Hi,
+ if (test_and_set_bit(BD_FREEZE_OP, &bdev->bd_state))
+ return ERR_PTR(-EBUSY);
+
+ sb = get_super_without_lock(bdev);
+
+ /* If super_block has been already frozen, return. */
+ if (sb && sb->s_frozen != SB_UNFROZEN) {
+ put_super(sb);
+ clear_bit(BD_FREEZE_OP, &bdev->bd_state);
+ return sb;
+ }
The BD_FREEZE_OP flag in the block_device already prevents multiple
freezes for a singe block device, so there is no need for this
additional check and the get_super_without_lock helper.
I think the above check is needed because BD_FREEZE_OP is used to
guarantee that the following two operations are done atomically.
1. Check if the filesystem has already been frozen.
(sb->s_frozen != SB_UNFROZEN)
2. Do the filesystem specific freeze operation
(write_super_lockfs)
The freeze operation can be done again for the frozen filesystem
unless the filesystem is checked.
And if write_super_lockfs doesn't consider that the filesystem
has already been frozen, a problem might occur.
down(&bdev->bd_mount_sem);
And with that flag bd_mount_sem is also obsolete for preventing the
multiple freeze operations.
bd_mount_sem is needed because it is used to guarantee that the freeze
operation never run while mounting (get_sb_bdev()).
We still need investigate what
synchronization we need vs unmount which also takes bd_mount_sem without
every having document what it exactly protects.
And s_umount (semaphore) is acquired in get_user() (called by freeze_bdev())
to guarantee that unmount never run while the filesystem is frozen.
Cheers, Takashi
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