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    SubjectRe: [PATCH block#for-2.6.31 2/3] block: set rq->resid_len to blk_rq_bytes() on issue
    Pete Zaitcev wrote:
    > On Sat, 16 May 2009 00:18:42 +0900, Tejun Heo <htejun@gmail.com> wrote:
    >
    >> In commit c3a4d78c580de4edc9ef0f7c59812fb02ceb037f, while introducing
    >> rq->resid_len, the default value of residue count was changed from
    >> full count to zero. []
    >
    > So it's not a residue anymore, right? You should've renamed it to
    > rq->count or something, then. Now we have this:

    It still is. It just is restoring the original behavior.

    >> +++ block/drivers/block/ub.c
    >> @@ -781,8 +781,7 @@ static void ub_rw_cmd_done(struct ub_dev
    >>
    >> if (cmd->error == 0) {
    >> if (blk_pc_request(rq)) {
    >> - if (cmd->act_len < blk_rq_bytes(rq))
    >> - rq->resid_len = blk_rq_bytes(rq) - cmd->act_len;
    >> + rq->resid_len -= min(cmd->act_len, rq->resid_len);
    >> scsi_status = 0;
    >
    > You are subtracting resid_len from itself. Just how in the world
    > can this be correct?
    >
    > Even it if is, in fact, correct, it's such an eggregious violation
    > of good style, that your good programmer's card is going to lose
    > a big coupon and have a hole punched in it.

    The original code was

    if (cmd->act_len >= rq->data_len)
    rq->data_len = 0;
    else
    rq->data_len -= cmd->act_len

    So, I could have written

    if (cmd->act_len >= rq->resid_len)
    rq->resid_len = 0;
    else
    rq->resid_len -= cmd->act_len

    Instead I wrote

    rq->resid_len -= min(cmd->act_len, rq->resid_len);

    It's just capping the amount to be subtracted so that resid_len
    doesn't underflow. What is so wrong or bad style about that?

    > This is not in Linus' tree yet, but I'm going to take a hard look
    > at this once it shows up.

    It would be great if you do before it hits Linus's tree.

    Thanks.

    --
    tejun


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