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    SubjectRe: [PATCH 01/13] blkcg: fix ref count issue with bio_blkcg() using task_css
    On Mon, Nov 26, 2018 at 04:19:34PM -0500, Dennis Zhou wrote:
    > The bio_blkcg() function turns out to be inconsistent and consequently
    > dangerous to use. The first part returns a blkcg where a reference is
    > owned by the bio meaning it does not need to be rcu protected. However,
    > the third case, the last line, is problematic:
    >
    > return css_to_blkcg(task_css(current, io_cgrp_id));
    >
    > This can race against task migration and the cgroup dying. It is also
    > semantically different as it must be called rcu protected and is
    > susceptible to failure when trying to get a reference to it.
    >
    > This patch adds association ahead of calling bio_blkcg() rather than
    > after. This makes association a required and explicit step along the
    > code paths for calling bio_blkcg(). In blk-iolatency, association is
    > moved above the bio_blkcg() call to ensure it will not return %NULL.
    >
    > BFQ uses the old bio_blkcg() function, but I do not want to address it
    > in this series due to the complexity. I have created a private version
    > documenting the inconsistency and noting not to use it.
    >
    > Signed-off-by: Dennis Zhou <dennis@kernel.org>
    > Acked-by: Tejun Heo <tj@kernel.org>

    Reviewed-by: Josef Bacik <josef@toxicpanda.com>

    Thanks,

    Josef

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