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    SubjectRe: [PATCH -v2] blk-mq: Start to fix memory ordering...
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    On 09/06/2017 02:00 AM, Peter Zijlstra wrote:
    >
    > Attempt to untangle the ordering in blk-mq. The patch introducing the
    > single smp_mb__before_atomic() is obviously broken in that it doesn't
    > clearly specify a pairing barrier and an obtained guarantee.
    >
    > The comment is further misleading in that it hints that the
    > deadline store and the COMPLETE store also need to be ordered, but
    > AFAICT there is no such dependency. However what does appear to be
    > important is the clear happening _after_ the store, and that worked by
    > pure accident.
    >
    > This clarifies blk_mq_start_request() -- we should not get there with
    > STARTING set -- this simplifies the code and makes the barrier usage
    > sane (the old code could be read to allow not having _any_ atomic after
    > the barrier, in which case the barrier hasn't got anything to order). We
    > then also introduce the missing pairing barrier for it.
    >
    > Also down-grade the barrier to smp_wmb(), this is cheaper for
    > PowerPC/ARM and doesn't cost anything extra on x86.
    >
    > And it documents the STARTING vs COMPLETE ordering. Although I've not
    > been entirely successful in reverse engineering the blk-mq state
    > machine so there might still be more funnies around timeout vs
    > requeue.
    >
    > If I got anything wrong, feel free to educate me by adding comments to
    > clarify things ;-)

    Sorry for the belated response on this, I spent some time and looked
    over everything. Looks solid to me.

    I'll queue this up for some testing, and also add a compile check to
    prevent us violating the need to have STARTED and COMPLETED be in
    the same byte of storage.

    --
    Jens Axboe

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