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    SubjectRe: [PATCH] Introduce freeze_super and thaw_super for the fsfreeze ioctl
    On Tue, Mar 23, 2010 at 10:22:00AM -0400, Josef Bacik wrote:
    > Currently the way we do freezing is by passing sb>s_bdev to freeze_bdev and then
    > letting it do all the work. But freezing is more of an fs thing, and doesn't
    > really have much to do with the bdev at all, all the work gets done with the
    > super. In btrfs we do not populate s_bdev, since we can have multiple bdev's
    > for one fs and setting s_bdev makes removing devices from a pool kind of tricky.
    > This means that freezing a btrfs filesystem fails, which causes us to corrupt
    > with things like tux-on-ice which use the fsfreeze mechanism. So instead of
    > populating sb->s_bdev with a random bdev in our pool, I've broken the actual fs
    > freezing stuff into freeze_super and thaw_super. These just take the
    > super_block that we're freezing and does the appropriate work. It's basically
    > just copy and pasted from freeze_bdev. I've then converted freeze_bdev over to
    > use the new super helpers. I've tested this with ext4 and btrfs and verified
    > everything continues to work the same as before.
    >
    > The only new gotcha is multiple calls to the fsfreeze ioctl will return EBUSY if
    > the fs is already frozen. I thought this was a better solution than adding a
    > freeze counter to the super_block, but if everybody hates this idea I'm open to
    > suggestions. Thanks,

    Locking is all wrong there. We don't need to worry about umount; we *already*
    have an active reference. And leaving a kernel object with semaphore held
    when ioctl returns is completely wrong.


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