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    SubjectRe: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk
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    On Mon, Jun 18, 2012 at 11:21 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
    > On Mon, Jun 18, 2012 at 02:53:10PM +0800, Asias He wrote:
    >> +static void virtblk_make_request(struct request_queue *q, struct bio *bio)
    >> +{
    >> +     struct virtio_blk *vblk = q->queuedata;
    >> +     unsigned int num, out = 0, in = 0;
    >> +     struct virtblk_req *vbr;
    >> +
    >> +     BUG_ON(bio->bi_phys_segments + 2 > vblk->sg_elems);
    >> +     BUG_ON(bio->bi_rw & (REQ_FLUSH | REQ_FUA));
    >> +
    >> +     vbr = virtblk_alloc_req(vblk, GFP_NOIO);
    >> +     if (!vbr) {
    >> +             bio_endio(bio, -ENOMEM);
    >> +             return;
    >> +     }
    >> +
    >> +     vbr->bio = bio;
    >> +     vbr->req = NULL;
    >> +     vbr->out_hdr.type = 0;
    >> +     vbr->out_hdr.sector = bio->bi_sector;
    >> +     vbr->out_hdr.ioprio = bio_prio(bio);
    >> +
    >> +     sg_set_buf(&vbr->sg[out++], &vbr->out_hdr, sizeof(vbr->out_hdr));
    >> +
    >> +     num = blk_bio_map_sg(q, bio, vbr->sg + out);
    >> +
    >> +     sg_set_buf(&vbr->sg[num + out + in++], &vbr->status,
    >> +                sizeof(vbr->status));
    >> +
    >> +     if (num) {
    >> +             if (bio->bi_rw & REQ_WRITE) {
    >> +                     vbr->out_hdr.type |= VIRTIO_BLK_T_OUT;
    >> +                     out += num;
    >> +             } else {
    >> +                     vbr->out_hdr.type |= VIRTIO_BLK_T_IN;
    >> +                     in += num;
    >> +             }
    >> +     }
    >> +
    >> +     spin_lock_irq(vblk->disk->queue->queue_lock);
    >> +     if (virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
    >> +                           GFP_ATOMIC) < 0) {
    >> +             spin_unlock_irq(vblk->disk->queue->queue_lock);
    >
    > Any implications of dropping lock like that?
    > E.g. for suspend. like we are still discussing with
    > unlocked kick?

    Since we aquired the lock in this function there should be no problem.
    Whatever protects against vblk or vblk->disk disappearing upon
    entering this function also protects after unlocking queue_lock.
    Otherwise all .make_request_fn() functions would be broken.

    I'd still like to understand the details though.

    Stefan
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