freeze_bdev — - lock a filesystem and force it into a consistent state
struct super_block * freeze_bdev ( | struct block_device * bdev); |
If a superblock is found on this device, we take the s_umount semaphore
on it to make sure nobody unmounts until the snapshot creation is done.
The reference counter (bd_fsfreeze_count) guarantees that only the last
unfreeze process can unfreeze the frozen filesystem actually when multiple
freeze requests arrive simultaneously. It counts up in freeze_bdev and
count down in thaw_bdev. When it becomes 0, thaw_bdev will unfreeze
actually.