fs: fix superblock iteration race
[linux-3.10.git] / fs / super.c
index d40d53a..938119a 100644 (file)
 
 #include <linux/module.h>
 #include <linux/slab.h>
-#include <linux/init.h>
-#include <linux/smp_lock.h>
 #include <linux/acct.h>
 #include <linux/blkdev.h>
-#include <linux/quotaops.h>
-#include <linux/namei.h>
 #include <linux/mount.h>
 #include <linux/security.h>
-#include <linux/syscalls.h>
-#include <linux/vfs.h>
 #include <linux/writeback.h>           /* for the emergency remount stuff */
 #include <linux/idr.h>
-#include <linux/kobject.h>
 #include <linux/mutex.h>
-#include <linux/file.h>
-#include <asm/uaccess.h>
+#include <linux/backing-dev.h>
 #include "internal.h"
 
 
@@ -54,7 +46,7 @@ DEFINE_SPINLOCK(sb_lock);
 static struct super_block *alloc_super(struct file_system_type *type)
 {
        struct super_block *s = kzalloc(sizeof(struct super_block),  GFP_USER);
-       static struct super_operations default_op;
+       static const struct super_operations default_op;
 
        if (s) {
                if (security_sb_alloc(s)) {
@@ -62,9 +54,6 @@ static struct super_block *alloc_super(struct file_system_type *type)
                        s = NULL;
                        goto out;
                }
-               INIT_LIST_HEAD(&s->s_dirty);
-               INIT_LIST_HEAD(&s->s_io);
-               INIT_LIST_HEAD(&s->s_more_io);
                INIT_LIST_HEAD(&s->s_files);
                INIT_LIST_HEAD(&s->s_instances);
                INIT_HLIST_HEAD(&s->s_anon);
@@ -95,16 +84,15 @@ static struct super_block *alloc_super(struct file_system_type *type)
                 * subclass.
                 */
                down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
-               s->s_count = S_BIAS;
+               s->s_count = 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);
                mutex_init(&s->s_dquot.dqio_mutex);
                mutex_init(&s->s_dquot.dqonoff_mutex);
                init_rwsem(&s->s_dquot.dqptr_sem);
                init_waitqueue_head(&s->s_wait_unfrozen);
                s->s_maxbytes = MAX_NON_LFS;
-               s->dq_op = sb_dquot_ops;
-               s->s_qcop = sb_quotactl_ops;
                s->s_op = &default_op;
                s->s_time_gran = 1000000000;
        }
@@ -129,39 +117,14 @@ static inline void destroy_super(struct super_block *s)
 /* Superblock refcounting  */
 
 /*
- * Drop a superblock's refcount.  Returns non-zero if the superblock was
- * destroyed.  The caller must hold sb_lock.
+ * Drop a superblock's refcount.  The caller must hold sb_lock.
  */
-static int __put_super(struct super_block *sb)
+void __put_super(struct super_block *sb)
 {
-       int ret = 0;
-
        if (!--sb->s_count) {
+               list_del_init(&sb->s_list);
                destroy_super(sb);
-               ret = 1;
-       }
-       return ret;
-}
-
-/*
- * Drop a superblock's refcount.
- * Returns non-zero if the superblock is about to be destroyed and
- * at least is already removed from super_blocks list, so if we are
- * making a loop through super blocks then we need to restart.
- * The caller must hold sb_lock.
- */
-int __put_super_and_need_restart(struct super_block *sb)
-{
-       /* check for race with generic_shutdown_super() */
-       if (list_empty(&sb->s_list)) {
-               /* super block is removed, need to restart... */
-               __put_super(sb);
-               return 1;
        }
-       /* can't be the last, since s_list is still in use */
-       sb->s_count--;
-       BUG_ON(sb->s_count == 0);
-       return 0;
 }
 
 /**
@@ -171,7 +134,7 @@ int __put_super_and_need_restart(struct super_block *sb)
  *     Drops a temporary reference, frees superblock if there's no
  *     references left.
  */
-static void put_super(struct super_block *sb)
+void put_super(struct super_block *sb)
 {
        spin_lock(&sb_lock);
        __put_super(sb);
@@ -180,57 +143,47 @@ static void put_super(struct super_block *sb)
 
