On Fri, Mar 15, 2013 at 09:14:07AM +0800, Asias He wrote:
> Currently, vs->vs_endpoint is used indicate if the endpoint is setup or
> not. It is set or cleared in vhost_scsi_set_endpoint() or
> vhost_scsi_clear_endpoint() under the vs->dev.mutex lock. However, when
> we check it in vhost_scsi_handle_vq(), we ignored the lock, this is
> wrong.
This one, I don't get. Why is it wrong? Could you please describe the
race codition you are trying to prevent?
> Instead of using the vs->vs_endpoint and the vs->dev.mutex lock to
> indicate the status of the endpoint, we use per virtqueue
> vq->private_data to indicate it. In this way, we can only take the
> vq->mutex lock which is per queue and make the concurrent multiqueue
> process having less lock contention. Further, in the read side of
> vq->private_data, we can even do not take only lock if it is accessed in
> the vhost worker thread, because it is protected by "vhost rcu".
But (unlike with -net) you never actually need the pointer. So why all
the complexity?
> Signed-off-by: Asias He <asias@xxxxxxxxxx>
> ---
> drivers/vhost/tcm_vhost.c | 46 ++++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 40 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> index 43fb11e..099feef 100644
> --- a/drivers/vhost/tcm_vhost.c
> +++ b/drivers/vhost/tcm_vhost.c
> @@ -67,7 +67,6 @@ struct vhost_scsi {
> /* Protected by vhost_scsi->dev.mutex */
> struct tcm_vhost_tpg *vs_tpg[VHOST_SCSI_MAX_TARGET];
> char vs_vhost_wwpn[TRANSPORT_IQN_LEN];
> - bool vs_endpoint;
>
> struct vhost_dev dev;
> struct vhost_virtqueue vqs[VHOST_SCSI_MAX_VQ];
> @@ -91,6 +90,24 @@ static int iov_num_pages(struct iovec *iov)
> ((unsigned long)iov->iov_base & PAGE_MASK)) >> PAGE_SHIFT;
> }
>
> +static bool tcm_vhost_check_endpoint(struct vhost_virtqueue *vq)
> +{
> + bool ret = false;
> +
> + /*
> + * We can handle the vq only after the endpoint is setup by calling the
> + * VHOST_SCSI_SET_ENDPOINT ioctl.
> + *
> + * TODO: Check that we are running from vhost_worker which acts
> + * as read-side critical section for vhost kind of RCU.
> + * See the comments in struct vhost_virtqueue in drivers/vhost/vhost.h
> + */
> + if (rcu_dereference_check(vq->private_data, 1))
> + ret = true;
> +
> + return ret;
> +}
> +
> static int tcm_vhost_check_true(struct se_portal_group *se_tpg)
> {
> return 1;
> @@ -581,8 +598,7 @@ static void vhost_scsi_handle_vq(struct vhost_scsi *vs,
> int head, ret;
> u8 target;
>
> - /* Must use ioctl VHOST_SCSI_SET_ENDPOINT */
> - if (unlikely(!vs->vs_endpoint))
> + if (!tcm_vhost_check_endpoint(vq))
> return;
>
> mutex_lock(&vq->mutex);
> @@ -781,8 +797,9 @@ static int vhost_scsi_set_endpoint(
> {
> struct tcm_vhost_tport *tv_tport;
> struct tcm_vhost_tpg *tv_tpg;
> + struct vhost_virtqueue *vq;
> bool match = false;
> - int index, ret;
> + int index, ret, i;
>
> mutex_lock(&vs->dev.mutex);
> /* Verify that ring has been setup correctly. */
> @@ -826,7 +843,13 @@ static int vhost_scsi_set_endpoint(
> if (match) {
> memcpy(vs->vs_vhost_wwpn, t->vhost_wwpn,
> sizeof(vs->vs_vhost_wwpn));
> - vs->vs_endpoint = true;
> + for (i = 0; i < VHOST_SCSI_MAX_VQ; i++) {
> + vq = &vs->vqs[i];
> + /* Flushing the vhost_work acts as synchronize_rcu */
> + mutex_lock(&vq->mutex);
> + rcu_assign_pointer(vq->private_data, vs);
> + mutex_unlock(&vq->mutex);
> + }
> ret = 0;
> } else {
> ret = -EEXIST;
> @@ -842,6 +865,8 @@ static int vhost_scsi_clear_endpoint(
> {
> struct tcm_vhost_tport *tv_tport;
> struct tcm_vhost_tpg *tv_tpg;
> + struct vhost_virtqueue *vq;
> + bool match = false;
> int index, ret, i;
> u8 target;
>
> @@ -877,9 +902,18 @@ static int vhost_scsi_clear_endpoint(
> }
> tv_tpg->tv_tpg_vhost_count--;
> vs->vs_tpg[target] = NULL;
> - vs->vs_endpoint = false;
> + match = true;
> mutex_unlock(&tv_tpg->tv_tpg_mutex);
> }
> + if (match) {
> + for (i = 0; i < VHOST_SCSI_MAX_VQ; i++) {
> + vq = &vs->vqs[i];
> + /* Flushing the vhost_work acts as synchronize_rcu */
> + mutex_lock(&vq->mutex);
> + rcu_assign_pointer(vq->private_data, NULL);
> + mutex_unlock(&vq->mutex);
> + }
> + }
> mutex_unlock(&vs->dev.mutex);
> return 0;
>
> --
> 1.8.1.4
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