Re: [PATCH v6 07/12] nvdimm: virtio_pmem: use READ_ONCE()/WRITE_ONCE() for wait flags

[Pankaj Gupta]

> Use READ_ONCE()/WRITE_ONCE() for the wait_event() flags (done and
> wq_buf_avail). They are observed by waiters without pmem_lock, so make
> the accesses explicit single loads/stores and avoid compiler
> reordering/caching across the wait/wake paths.
>
> Signed-off-by: Li Chen <me@linux.beauty>

Acked-by: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx

> ---
> v2->v3:
> - Split out READ_ONCE()/WRITE_ONCE() updates from patch 3/7 (no functional
>   change intended).
> v3->v4:
> - Rebased onto v7.1-rc7 and renumbered after the flush error patches.
>
>  drivers/nvdimm/nd_virtio.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> index 8ed4d6b3a9284..da829e9f4bdff 100644
> --- a/drivers/nvdimm/nd_virtio.c
> +++ b/drivers/nvdimm/nd_virtio.c
> @@ -18,9 +18,9 @@ static void virtio_pmem_wake_one_waiter(struct virtio_pmem *vpmem)
>
>         req_buf = list_first_entry(&vpmem->req_list,
>                                    struct virtio_pmem_request, list);
> -       req_buf->wq_buf_avail = true;
> +       list_del_init(&req_buf->list);
> +       WRITE_ONCE(req_buf->wq_buf_avail, true);
>         wake_up(&req_buf->wq_buf);
> -       list_del(&req_buf->list);
>  }
>
>   /* The interrupt handler */
> @@ -34,7 +34,7 @@ void virtio_pmem_host_ack(struct virtqueue *vq)
>         spin_lock_irqsave(&vpmem->pmem_lock, flags);
>         while ((req_data = virtqueue_get_buf(vq, &len)) != NULL) {
>                 virtio_pmem_wake_one_waiter(vpmem);
> -               req_data->done = true;
> +               WRITE_ONCE(req_data->done, true);
>                 wake_up(&req_data->host_acked);
>         }
>         spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> @@ -66,7 +66,7 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
>         if (!req_data)
>                 return -ENOMEM;
>
> -       req_data->done = false;
> +       WRITE_ONCE(req_data->done, false);
>         init_waitqueue_head(&req_data->host_acked);
>         init_waitqueue_head(&req_data->wq_buf);
>         INIT_LIST_HEAD(&req_data->list);
> @@ -87,12 +87,12 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
>                                         GFP_ATOMIC)) == -ENOSPC) {
>
>                 dev_info(&vdev->dev, "failed to send command to virtio pmem device, no free slots in the virtqueue\n");
> -               req_data->wq_buf_avail = false;
> +               WRITE_ONCE(req_data->wq_buf_avail, false);
>                 list_add_tail(&req_data->list, &vpmem->req_list);
>                 spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
>
>                 /* A host response results in "host_ack" getting called */
> -               wait_event(req_data->wq_buf, req_data->wq_buf_avail);
> +               wait_event(req_data->wq_buf, READ_ONCE(req_data->wq_buf_avail));
>                 spin_lock_irqsave(&vpmem->pmem_lock, flags);
>         }
>         err1 = virtqueue_kick(vpmem->req_vq);
> @@ -106,7 +106,7 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
>                 err = -EIO;
>         } else {
>                 /* A host response results in "host_ack" getting called */
> -               wait_event(req_data->host_acked, req_data->done);
> +               wait_event(req_data->host_acked, READ_ONCE(req_data->done));
>                 err = le32_to_cpu(req_data->resp.ret);
>         }
>
> --
> 2.52.0