 
 /**
- *     deactivate_super        -       drop an active reference to superblock
+ *     deactivate_locked_super -       drop an active reference to superblock
  *     @s: superblock to deactivate
  *
- *     Drops an active reference to superblock, acquiring a temprory one if
- *     there is no active references left.  In that case we lock superblock,
+ *     Drops an active reference to superblock, converting it into a temprory
+ *     one if there is no other active references left.  In that case we
  *     tell fs driver to shut it down and drop the temporary reference we
  *     had just acquired.
+ *
+ *     Caller holds exclusive lock on superblock; that lock is released.
  */
-void deactivate_super(struct super_block *s)
+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);
-               down_write(&s->s_umount);
+       if (atomic_dec_and_test(&s->s_active)) {
                fs->kill_sb(s);
                put_filesystem(fs);
                put_super(s);
+       } else {
+               up_write(&s->s_umount);
        }
 }
 
-EXPORT_SYMBOL(deactivate_super);
+EXPORT_SYMBOL(deactivate_locked_super);
 
 /**
- *     deactivate_locked_super -       drop an active reference to superblock
+ *     deactivate_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.
+ *     Variant of deactivate_locked_super(), except that superblock is *not*
+ *     locked by caller.  If we are going to drop the final active reference,
+ *     lock will be acquired prior to that.
  */
-void deactivate_locked_super(struct super_block *s)
+void deactivate_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);
+        if (!atomic_add_unless(&s->s_active, -1, 1)) {
+               down_write(&s->s_umount);
+               deactivate_locked_super(s);
        }
 }
 
-EXPORT_SYMBOL(deactivate_locked_super);
+EXPORT_SYMBOL(deactivate_super);
 
 /**
  *     grab_super - acquire an active reference
@@ -245,22 +198,17 @@ EXPORT_SYMBOL(deactivate_locked_super);
  */
 static int grab_super(struct super_block *s) __releases(sb_lock)
 {
+       if (atomic_inc_not_zero(&s->s_active)) {
+               spin_unlock(&sb_lock);
+               return 1;
+       }
+       /* it's going away */
        s->s_count++;
        spin_unlock(&sb_lock);
+       /* wait for it to die */
        down_write(&s->s_umount);
-       if (s->s_root) {
-               spin_lock(&sb_lock);
-               if (s->s_count > S_BIAS) {
-                       atomic_inc(&s->s_active);
-                       s->s_count--;
-                       spin_unlock(&sb_lock);
-                       return 1;
-               }
-               spin_unlock(&sb_lock);
-       }
        up_write(&s->s_umount);
        put_super(s);
-       yield();
        return 0;
 }
 
@@ -323,8 +271,7 @@ void generic_shutdown_super(struct super_block *sb)
        }
        spin_lock(&sb_lock);
        /* should be initialized for __put_super_and_need_restart() */
-       list_del_init(&sb->s_list);
-       list_del(&sb->s_instances);
+       list_del_init(&sb->s_instances);
        spin_unlock(&sb_lock);
        up_write(&sb->s_umount);
 }
@@ -359,6 +306,7 @@ retry:
                                up_write(&s->s_umount);
                                destroy_super(s);
                        }
+                       down_write(&old->s_umount);
                        return old;
                }
        }
@@ -410,11 +358,12 @@ EXPORT_SYMBOL(drop_super);
  */
 void sync_supers(void)
 {
-       struct super_block *sb;
+       struct super_block *sb, *n;
 
        spin_lock(&sb_lock);
-restart:
-       list_for_each_entry(sb, &super_blocks, s_list) {
+       list_for_each_entry_safe(sb, n, &super_blocks, s_list) {
+               if (list_empty(&sb->s_instances))
+                       continue;
                if (sb->s_op->write_super && sb->s_dirt) {
                        sb->s_count++;
                        spin_unlock(&sb_lock);
@@ -425,14 +374,47 @@ restart:
                        up_read(&sb->s_umount);
 
