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    SubjectRe: [RFC][PATCH] Move bv_offset/bv_len update after bio_endio in __end_that_request_first
    On Thu, Jan 01 2004, Christophe Saout wrote:
    > Hi!
    >
    > Can we move the update of bio_index(bio)->bv_offset and bv_len after the
    > bio_endio call in __end_that_request_first please (if a bvec is partially
    > completed)?
    >
    > The bi_idx is currently also updated after the bio_endio call.
    >
    > Currently the bi_end_io function cannot exactly determine whether a bvec
    > was completed or not.
    >
    > Think of the following situation:
    >
    > bv_offset is 0 and bv_len is 4096, now the driver completes 2048 bytes of
    > that bvec.
    >
    > At the moment bv_offset and bv_len are set to 2048 first. The bi_end_io
    > function can't distinguish between this situation and the situation where
    > bv_offset and bv_len were 2048 before and that bvec was completed (because
    > bi_idx is incremented afterwards).
    >
    > This shouldn't break any user since most users are waiting for the whole
    > bio to complete with if (bio->bi_size > 0) return 1;.
    >
    > I need this because I want to release buffers as soon as possible. The
    > incoming bio can get split by my driver due to problems allocating buffers.
    > If the partial bio returns and can't release its buffers immediately the
    > whole thing might deadlock.
    >
    > That's why I need to know exactly how many and which bvecs were completed
    > in my bi_end_io function.
    >
    > Or do you think it is safer to count backwards using bi_vcnt and bi_size?

    I'm inclined to thinking that, indeed. Those two fields have a more well
    established usage, so I think you'll be better off doing that in the
    long run.

    --
    Jens Axboe

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