[RFC PATCH 3/4] fs/super.c: introduce reverse superblock iterator and use it in emergency remount

[James Bottomley <James.Bottomley@HansenPartnership.com>]

Originally proposed by Amir as an extract from the android kernel:

https://lore.kernel.org/linux-fsdevel/CAA2m6vfatWKS1CQFpaRbii2AXiZFvQUjVvYhGxWTSpz+2rxDyg@mail.gmail.com/

Since suspend/resume requires a reverse iterator, I'm dusting it off.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
---
 fs/super.c | 48 +++++++++++++++++++++++++++++-------------------
 1 file changed, 29 insertions(+), 19 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 5a7db4a556e3..76785509d906 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -887,28 +887,38 @@ void drop_super_exclusive(struct super_block *sb)
 }
 EXPORT_SYMBOL(drop_super_exclusive);
 
+#define ITERATE_SUPERS(f, rev)					\
+	struct super_block *sb, *p = NULL;			\
+								\
+	spin_lock(&sb_lock);					\
+								\
+	list_for_each_entry##rev(sb, &super_blocks, s_list) {	\
+		if (super_flags(sb, SB_DYING))			\
+			continue;				\
+		sb->s_count++;					\
+		spin_unlock(&sb_lock);				\
+								\
+		f(sb);						\
+								\
+		spin_lock(&sb_lock);				\
+		if (p)						\
+			__put_super(p);				\
+		p = sb;						\
+	}							\
+	if (p)							\
+		__put_super(p);					\
+	spin_unlock(&sb_lock);
+
 static void __iterate_supers(void (*f)(struct super_block *))
 {
-	struct super_block *sb, *p = NULL;
-
-	spin_lock(&sb_lock);
-	list_for_each_entry(sb, &super_blocks, s_list) {
-		if (super_flags(sb, SB_DYING))
-			continue;
-		sb->s_count++;
-		spin_unlock(&sb_lock);
-
-		f(sb);
+	ITERATE_SUPERS(f,)
+}
 
-		spin_lock(&sb_lock);
-		if (p)
-			__put_super(p);
-		p = sb;
-	}
-	if (p)
-		__put_super(p);
-	spin_unlock(&sb_lock);
+static void __iterate_supers_rev(void (*f)(struct super_block *))
+{
+	ITERATE_SUPERS(f, _reverse)
 }
+
 /**
  *	iterate_supers - call function for all active superblocks
  *	@f: function to call
@@ -1132,7 +1142,7 @@ static void do_emergency_remount_callback(struct super_block *sb)
 
 static void do_emergency_remount(struct work_struct *work)
 {
-	__iterate_supers(do_emergency_remount_callback);
+	__iterate_supers_rev(do_emergency_remount_callback);
 	kfree(work);
 	printk("Emergency Remount complete\n");
 }
-- 
2.43.0