                        spin_lock(&sb_lock);
-                       if (__put_super_and_need_restart(sb))
-                               goto restart;
+                       /* lock was dropped, must reset next */
+                       list_safe_reset_next(sb, n, s_list);
+                       __put_super(sb);
                }
        }
        spin_unlock(&sb_lock);
 }
 
 /**
+ *     iterate_supers - call function for all active superblocks
+ *     @f: function to call
+ *     @arg: argument to pass to it
+ *
+ *     Scans the superblock list and calls given function, passing it
+ *     locked superblock and given argument.
+ */
+void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
+{
+       struct super_block *sb, *n;
+
+       spin_lock(&sb_lock);
+       list_for_each_entry_safe(sb, n, &super_blocks, s_list) {
+               if (list_empty(&sb->s_instances))
+                       continue;
+               sb->s_count++;
+               spin_unlock(&sb_lock);
+
+               down_read(&sb->s_umount);
+               if (sb->s_root)
+                       f(sb, arg);
+               up_read(&sb->s_umount);
+
+               spin_lock(&sb_lock);
+               /* lock was dropped, must reset next */
+               list_safe_reset_next(sb, n, s_list);
+               __put_super(sb);
+       }
+       spin_unlock(&sb_lock);
+}
+
+/**
  *     get_super - get the superblock of a device
  *     @bdev: device to get the superblock for
  *     
@@ -440,7 +422,7 @@ restart:
  *     mounted on the device given. %NULL is returned if no match is found.
  */
 
-struct super_block * get_super(struct block_device *bdev)
+struct super_block *get_super(struct block_device *bdev)
 {
        struct super_block *sb;
 
@@ -450,17 +432,20 @@ struct super_block * get_super(struct block_device *bdev)
        spin_lock(&sb_lock);
 rescan:
        list_for_each_entry(sb, &super_blocks, s_list) {
+               if (list_empty(&sb->s_instances))
+                       continue;
                if (sb->s_bdev == bdev) {
                        sb->s_count++;
                        spin_unlock(&sb_lock);
                        down_read(&sb->s_umount);
+                       /* still alive? */
                        if (sb->s_root)
                                return sb;
                        up_read(&sb->s_umount);
-                       /* restart only when sb is no longer on the list */
+                       /* nope, got unmounted */
                        spin_lock(&sb_lock);
-                       if (__put_super_and_need_restart(sb))
-                               goto rescan;
+                       __put_super(sb);
+                       goto rescan;
                }
        }
        spin_unlock(&sb_lock);
@@ -468,55 +453,65 @@ rescan:
 }
 
 EXPORT_SYMBOL(get_super);
+
+/**
+ * get_active_super - get an active reference to the superblock of a device
+ * @bdev: device to get the superblock for
+ *
+ * Scans the superblock list and finds the superblock of the file system
+ * mounted on the device given.  Returns the superblock with an active
+ * reference or %NULL if none was found.
+ */
+struct super_block *get_active_super(struct block_device *bdev)
+{
+       struct super_block *sb;
+
+       if (!bdev)
+               return NULL;
+
+restart:
+       spin_lock(&sb_lock);
+       list_for_each_entry(sb, &super_blocks, s_list) {
+               if (list_empty(&sb->s_instances))
+                       continue;
+               if (sb->s_bdev == bdev) {
+                       if (grab_super(sb)) /* drops sb_lock */
+                               return sb;
+                       else
+                               goto restart;
+               }
+       }
+       spin_unlock(&sb_lock);
+       return NULL;
+}
  
