Comment 3 for bug 1775235

Hi Joseph,

Thanks for looking at this so promptly. I have downloaded and tested your kernel. It appears to work well both with scsi mq enabled and disabled (over virtio-scsi). I also ran some basic io integrity tests and didn't spot any problems.

Before you commit this, may I propose a v2 of my own patch which is functionally identical but slightly more elegant? It does the atomic_inc() within virtscsi_pick_vq_mq(). That's probably preferable given the other uses of atomic_*() for non-mq are done within virtscsi_pick_vq().

Have a look:
-------------8<-------------
--- old/linux-4.4.0/drivers/scsi/virtio_scsi.c 2018-06-04 10:23:07.000000000 -0700
+++ new/linux-4.4.0/drivers/scsi/virtio_scsi.c 2018-06-07 06:20:58.596764040 -0700
@@ -588,11 +588,13 @@
 }

 static struct virtio_scsi_vq *virtscsi_pick_vq_mq(struct virtio_scsi *vscsi,
+ struct virtio_scsi_target_state *tgt,
                                                  struct scsi_cmnd *sc)
 {
        u32 tag = blk_mq_unique_tag(sc->request);
        u16 hwq = blk_mq_unique_tag_to_hwq(tag);

+ atomic_inc(&tgt->reqs);
        return &vscsi->req_vqs[hwq];
 }

@@ -642,7 +644,7 @@
        struct virtio_scsi_vq *req_vq;

        if (shost_use_blk_mq(sh))
- req_vq = virtscsi_pick_vq_mq(vscsi, sc);
+ req_vq = virtscsi_pick_vq_mq(vscsi, tgt, sc);
        else
                req_vq = virtscsi_pick_vq(vscsi, tgt);

Signed-off-by: Felipe Franciosi <email address hidden>

-------------8<-------------

I'm happy to test it again if you'd like, but it should be functionally identical.