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    SubjectRe: [patch-mm] Workaround for RAID breakage
    On Mon, Jan 15 2007, Thomas Gleixner wrote:
    > On Mon, 2007-01-15 at 09:08 +0100, Thomas Gleixner wrote:
    > > > Thomas saw something similar yesterday and he the partial results that
    > > > git.block (between rc2-mm1 and rc4-mm1) breaks certain disk drivers or
    > > > filesystems drivers. For me it worked fine, so it must be only on some
    > > > combinations. The changes to ll_rw_block.c look quite extensive.
    > >
    > > Yes. Jens Axboe confirmed yesterday that the plug changes broke RAID.
    >
    > I tracked this down and found two problems:
    >
    > - The new plug/unplug code does not check for underruns. That allows the
    > plug count (ioc->plugged) to become negative. This gets triggered from
    > various places.
    >
    > AFAICS this is intentional to avoid checks all over the place, but the
    > underflow check is missing. All we need to do is make sure, that in case
    > of ioc->plugged == 0 we return early and bug, if there is either a queue
    > plugged in or the plugged_list is not empty.
    >
    > Jens ?

    It should not go negative, that would be a bug elsewhere. So it's
    interesting if it does, we should definitely put a WARN_ON() check in
    there for that.

    > - The raid1 code has no bitmap set in remount r/w. So the
    > pending_bio_list gets not processed for quite a time. The workaround is
    > to kick mddev->thread, so the list is processed. Not sure about that.
    >
    > Neil ?

    Super, thanks for that Thomas! I'll merge it in the plug branch.

    --
    Jens Axboe

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