-struct super_block * user_get_super(dev_t dev)
+struct super_block *user_get_super(dev_t dev)
 {
        struct super_block *sb;
 
        spin_lock(&sb_lock);
 rescan:
        list_for_each_entry(sb, &super_blocks, s_list) {
+               if (list_empty(&sb->s_instances))
+                       continue;
                if (sb->s_dev ==  dev) {
                        sb->s_count++;
                        spin_unlock(&sb_lock);
                        down_read(&sb->s_umount);
+                       /* still alive? */
                        if (sb->s_root)
                                return sb;
                        up_read(&sb->s_umount);
-                       /* restart only when sb is no longer on the list */
+                       /* nope, got unmounted */
                        spin_lock(&sb_lock);
-                       if (__put_super_and_need_restart(sb))
-                               goto rescan;
+                       __put_super(sb);
+                       goto rescan;
                }
        }
        spin_unlock(&sb_lock);
        return NULL;
 }
 
-SYSCALL_DEFINE2(ustat, unsigned, dev, struct ustat __user *, ubuf)
-{
-        struct super_block *s;
-        struct ustat tmp;
-        struct kstatfs sbuf;
-       int err = -EINVAL;
-
-        s = user_get_super(new_decode_dev(dev));
-        if (s == NULL)
-                goto out;
-       err = vfs_statfs(s->s_root, &sbuf);
-       drop_super(s);
-       if (err)
-               goto out;
-
-        memset(&tmp,0,sizeof(struct ustat));
-        tmp.f_tfree = sbuf.f_bfree;
-        tmp.f_tinode = sbuf.f_ffree;
-
-        err = copy_to_user(ubuf,&tmp,sizeof(struct ustat)) ? -EFAULT : 0;
-out:
-       return err;
-}
-
 /**
  *     do_remount_sb - asks filesystem to change mount options.
  *     @sb:    superblock in question
@@ -529,29 +524,31 @@ out:
 int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
 {
        int retval;
-       int remount_rw;
-       
+       int remount_ro;
+
+       if (sb->s_frozen != SB_UNFROZEN)
+               return -EBUSY;
+
 #ifdef CONFIG_BLOCK
        if (!(flags & MS_RDONLY) && bdev_read_only(sb->s_bdev))
                return -EACCES;
 #endif
+
        if (flags & MS_RDONLY)
                acct_auto_close(sb);
        shrink_dcache_sb(sb);
        sync_filesystem(sb);
 
+       remount_ro = (flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY);
+
        /* If we are remounting RDONLY and current sb is read/write,
           make sure there are no rw files opened */
-       if ((flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY)) {
+       if (remount_ro) {
                if (force)
                        mark_files_ro(sb);
                else if (!fs_may_remount_ro(sb))
                        return -EBUSY;
-               retval = vfs_dq_off(sb, 1);
-               if (retval < 0 && retval != -ENOSYS)
-                       return -EBUSY;
        }
-       remount_rw = !(flags & MS_RDONLY) && (sb->s_flags & MS_RDONLY);
 
        if (sb->s_op->remount_fs) {
                retval = sb->s_op->remount_fs(sb, &flags, data);
@@ -559,31 +556,42 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
                        return retval;
        }
        sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);
-       if (remount_rw)
-               vfs_dq_quota_on_remount(sb);
+
+       /*
+        * Some filesystems modify their metadata via some other path than the
+        * bdev buffer cache (eg. use a private mapping, or directories in
+        * pagecache, etc). Also file data modifications go via their own
+        * mappings. So If we try to mount readonly then copy the filesystem
+        * from bdev, we could get stale data, so invalidate it to give a best
+        * effort at coherency.
+        */
+       if (remount_ro && sb->s_bdev)
+               invalidate_bdev(sb->s_bdev);
        return 0;
 }
 
 static void do_emergency_remount(struct work_struct *work)
 {
-       struct super_block *sb;
+       struct super_block *sb, *n;
 
