[PATCH RFC v2 02/18] super: convert s_count to refcount_t s_passive

[Christian Brauner <brauner@kernel.org>]

The superblock carries two counters: s_active, the active reference
count that keeps the filesystem usable, and s_count, the passive
reference count that merely keeps the structure itself alive. Turn the
passive count into a refcount_t and rename it to s_passive to make the
pairing with s_active obvious.

Everything is still serialized by sb_lock, so there is no functional
change; the conversion buys the usual refcount_t saturation and
underflow checking. The following patches start dropping passive
references without holding sb_lock and make the device-to-superblock
table hold one passive reference per registered entry, which a plain
integer cannot support.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/super.c                     | 18 +++++++++---------
 include/linux/fs/super_types.h |  2 +-
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index a8fd61136aaf..25dd72b550e0 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -102,7 +102,7 @@ static bool super_flags(const struct super_block *sb, unsigned int flags)
  * creation will succeed and SB_BORN is set by vfs_get_tree() or we're
  * woken and we'll see SB_DYING.
  *
- * The caller must have acquired a temporary reference on @sb->s_count.
+ * The caller must have acquired a temporary reference on @sb->s_passive.
  *
  * Return: The function returns true if SB_BORN was set and with
  *         s_umount held. The function returns false if SB_DYING was
@@ -367,7 +367,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	spin_lock_init(&s->s_inode_wblist_lock);
 	fserror_mount(s);
 
-	s->s_count = 1;
+	refcount_set(&s->s_passive, 1);
 	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);
@@ -407,7 +407,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
  */
 static void __put_super(struct super_block *s)
 {
-	if (!--s->s_count) {
+	if (refcount_dec_and_test(&s->s_passive)) {
 		list_del_init(&s->s_list);
 		WARN_ON(s->s_dentry_lru.node);
 		WARN_ON(s->s_inode_lru.node);
@@ -529,7 +529,7 @@ static bool grab_super(struct super_block *sb)
 {
 	bool locked;
 
-	sb->s_count++;
+	refcount_inc(&sb->s_passive);
 	spin_unlock(&sb_lock);
 	locked = super_lock_excl(sb);
 	if (locked) {
@@ -556,7 +556,7 @@ static bool grab_super(struct super_block *sb)
  *	lock held in read mode in case of success. On successful return,
  *	the caller must drop the s_umount lock when done.
  *
- *	Note that unlike get_super() et.al. this one does *not* bump ->s_count.
+ *	Note that unlike get_super() et.al. this one does *not* bump ->s_passive.
  *	The reason why it's safe is that we are OK with doing trylock instead
  *	of down_read().  There's a couple of places that are OK with that, but
  *	it's very much not a general-purpose interface.
@@ -858,7 +858,7 @@ static void __iterate_supers(void (*f)(struct super_block *, void *), void *arg,
 	     sb = next_super(sb, flags)) {
 		if (super_flags(sb, SB_DYING))
 			continue;
-		sb->s_count++;
+		refcount_inc(&sb->s_passive);
 		spin_unlock(&sb_lock);
 
 		if (flags & SUPER_ITER_UNLOCKED) {
@@ -903,7 +903,7 @@ void iterate_supers_type(struct file_system_type *type,
 		if (super_flags(sb, SB_DYING))
 			continue;
 
-		sb->s_count++;
+		refcount_inc(&sb->s_passive);
 		spin_unlock(&sb_lock);
 
 		locked = super_lock_shared(sb);
@@ -935,7 +935,7 @@ struct super_block *user_get_super(dev_t dev, bool excl)
 		if (sb->s_dev != dev)
 			continue;
 
-		sb->s_count++;
+		refcount_inc(&sb->s_passive);
 		spin_unlock(&sb_lock);
 
 		locked = super_lock(sb, excl);
@@ -1369,7 +1369,7 @@ static struct super_block *bdev_super_lock(struct block_device *bdev, bool excl)
 
 	/* Make sure sb doesn't go away from under us */
 	spin_lock(&sb_lock);
-	sb->s_count++;
+	refcount_inc(&sb->s_passive);
 	spin_unlock(&sb_lock);
 
 	mutex_unlock(&bdev->bd_holder_lock);
diff --git a/include/linux/fs/super_types.h b/include/linux/fs/super_types.h
index ef7941e9dc79..68747182abf9 100644
--- a/include/linux/fs/super_types.h
+++ b/include/linux/fs/super_types.h
@@ -145,7 +145,7 @@ struct super_block {
 	unsigned long				s_magic;
 	struct dentry				*s_root;
 	struct rw_semaphore			s_umount;
-	int					s_count;
+	refcount_t				s_passive;
 	atomic_t				s_active;
 #ifdef CONFIG_SECURITY
 	void					*s_security;

-- 
2.47.3