[PATCH] fs: reintroduce freezing nesting

[Florian Margaine <florian@margaine.com>]

The behavior was removed in 18e9e5104fcd9a973ffe3eed3816c87f2a1b6cd2
noting that this was a better idea than using a counter. However, this
behavior is actually wanted if multiple applications want to freeze
concurrently while remaining non-racy.

This patch reintroduces this feature by using a counter.

---
 fs/super.c         | 15 +++++++++++----
 include/linux/fs.h |  1 +
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 74914b1..9fa8ca1 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -231,6 +231,7 @@ static struct super_block *alloc_super(struct
file_system_type *type, int flags)
 	 */
 	down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
 	s->s_count = 1;
+	s->s_freezers = 0;
 	atomic_set(&s->s_active, 1);
 	mutex_init(&s->s_vfs_rename_mutex);
 	lockdep_set_class(&s->s_vfs_rename_mutex, &type-
>s_vfs_rename_key);
@@ -1275,12 +1276,12 @@ int freeze_super(struct super_block *sb)
 {
 	int ret;
 
+	sb->s_freezers++;
+	if (sb->s_writers.frozen != SB_UNFROZEN)
+		return 0;
+
 	atomic_inc(&sb->s_active);
 	down_write(&sb->s_umount);
-	if (sb->s_writers.frozen != SB_UNFROZEN) {
-		deactivate_locked_super(sb);
-		return -EBUSY;
-	}
 
 	if (!(sb->s_flags & MS_BORN)) {
 		up_write(&sb->s_umount);
@@ -1338,14 +1339,20 @@ EXPORT_SYMBOL(freeze_super);
  * @sb: the super to thaw
  *
  * Unlocks the filesystem and marks it writeable again after
freeze_super().
+ * Since nesting freezes is allowed, only the last freeze actually
unlocks.
  */
 int thaw_super(struct super_block *sb)
 {
 	int error;
 
+	sb->s_freezers--;
+	if (sb->s_freezers > 0)
+		return 0;
+
 	down_write(&sb->s_umount);
 	if (sb->s_writers.frozen == SB_UNFROZEN) {
 		up_write(&sb->s_umount);
+		sb->s_freezers++;
 		return -EINVAL;
 	}
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e514f76..c045e2a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1333,6 +1333,7 @@ struct super_block {
 	struct quota_info	s_dquot;	/* Diskquota specific
options */
 
 	struct sb_writers	s_writers;
+	int			s_freezers;
 
 	char s_id[32];				/* Informational
name */
 	u8 s_uuid[16];				/* UUID */
-- 
2.8.0