[PATCH 1/2] vfs: Provide function to get superblock and wait for it to thaw

[Jan Kara <jack@xxxxxxx>]

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.

Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/super.c         |   50 ++++++++++++++++++++++++++++++++++++++------------
 include/linux/fs.h |    1 +
 2 files changed, 39 insertions(+), 12 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 6015c02..1f657d6 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -593,15 +593,8 @@ void iterate_supers_type(struct file_system_type *type,
 
 EXPORT_SYMBOL(iterate_supers_type);
 
-/**
- *	get_super - get 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. %NULL is returned if no match is found.
- */
-
-struct super_block *get_super(struct block_device *bdev)
+static struct super_block *__get_super(struct block_device *bdev,
+				       bool wait_thaw)
 {
 	struct super_block *sb;
 
@@ -618,9 +611,13 @@ rescan:
 			spin_unlock(&sb_lock);
 			down_read(&sb->s_umount);
 			/* still alive? */
-			if (sb->s_root && (sb->s_flags & MS_BORN))
-				return sb;
-			up_read(&sb->s_umount);
+			if (sb->s_root && (sb->s_flags & MS_BORN)) {
+				if (!wait_thaw || sb->s_frozen == SB_UNFROZEN)
+					return sb;
+				up_read(&sb->s_umount);
+				vfs_check_frozen(sb, SB_FREEZE_WRITE);
+			} else
+				up_read(&sb->s_umount);
 			/* nope, got unmounted */
 			spin_lock(&sb_lock);
 			__put_super(sb);
@@ -631,9 +628,38 @@ rescan:
 	return NULL;
 }
 
+/**
+ *	get_super - get 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. %NULL is returned if no match is found.
+ *	Note that the returned superblock may be frozen.
+ */
+
+struct super_block *get_super(struct block_device *bdev)
+{
+	return __get_super(bdev, false);
+}
 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)
+{
+	return __get_super(bdev, true);
+}
+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
  *
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 386da09..69cd5bb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2496,6 +2496,7 @@ extern void get_filesystem(struct file_system_type *fs);
 extern void put_filesystem(struct file_system_type *fs);
 extern struct file_system_type *get_fs_type(const char *name);
 extern struct super_block *get_super(struct block_device *);
+extern struct super_block *get_super_thawed(struct block_device *);
 extern struct super_block *get_active_super(struct block_device *bdev);
 extern void drop_super(struct super_block *sb);
 extern void iterate_supers(void (*)(struct super_block *, void *), void *);
-- 
1.7.1

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