[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

[dm-devel] Re: [RFC PATCH 1/3] Implement generic freeze feature



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


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]