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    SubjectRe: [PATCH] Freeze bdevs when freezing processes.
    Hi Nigel,

    On Thu, Oct 26, 2006 at 06:18:29PM +1000, Nigel Cunningham wrote:
    > On Thu, 2006-10-26 at 17:30 +1000, David Chinner wrote:
    > > We have daemons running in the background that can definitely do stuff
    > > after a sync. hmm - one does try_to_freeze() after a wakeup, the
    > > other does:
    > >
    > > if (unlikely(freezing(current))) {
    > > set_bit(XBT_FORCE_SLEEP, &target->bt_flags);
    > > refrigerator();
    > > } else {
    > > clear_bit(XBT_FORCE_SLEEP, &target->bt_flags);
    > > }
    > >
    > > before it goes to sleep. So that one (xfsbufd - metadata buffer flushing)
    > > can definitely wake up after the sync and do work, and the other could if
    > > the kernel thread freeze occurs after the sync.
    > >
    > > Another good question at this point - exactly how should we be putting
    > > these thread to to sleep? Are both these valid methods for freezing them?
    > > And should we be freezing when we wake up instead of before we go to
    > > sleep? i.e. what are teh rules we are supposed to be following?
    > As you have them at the moment, the threads seem to be freezing fine.
    > The issue I've seen in the past related not to threads but to timer
    > based activity. Admittedly it was 2.6.14 when I last looked at it, but
    > there used to be a possibility for XFS to submit I/O from a timer when
    > the threads are frozen but the bdev isn't frozen. Has that changed?

    I didn't think we've ever done that - periodic or delayed operations
    are passed off to the kernel threads to execute. A stack trace
    (if you still have it) would be really help here.

    Hmmm - we have a couple of per-cpu work queues as well that are
    used on I/O completion and that can, in some circumstances,
    trigger new transactions. If we are only flush metadata, then
    I don't think that any more I/o will be issued, but I could be
    wrong (maze of twisty passages).


    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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