduced in the subsequent
patches. Introduce a helper macro sbi_array_rcu_deref() to be used to
provide rcu protected access to such fields.

Also update the current s_group_desc access site to use the macro.

Signed-off-by: Suraj Jitindar Singh <surajjs@amazon.com>
Cc: stable@vger-kernel.org
---
 fs/ext4/balloc.c | 11 +++++------
 fs/ext4/ext4.h   | 17 +++++++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index 5368bf67300b..8fd0b3cdab4c 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -281,14 +281,13 @@ struct ext4_group_desc * ext4_get_group_desc(struct super_block *sb,
 
 	group_desc = block_group >> EXT4_DESC_PER_BLOCK_BITS(sb);
 	offset = block_group & (EXT4_DESC_PER_BLOCK(sb) - 1);
-	rcu_read_lock();
-	bh_p = rcu_dereference(sbi->s_group_desc)[group_desc];
+	bh_p = sbi_array_rcu_deref(sbi, s_group_desc, group_desc);
 	/*
-	 * We can unlock here since the pointer being dereferenced won't be
-	 * dereferenced again. By looking at the usage in add_new_gdb() the
-	 * value isn't modified, just the pointer, and so it remains valid.
+	 * sbi_array_rcu_deref returns with rcu unlocked, this is ok since
+	 * the pointer being dereferenced won't be dereferenced again. By
+	 * looking at the usage in add_new_gdb() the value isn't modified,
+	 * just the pointer, and so it remains valid.
 	 */
-	rcu_read_unlock();
 	if (!bh_p) {
 		ext4_error(sb, "Group descriptor not loaded - "
 			   "block_group = %u, group_desc = %u, desc = %u",
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 149ee0ab6d64..236fc6500340 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1576,6 +1576,23 @@ static inline int ext4_valid_inum(struct super_block *sb, unsigned long ino)
 		 ino <= le32_to_cpu(EXT4_SB(sb)->s_es->s_inodes_count));
 }
 
+/*
+ * Returns: sbi->field[index]
+ * Used to access an array element from the following sbi fields which require
+ * rcu protection to avoid dereferencing an invalid pointer due to reassignment
+ * - s_group_desc
+ * - s_group_info
+ * - s_flex_group
+ */
+#define sbi_array_rcu_deref(sbi, field, index)				   \
+({									   \
+	typeof(*((sbi)->field)) _v;					   \
+	rcu_read_lock();						   \
+	_v = ((typeof((sbi)->field))rcu_dereference((sbi)->field))[index]; \
+	rcu_read_unlock();						   \
+	_v;								   \
+})
+
 /*
  * Simulate_fail codes
  */
-- 
2.17.1