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super: convert s_count to refcount_t s_passive

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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.

Link: https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-2-7df6b864028e@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2026-06-16 16:08:18 +02:00
parent d152447614
commit 3ec9800c2d
2 changed files with 10 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

18
fs/super.c
View File

@@ -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);

2
include/linux/fs/super_types.h
View File

@@ -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;