btrfs: make module init/exit match their sequence

[BACKGROUND]
In theory init_btrfs_fs() and exit_btrfs_fs() should match their
sequence, thus normally they should look like this:

    init_btrfs_fs()   |   exit_btrfs_fs()
----------------------+------------------------
    init_A();         |
    init_B();         |
    init_C();         |
                      |   exit_C();
                      |   exit_B();
                      |   exit_A();

So is for the error path of init_btrfs_fs().

But it's not the case, some exit functions don't match their init
functions sequence in init_btrfs_fs().

Furthermore in init_btrfs_fs(), we need to have a new error label for
each new init function we added.  This is not really expandable,
especially recently we may add several new functions to init_btrfs_fs().

[ENHANCEMENT]
The patch will introduce the following things to enhance the situation:

- struct init_sequence
  Just a wrapper of init and exit function pointers.

  The init function must use int type as return value, thus some init
  functions need to be updated to return 0.

  The exit function can be NULL, as there are some init sequence just
  outputting a message.

- struct mod_init_seq[] array
  This is a const array, recording all the initialization we need to do
  in init_btrfs_fs(), and the order follows the old init_btrfs_fs().

- bool mod_init_result[] array
  This is a bool array, recording if we have initialized one entry in
  mod_init_seq[].

  The reason to split mod_init_seq[] and mod_init_result[] is to avoid
  section mismatch in reference.

  All init function are in .init.text, but if mod_init_seq[] records
  the @initialized member it can no longer be const, thus will be put
  into .data section, and cause modpost warning.

For init_btrfs_fs() we just call all init functions in their order in
mod_init_seq[] array, and after each call, setting corresponding
mod_init_result[] to true.

For exit_btrfs_fs() and error handling path of init_btrfs_fs(), we just
iterate mod_init_seq[] in reverse order, and skip all uninitialized
entry.

With this patch, init_btrfs_fs()/exit_btrfs_fs() will be much easier to
expand and will always follow the strict order.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/compression.c

@@ -1243,12 +1243,13 @@ int btrfs_decompress(int type, unsigned char *data_in, struct page *dest_page,
 	return ret;
 }
 
-void __init btrfs_init_compress(void)
+int __init btrfs_init_compress(void)
 {
 	btrfs_init_workspace_manager(BTRFS_COMPRESS_NONE);
 	btrfs_init_workspace_manager(BTRFS_COMPRESS_ZLIB);
 	btrfs_init_workspace_manager(BTRFS_COMPRESS_LZO);
 	zstd_init_workspace_manager();
+	return 0;
 }
 
 void __cold btrfs_exit_compress(void)

fs/btrfs/compression.h

@@ -77,7 +77,7 @@ static inline unsigned int btrfs_compress_level(unsigned int type_level)
 	return ((type_level & 0xF0) >> 4);
 }
 
-void __init btrfs_init_compress(void);
+int __init btrfs_init_compress(void);
 void __cold btrfs_exit_compress(void);
 
 int btrfs_compress_pages(unsigned int type_level, struct address_space *mapping,

fs/btrfs/props.c

@@ -454,7 +454,7 @@ int btrfs_inode_inherit_props(struct btrfs_trans_handle *trans,
 	return 0;
 }
 
-void __init btrfs_props_init(void)
+int __init btrfs_props_init(void)
 {
 	int i;
 
@@ -464,5 +464,6 @@ void __init btrfs_props_init(void)
 
 		hash_add(prop_handlers_ht, &p->node, h);
 	}
+	return 0;
 }
 

fs/btrfs/props.h

@@ -8,7 +8,7 @@
 
 #include "ctree.h"
 
-void __init btrfs_props_init(void);
+int __init btrfs_props_init(void);
 
 int btrfs_set_prop(struct btrfs_trans_handle *trans, struct inode *inode,
 		   const char *name, const char *value, size_t value_len,

fs/btrfs/super.c

@@ -2694,7 +2694,7 @@ static __cold void btrfs_interface_exit(void)
 	misc_deregister(&btrfs_misc);
 }
 