        spin_lock(&sb_lock);
-       list_for_each_entry(sb, &super_blocks, s_list) {
+       list_for_each_entry_safe(sb, n, &super_blocks, s_list) {
+               if (list_empty(&sb->s_instances))
+                       continue;
                sb->s_count++;
                spin_unlock(&sb_lock);
                down_write(&sb->s_umount);
                if (sb->s_root && sb->s_bdev && !(sb->s_flags & MS_RDONLY)) {
                        /*
-                        * ->remount_fs needs lock_kernel().
-                        *
                         * What lock protects sb->s_flags??
                         */
                        do_remount_sb(sb, MS_RDONLY, NULL, 1);
                }
                up_write(&sb->s_umount);
-               put_super(sb);
                spin_lock(&sb_lock);
+               /* lock was dropped, must reset next */
+               list_safe_reset_next(sb, n, s_list);
+               __put_super(sb);
        }
        spin_unlock(&sb_lock);
        kfree(work);
@@ -608,6 +616,7 @@ void emergency_remount(void)
 
 static DEFINE_IDA(unnamed_dev_ida);
 static DEFINE_SPINLOCK(unnamed_dev_lock);/* protects the above */
+static int unnamed_dev_start = 0; /* don't bother trying below it */
 
 int set_anon_super(struct super_block *s, void *data)
 {
@@ -618,7 +627,9 @@ int set_anon_super(struct super_block *s, void *data)
        if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)
                return -ENOMEM;
        spin_lock(&unnamed_dev_lock);
-       error = ida_get_new(&unnamed_dev_ida, &dev);
+       error = ida_get_new_above(&unnamed_dev_ida, unnamed_dev_start, &dev);
+       if (!error)
+               unnamed_dev_start = dev + 1;
        spin_unlock(&unnamed_dev_lock);
        if (error == -EAGAIN)
                /* We raced and lost with another CPU. */
@@ -629,10 +640,13 @@ int set_anon_super(struct super_block *s, void *data)
        if ((dev & MAX_ID_MASK) == (1 << MINORBITS)) {
                spin_lock(&unnamed_dev_lock);
                ida_remove(&unnamed_dev_ida, dev);
+               if (unnamed_dev_start > dev)
+                       unnamed_dev_start = dev;
                spin_unlock(&unnamed_dev_lock);
                return -EMFILE;
        }
        s->s_dev = MKDEV(0, dev & MINORMASK);
+       s->s_bdi = &noop_backing_dev_info;
        return 0;
 }
 
@@ -645,6 +659,8 @@ void kill_anon_super(struct super_block *sb)
        generic_shutdown_super(sb);
        spin_lock(&unnamed_dev_lock);
        ida_remove(&unnamed_dev_ida, slot);
+       if (slot < unnamed_dev_start)
+               unnamed_dev_start = slot;
        spin_unlock(&unnamed_dev_lock);
 }
 
@@ -703,6 +719,12 @@ static int set_bdev_super(struct super_block *s, void *data)
 {
        s->s_bdev = data;
        s->s_dev = s->s_bdev->bd_dev;
+
+       /*
+        * We set the bdi here to the queue backing, file systems can
+        * overwrite this in ->fill_super()
+        */
+       s->s_bdi = &bdev_get_queue(s->s_bdev)->backing_dev_info;
        return 0;
 }
 
@@ -733,9 +755,14 @@ int get_sb_bdev(struct file_system_type *fs_type,
         * will protect the lockfs code from trying to start a snapshot
         * while we are mounting
         */
-       down(&bdev->bd_mount_sem);
+       mutex_lock(&bdev->bd_fsfreeze_mutex);
+       if (bdev->bd_fsfreeze_count > 0) {
+               mutex_unlock(&bdev->bd_fsfreeze_mutex);
+               error = -EBUSY;
+               goto error_bdev;
+       }
        s = sget(fs_type, test_bdev_super, set_bdev_super, bdev);
-       up(&bdev->bd_mount_sem);
+       mutex_unlock(&bdev->bd_fsfreeze_mutex);
        if (IS_ERR(s))
                goto error_s;
 
@@ -840,8 +867,9 @@ int get_sb_single(struct file_system_type *fs_type,
                        return error;
                }
                s->s_flags |= MS_ACTIVE;
+       } else {
+               do_remount_sb(s, flags, data, 0);
        }
-       do_remount_sb(s, flags, data, 0);
        simple_set_mnt(mnt, s);
        return 0;
 }
@@ -863,6 +891,9 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
        if (!mnt)
                goto out;
 
