vfs: Provide function to get superblock and wait for it to thaw

In quota code we need to find a superblock corresponding to a device and wait
for superblock to be unfrozen. However this waiting has to happen without
s_umount semaphore because that is required for superblock to thaw. So provide
a function in VFS for this to keep dances with s_umount where they belong.

[AV: implementation switched to saner variant]

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 22 insertions, 0 deletions

diff --git a/fs/super.c b/fs/super.c
index 6015c02296b7..6277ec6cb60a 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -634,6 +634,28 @@ rescan:
 EXPORT_SYMBOL(get_super);
 
 /**
+ *      get_super_thawed - get thawed 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. The superblock is returned once it is thawed
+ *      (or immediately if it was not frozen). %NULL is returned if no match
+ *      is found.
+ */
+struct super_block *get_super_thawed(struct block_device *bdev)
+{
+        while (1) {
+                struct super_block *s = get_super(bdev);
+                if (!s || s->s_frozen == SB_UNFROZEN)
+                        return s;
+                up_read(&s->s_umount);
+                vfs_check_frozen(s, SB_FREEZE_WRITE);
+                put_super(s);
+        }
+}
+EXPORT_SYMBOL(get_super_thawed);
+
+/**
  * get_active_super - get an active reference to the superblock of a device
  * @bdev: device to get the superblock for
  *