[RFC 1/5] fs: add iterate_supers_reverse()

[Luis R. Rodriguez]

There are use cases where we wish to traverse the superblock list
in reverse order. This particular implementation will also enable
to capture errors.

Signed-off-by: Luis R. Rodriguez <mcg...@kernel.org>
---
 fs/super.c         | 43 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  1 +
 2 files changed, 44 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 02da00410de8..d45e92d9a38f 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -609,6 +609,49 @@ void iterate_supers(void (*f)(struct super_block *, void 
*), void *arg)
        spin_unlock(&sb_lock);
 }
 
+/**
+ *     iterate_supers_reverse - call function for active superblocks in reverse
+ *     @f: function to call
+ *     @arg: argument to pass to it
+ *
+ *     Scans the superblock list and calls given function, passing it
+ *     locked superblock and given argument, in reverse order. Returns if
+ *     an error occurred.
+ */
+int iterate_supers_reverse(int (*f)(struct super_block *, void *), void *arg)
+{
+       struct super_block *sb, *p = NULL;
+       int error = 0;
+
+       spin_lock(&sb_lock);
+       list_for_each_entry_reverse(sb, &super_blocks, s_list) {
+               if (hlist_unhashed(&sb->s_instances))
+                       continue;
+               sb->s_count++;
+               spin_unlock(&sb_lock);
+
+               down_read(&sb->s_umount);
+               if (sb->s_root && (sb->s_flags & SB_BORN)) {
+                       error = f(sb, arg);
+                       if (error) {
+                               spin_lock(&sb_lock);
+                               break;
+                       }
+               }
+               up_read(&sb->s_umount);
+
+               spin_lock(&sb_lock);
+               if (p)
+                       __put_super(p);
+               p = sb;
+       }
+       if (p)
+               __put_super(p);
+       spin_unlock(&sb_lock);
+
+       return error;
+}
+
 /**
  *     iterate_supers_type - call function for superblocks of given type
  *     @type: fs type
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 46796b2f956b..cd084792cf39 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3097,6 +3097,7 @@ extern struct super_block *get_active_super(struct 
block_device *bdev);
 extern void drop_super(struct super_block *sb);
 extern void drop_super_exclusive(struct super_block *sb);
 extern void iterate_supers(void (*)(struct super_block *, void *), void *);
+extern int iterate_supers_reverse(int (*)(struct super_block *, void *), void 
*);
 extern void iterate_supers_type(struct file_system_type *,
                                void (*)(struct super_block *, void *), void *);
 
-- 
2.14.0