+       if (flags & MS_KERNMOUNT)
+               mnt->mnt_flags = MNT_INTERNAL;
+
        if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {
                secdata = alloc_secdata();
                if (!secdata)
@@ -877,10 +908,21 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
        if (error < 0)
                goto out_free_secdata;
        BUG_ON(!mnt->mnt_sb);
+       WARN_ON(!mnt->mnt_sb->s_bdi);
 
-       error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);
-       if (error)
-               goto out_sb;
+       error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);
+       if (error)
+               goto out_sb;
+
+       /*
+        * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
+        * but s_maxbytes was an unsigned long long for many releases. Throw
+        * this warning for a little while to try and catch filesystems that
+        * violate this rule. This warning should be either removed or
+        * converted to a BUG() in 2.6.34.
+        */
+       WARN((mnt->mnt_sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
+               "negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);
 
        mnt->mnt_mountpoint = mnt->mnt_root;
        mnt->mnt_parent = mnt;
@@ -900,6 +942,96 @@ out:
 
 EXPORT_SYMBOL_GPL(vfs_kern_mount);
 
+/**
+ * freeze_super - lock the filesystem and force it into a consistent state
+ * @sb: the super to lock
+ *
+ * Syncs the super to make sure the filesystem is consistent and calls the fs's
+ * freeze_fs.  Subsequent calls to this without first thawing the fs will return
+ * -EBUSY.
+ */
+int freeze_super(struct super_block *sb)
+{
+       int ret;
+
+       atomic_inc(&sb->s_active);
+       down_write(&sb->s_umount);
+       if (sb->s_frozen) {
+               deactivate_locked_super(sb);
+               return -EBUSY;
+       }
+
+       if (sb->s_flags & MS_RDONLY) {
+               sb->s_frozen = SB_FREEZE_TRANS;
+               smp_wmb();
+               up_write(&sb->s_umount);
+               return 0;
+       }
+
+       sb->s_frozen = SB_FREEZE_WRITE;
+       smp_wmb();
+
+       sync_filesystem(sb);
+
+       sb->s_frozen = SB_FREEZE_TRANS;
+       smp_wmb();
+
+       sync_blockdev(sb->s_bdev);
+       if (sb->s_op->freeze_fs) {
+               ret = sb->s_op->freeze_fs(sb);
+               if (ret) {
+                       printk(KERN_ERR
+                               "VFS:Filesystem freeze failed\n");
+                       sb->s_frozen = SB_UNFROZEN;
+                       deactivate_locked_super(sb);
+                       return ret;
+               }
+       }
+       up_write(&sb->s_umount);
+       return 0;
+}
+EXPORT_SYMBOL(freeze_super);
+
+/**
+ * thaw_super -- unlock filesystem
+ * @sb: the super to thaw
+ *
+ * Unlocks the filesystem and marks it writeable again after freeze_super().
+ */
+int thaw_super(struct super_block *sb)
+{
+       int error;
+
+       down_write(&sb->s_umount);
+       if (sb->s_frozen == SB_UNFROZEN) {
+               up_write(&sb->s_umount);
+               return -EINVAL;
+       }
+
+       if (sb->s_flags & MS_RDONLY)
+               goto out;
+
+       if (sb->s_op->unfreeze_fs) {
+               error = sb->s_op->unfreeze_fs(sb);
+               if (error) {
+                       printk(KERN_ERR
+                               "VFS:Filesystem thaw failed\n");
+                       sb->s_frozen = SB_FREEZE_TRANS;
+                       up_write(&sb->s_umount);
+                       return error;
+               }
+       }
+
+out:
+       sb->s_frozen = SB_UNFROZEN;
+       smp_wmb();
+       wake_up(&sb->s_wait_unfrozen);
+       deactivate_locked_super(sb);
+
+       return 0;
+}
+EXPORT_SYMBOL(thaw_super);
+
 static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
 {
        int err;