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Subject[PATCH v2 1/2] fs/super: function to prevent super re-use
Date
The function is called from generic_shutdown_super upon super
destruction, but also can be called by a filesystem during unmount to
ensure a new superblock is created upon the next mount. This is
necessary for the cases, where force unmount of a filesystem renders
superblock disfunctional, in a state that can not be re-used (e.g. FUSE
severes the connection of the userspace daemon to the superblock which
prevents any further communications between them).

Signed-off-by: Daniil Lunev <dlunev@chromium.org>
---

Changes in v2:
- Remove super from list of superblocks instead of using a flag

fs/super.c | 51 ++++++++++++++++++++++++++++++++++++----------
include/linux/fs.h | 1 +
2 files changed, 41 insertions(+), 11 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index f1d4a193602d6..f23e45434de15 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -422,6 +422,45 @@ bool trylock_super(struct super_block *sb)
return false;
}

+/**
+ * retire_locked_super - prevent superblock from re-use
+ * @sb: superblock that shouldn't be re-used
+ *
+ * retire_locked_super excludes the super from the list of superblocks and
+ * removes the link between superblock and it bdi. After a call to this
+ * function, any subsequent mount will allocate a new superblock, even if
+ * the retired one would have been re-used otherwise. It is safe to call
+ * the function multiple times for the same superblock, any subsequent
+ * invocations after the first one have no effect.
+ */
+static void retire_locked_super(struct super_block *sb)
+{
+ spin_lock(&sb_lock);
+ hlist_del_init(&sb->s_instances);
+ spin_unlock(&sb_lock);
+ up_write(&sb->s_umount);
+ if (sb->s_bdi != &noop_backing_dev_info) {
+ if (sb->s_iflags & SB_I_PERSB_BDI)
+ bdi_unregister(sb->s_bdi);
+ bdi_put(sb->s_bdi);
+ sb->s_bdi = &noop_backing_dev_info;
+ }
+}
+
+/**
+ * retire_super - prevent superblock from re-use
+ * @sb: superblock that shouldn't be re-used
+ *
+ * Variant of retire_locked_super, except that superblock is *not* locked
+ * by caller.
+ */
+void retire_super(struct super_block *sb)
+{
+ down_write(&sb->s_umount);
+ retire_locked_super(sb);
+}
+EXPORT_SYMBOL(retire_super);
+
/**
* generic_shutdown_super - common helper for ->kill_sb()
* @sb: superblock to kill
@@ -467,17 +506,7 @@ void generic_shutdown_super(struct super_block *sb)
sb->s_id);
}
}
- spin_lock(&sb_lock);
- /* should be initialized for __put_super_and_need_restart() */
- hlist_del_init(&sb->s_instances);
- spin_unlock(&sb_lock);
- up_write(&sb->s_umount);
- if (sb->s_bdi != &noop_backing_dev_info) {
- if (sb->s_iflags & SB_I_PERSB_BDI)
- bdi_unregister(sb->s_bdi);
- bdi_put(sb->s_bdi);
- sb->s_bdi = &noop_backing_dev_info;
- }
+ retire_locked_super(sb);
}

EXPORT_SYMBOL(generic_shutdown_super);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index bbde95387a23a..dc9151b7f0102 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2424,6 +2424,7 @@ extern struct dentry *mount_nodev(struct file_system_type *fs_type,
int flags, void *data,
int (*fill_super)(struct super_block *, void *, int));
extern struct dentry *mount_subtree(struct vfsmount *mnt, const char *path);
+void retire_super(struct super_block *sb);
void generic_shutdown_super(struct super_block *sb);
void kill_block_super(struct super_block *sb);
void kill_anon_super(struct super_block *sb);
--
2.31.0
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