-static void __init btrfs_print_mod_info(void)
+static int __init btrfs_print_mod_info(void)
 {
 	static const char options[] = ""
 #ifdef CONFIG_BTRFS_DEBUG
@@ -2721,143 +2721,133 @@ static void __init btrfs_print_mod_info(void)
 #endif
 			;
 	pr_info("Btrfs loaded, crc32c=%s%s\n", crc32c_impl(), options);
+	return 0;
+}
+
+static int register_btrfs(void)
+{
+	return register_filesystem(&btrfs_fs_type);
+}
+
+static void unregister_btrfs(void)
+{
+	unregister_filesystem(&btrfs_fs_type);
+}
+
+/* Helper structure for long init/exit functions. */
+struct init_sequence {
+	int (*init_func)(void);
+	/* Can be NULL if the init_func doesn't need cleanup. */
+	void (*exit_func)(void);
+};
+
+static const struct init_sequence mod_init_seq[] = {
+	{
+		.init_func = btrfs_props_init,
+		.exit_func = NULL,
+	}, {
+		.init_func = btrfs_init_sysfs,
+		.exit_func = btrfs_exit_sysfs,
+	}, {
+		.init_func = btrfs_init_compress,
+		.exit_func = btrfs_exit_compress,
+	}, {
+		.init_func = btrfs_init_cachep,
+		.exit_func = btrfs_destroy_cachep,
+	}, {
+		.init_func = btrfs_transaction_init,
+		.exit_func = btrfs_transaction_exit,
+	}, {
+		.init_func = btrfs_ctree_init,
+		.exit_func = btrfs_ctree_exit,
+	}, {
+		.init_func = btrfs_free_space_init,
+		.exit_func = btrfs_free_space_exit,
+	}, {
+		.init_func = extent_state_init_cachep,
+		.exit_func = extent_state_free_cachep,
+	}, {
+		.init_func = extent_buffer_init_cachep,
+		.exit_func = extent_buffer_free_cachep,
+	}, {
+		.init_func = btrfs_bioset_init,
+		.exit_func = btrfs_bioset_exit,
+	}, {
+		.init_func = extent_map_init,
+		.exit_func = extent_map_exit,
+	}, {
+		.init_func = ordered_data_init,
+		.exit_func = ordered_data_exit,
+	}, {
+		.init_func = btrfs_delayed_inode_init,
+		.exit_func = btrfs_delayed_inode_exit,
+	}, {
+		.init_func = btrfs_auto_defrag_init,
+		.exit_func = btrfs_auto_defrag_exit,
+	}, {
+		.init_func = btrfs_delayed_ref_init,
+		.exit_func = btrfs_delayed_ref_exit,
+	}, {
+		.init_func = btrfs_prelim_ref_init,
+		.exit_func = btrfs_prelim_ref_exit,
+	}, {
+		.init_func = btrfs_interface_init,
+		.exit_func = btrfs_interface_exit,
+	}, {
+		.init_func = btrfs_print_mod_info,
+		.exit_func = NULL,
+	}, {
+		.init_func = btrfs_run_sanity_tests,
+		.exit_func = NULL,
+	}, {
+		.init_func = register_btrfs,
+		.exit_func = unregister_btrfs,
+	}
+};
+
+static bool mod_init_result[ARRAY_SIZE(mod_init_seq)];
+
+static void __exit exit_btrfs_fs(void)
+{
+	int i;
+
+	for (i = ARRAY_SIZE(mod_init_seq) - 1; i >= 0; i--) {
+		if (!mod_init_result[i])
+			continue;
+		if (mod_init_seq[i].exit_func)
+			mod_init_seq[i].exit_func();
+		mod_init_result[i] = false;
+	}
 }
 
