New helper: deactivate_locked_super()

Does equivalent of up_write(&s->s_umount); deactivate_super(s);
However, it does not does not unlock it until it's all over.
As the result, it's safe to use to dispose of new superblock on ->get_sb()
failure exits - nobody will see the sucker until it's all over.
Equivalent using up_write/deactivate_super is safe for that purpose
if superblock is either	safe to use or has NULL ->s_root when we unlock.
Normally filesystems take the required precautions, but
	a) we do have bugs in that area in some of them.
	b) up_write/deactivate_super sequence is extremely common,
so the helper makes sense anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 35 insertions, 12 deletions

diff --git a/fs/super.c b/fs/super.c
index 786fe7d72790..a9dc4c33ef4d 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -208,6 +208,34 @@ void deactivate_super(struct super_block *s)
 EXPORT_SYMBOL(deactivate_super);
 
 /**
+ *      deactivate_locked_super -       drop an active reference to superblock
+ *      @s: superblock to deactivate
+ *
+ *      Equivalent of up_write(&s->s_umount); deactivate_super(s);, except that
+ *      it does not unlock it until it's all over.  As the result, it's safe to
+ *      use to dispose of new superblock on ->get_sb() failure exits - nobody
+ *      will see the sucker until it's all over.  Equivalent using up_write +
+ *      deactivate_super is safe for that purpose only if superblock is either
+ *      safe to use or has NULL ->s_root when we unlock.
+ */
+void deactivate_locked_super(struct super_block *s)
+{
+        struct file_system_type *fs = s->s_type;
+        if (atomic_dec_and_lock(&s->s_active, &sb_lock)) {
+                s->s_count -= S_BIAS-1;
+                spin_unlock(&sb_lock);
+                vfs_dq_off(s, 0);
+                fs->kill_sb(s);
+                put_filesystem(fs);
+                put_super(s);
+        } else {
+                up_write(&s->s_umount);
+        }
+}
+
+EXPORT_SYMBOL(deactivate_locked_super);
+
+/**
  *      grab_super - acquire an active reference
  *      @s: reference we are trying to make active
  *
@@ -797,8 +825,7 @@ int get_sb_ns(struct file_system_type *fs_type, int flags, void *data,
                 sb->s_flags = flags;
                 err = fill_super(sb, data, flags & MS_SILENT ? 1 : 0);
                 if (err) {
-                        up_write(&sb->s_umount);
-                        deactivate_super(sb);
+                        deactivate_locked_super(sb);
                         return err;
                 }
 
@@ -854,8 +881,7 @@ int get_sb_bdev(struct file_system_type *fs_type,
 
         if (s->s_root) {
                 if ((flags ^ s->s_flags) & MS_RDONLY) {
-                        up_write(&s->s_umount);
-                        deactivate_super(s);
+                        deactivate_locked_super(s);
                         error = -EBUSY;
                         goto error_bdev;
                 }
@@ -870,8 +896,7 @@ int get_sb_bdev(struct file_system_type *fs_type,
                 sb_set_blocksize(s, block_size(bdev));
                 error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
                 if (error) {
-                        up_write(&s->s_umount);
-                        deactivate_super(s);
+                        deactivate_locked_super(s);
                         goto error;
                 }
 
@@ -921,8 +946,7 @@ int get_sb_nodev(struct file_system_type *fs_type,
 
         error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
         if (error) {
-                up_write(&s->s_umount);
-                deactivate_super(s);
+                deactivate_locked_super(s);
                 return error;
         }
         s->s_flags |= MS_ACTIVE;
@@ -952,8 +976,7 @@ int get_sb_single(struct file_system_type *fs_type,
                 s->s_flags = flags;
                 error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);
                 if (error) {
-                        up_write(&s->s_umount);
-                        deactivate_super(s);
+                        deactivate_locked_super(s);
                         return error;
                 }
                 s->s_flags |= MS_ACTIVE;
@@ -1006,8 +1029,7 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
         return mnt;
 out_sb:
         dput(mnt->mnt_root);
-        up_write(&mnt->mnt_sb->s_umount);
-        deactivate_super(mnt->mnt_sb);
+        deactivate_locked_super(mnt->mnt_sb);
 out_free_secdata:
         free_secdata(secdata);
 out_mnt:
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 5bed436f4353..11484d08042c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1775,6 +1775,7 @@ void kill_block_super(struct super_block *sb);
 void kill_anon_super(struct super_block *sb);
 void kill_litter_super(struct super_block *sb);
 void deactivate_super(struct super_block *sb);
+void deactivate_locked_super(struct super_block *sb);
 int set_anon_super(struct super_block *s, void *data);
 struct super_block *sget(struct file_system_type *type,
                         int (*test)(struct super_block *,void *),