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    SubjectRe: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex
    On Fri, Nov 10, 2006 at 12:11:46AM +0100, Pavel Machek wrote:
    > ? Not sure if I quite understand, but if dm breaks sync... something
    > is teribly wrong with dm. And we do simple sys_sync()... so I do not
    > think we have a problem.

    If you want to handle arbitrary kernel state, you might have a device-mapper
    device somewhere lower down the stack of devices that is queueing any I/O
    that reaches it. So anything waiting for I/O completion will wait until
    the dm process that suspended that device has finished whatever it is doing
    - and that might be a quick thing carried out by a userspace lvm tool, or
    a long thing carried out by an administrator using dmsetup.

    I'm guessing you need a way of detecting such state lower down the stack
    then optionally either aborting the operation telling the user it can't be
    done at present; waiting for however long it takes (perhaps for ever if
    the admin disappeared); or more probably skipping those devices on a
    'best endeavours' basis.

    I'm suggesting sysfs or something built on bd_claim might offer a mechanism
    for traversing the stack.

    Alasdair
    --
    agk@redhat.com
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