 static int __init init_btrfs_fs(void)
 {
-	int err;
-
-	btrfs_props_init();
-
-	err = btrfs_init_sysfs();
-	if (err)
-		return err;
-
-	btrfs_init_compress();
-
-	err = btrfs_init_cachep();
-	if (err)
-		goto free_compress;
-
-	err = btrfs_transaction_init();
-	if (err)
-		goto free_cachep;
-
-	err = btrfs_ctree_init();
-	if (err)
-		goto free_transaction;
-
-	err = btrfs_free_space_init();
-	if (err)
-		goto free_ctree;
-
-	err = extent_state_init_cachep();
-	if (err)
-		goto free_free_space;
-
-	err = extent_buffer_init_cachep();
-	if (err)
-		goto free_extent_cachep;
-
-	err = btrfs_bioset_init();
-	if (err)
-		goto free_eb_cachep;
-
-	err = extent_map_init();
-	if (err)
-		goto free_bioset;
-
-	err = ordered_data_init();
-	if (err)
-		goto free_extent_map;
-
-	err = btrfs_delayed_inode_init();
-	if (err)
-		goto free_ordered_data;
-
-	err = btrfs_auto_defrag_init();
-	if (err)
-		goto free_delayed_inode;
-
-	err = btrfs_delayed_ref_init();
-	if (err)
-		goto free_auto_defrag;
-
-	err = btrfs_prelim_ref_init();
-	if (err)
-		goto free_delayed_ref;
-
-	err = btrfs_interface_init();
-	if (err)
-		goto free_prelim_ref;
-
-	btrfs_print_mod_info();
-
-	err = btrfs_run_sanity_tests();
-	if (err)
-		goto unregister_ioctl;
-
-	err = register_filesystem(&btrfs_fs_type);
-	if (err)
-		goto unregister_ioctl;
+	int ret;
+	int i;
 
+	for (i = 0; i < ARRAY_SIZE(mod_init_seq); i++) {
+		ASSERT(!mod_init_result[i]);
+		ret = mod_init_seq[i].init_func();
+		if (ret < 0)
+			goto error;
+		mod_init_result[i] = true;
+	}
 	return 0;
 
-unregister_ioctl:
-	btrfs_interface_exit();
-free_prelim_ref:
-	btrfs_prelim_ref_exit();
-free_delayed_ref:
-	btrfs_delayed_ref_exit();
-free_auto_defrag:
-	btrfs_auto_defrag_exit();
-free_delayed_inode:
-	btrfs_delayed_inode_exit();
-free_ordered_data:
-	ordered_data_exit();
-free_extent_map:
-	extent_map_exit();
-free_bioset:
-	btrfs_bioset_exit();
-free_eb_cachep:
-	extent_buffer_free_cachep();
-free_extent_cachep:
-	extent_state_free_cachep();
-free_free_space:
-	btrfs_free_space_exit();
-free_ctree:
-	btrfs_ctree_exit();
-free_transaction:
-	btrfs_transaction_exit();
-free_cachep:
-	btrfs_destroy_cachep();
-free_compress:
-	btrfs_exit_compress();
-	btrfs_exit_sysfs();
-
-	return err;
+error:
+	/*
+	 * If we call exit_btrfs_fs() it would cause section mismatch.
+	 * As init_btrfs_fs() belongs to .init.text, while exit_btrfs_fs()
+	 * belongs to .exit.text.
+	 */
+	for (i = ARRAY_SIZE(mod_init_seq) - 1; i >= 0; i--) {
+		if (!mod_init_result[i])
+			continue;
+		if (mod_init_seq[i].exit_func)
+			mod_init_seq[i].exit_func();
+		mod_init_result[i] = false;
 	}
-
-static void __exit exit_btrfs_fs(void)
-{
-	btrfs_free_space_exit();
-	btrfs_ctree_exit();
-	btrfs_transaction_exit();
-	btrfs_destroy_cachep();
-	btrfs_delayed_ref_exit();
-	btrfs_auto_defrag_exit();
-	btrfs_delayed_inode_exit();
-	btrfs_prelim_ref_exit();
-	ordered_data_exit();
-	extent_map_exit();
-	btrfs_bioset_exit();
-	extent_state_free_cachep();
-	extent_buffer_free_cachep();
-	btrfs_interface_exit();
-	unregister_filesystem(&btrfs_fs_type);
-	btrfs_exit_sysfs();
-	btrfs_cleanup_fs_uuids();
-	btrfs_exit_compress();
+	return ret;
 }
 
 late_initcall(init_btrfs_fs);