erofs: kill erofs_{init,exit}_inode_cache

As Christoph said [1] "having this function seems
entirely pointless", let's kill those.

filesystem                              function name
ext2,f2fs,ext4,isofs,squashfs,cifs,...  init_inodecache

In addition, add a necessary "rcu_barrier()" on exit_fs();

[1] https://lore.kernel.org/r/20190829101545.GC20598@infradead.org/Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-9-gaoxiang25@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/erofs/super.c

@@ -23,21 +23,6 @@ static void init_once(void *ptr)
 	inode_init_once(&vi->vfs_inode);
 }
 
-static int __init erofs_init_inode_cache(void)
-{
-	erofs_inode_cachep = kmem_cache_create("erofs_inode",
-					       sizeof(struct erofs_vnode), 0,
-					       SLAB_RECLAIM_ACCOUNT,
-					       init_once);
-
-	return erofs_inode_cachep ? 0 : -ENOMEM;
-}
-
-static void erofs_exit_inode_cache(void)
-{
-	kmem_cache_destroy(erofs_inode_cachep);
-}
-
 static struct inode *alloc_inode(struct super_block *sb)
 {
 	struct erofs_vnode *vi =
@@ -531,9 +516,14 @@ static int __init erofs_module_init(void)
 	erofs_check_ondisk_layout_definitions();
 	infoln("initializing erofs " EROFS_VERSION);
 
-	err = erofs_init_inode_cache();
-	if (err)
+	erofs_inode_cachep = kmem_cache_create("erofs_inode",
+					       sizeof(struct erofs_vnode), 0,
+					       SLAB_RECLAIM_ACCOUNT,
+					       init_once);
+	if (!erofs_inode_cachep) {
+		err = -ENOMEM;
 		goto icache_err;
+	}
 
 	err = erofs_init_shrinker();
 	if (err)
@@ -555,7 +545,7 @@ static int __init erofs_module_init(void)
 zip_err:
 	erofs_exit_shrinker();
 shrinker_err:
-	erofs_exit_inode_cache();
+	kmem_cache_destroy(erofs_inode_cachep);
 icache_err:
 	return err;
 }
@@ -565,7 +555,10 @@ static void __exit erofs_module_exit(void)
 	unregister_filesystem(&erofs_fs_type);
 	z_erofs_exit_zip_subsystem();
 	erofs_exit_shrinker();
-	erofs_exit_inode_cache();
+
+	/* Ensure all RCU free inodes are safe before cache is destroyed. */
+	rcu_barrier();
+	kmem_cache_destroy(erofs_inode_cachep);
 	infoln("successfully finalize erofs");
 }