On Thu, Mar 28, 2013 at 04:10:02PM +0800, Asias He wrote:
> On Thu, Mar 28, 2013 at 08:16:59AM +0200, Michael S. Tsirkin wrote:
> > On Thu, Mar 28, 2013 at 10:17:28AM +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.
> > >
> > > 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".
> > >
> > > Signed-off-by: Asias He <asias@xxxxxxxxxx>
> > > ---
> > > drivers/vhost/tcm_vhost.c | 38 +++++++++++++++++++++++++++++++++-----
> > > 1 file changed, 33 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > > index 5e3d4487..0524267 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;
> > >
> >
> > I would just move the check to under vq mutex,
> > and avoid rcu completely. In vhost-net we are using
> > private data outside lock so we can't do this,
> > no such issue here.
>
> Are you talking about:
>
> handle_tx:
> /* TODO: check that we are running from vhost_worker? */
> sock = rcu_dereference_check(vq->private_data, 1);
> if (!sock)
> return;
>
> wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> if (wmem >= sock->sk->sk_sndbuf) {
> mutex_lock(&vq->mutex);
> tx_poll_start(net, sock);
> mutex_unlock(&vq->mutex);
> return;
> }
> mutex_lock(&vq->mutex);
>
> Why not do the atomic_read and tx_poll_start under the vq->mutex, and thus do
> the check under the lock as well.
>
> handle_rx:
> mutex_lock(&vq->mutex);
>
> /* TODO: check that we are running from vhost_worker? */
> struct socket *sock = rcu_dereference_check(vq->private_data, 1);
>
> if (!sock)
> return;
>
> mutex_lock(&vq->mutex);
>
> Can't we can do the check under the vq->mutex here?
>
> The rcu is still there but it makes the code easier to read. IMO, If we want to
> use rcu, use it explicitly and avoid the vhost rcu completely.
The point is to make spurios wakeups as lightweight as possible.
The seemed to happen a lot with -net.
Should not happen with -scsi at all.
> > > mutex_lock(&vq->mutex);
> > > @@ -829,11 +845,12 @@ static int vhost_scsi_set_endpoint(
> > > sizeof(vs->vs_vhost_wwpn));
> > > 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);
> > > vhost_init_used(vq);
> > > mutex_unlock(&vq->mutex);
> > > }
> > > - vs->vs_endpoint = true;
> > > ret = 0;
> > > } else {
> > > ret = -EEXIST;
> >
> >
> > There's also some weird smp_mb__after_atomic_inc() with no
> > atomic in sight just above ... Nicholas what was the point there?
> >
> >
> > > @@ -849,6 +866,8 @@ 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;
> > >
> > > @@ -884,9 +903,18 @@ 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);
> > > + }
> > > + }
> >
> > I'm trying to understand what's going on here.
> > Does vhost_scsi only have a single target?
> > Because the moment you clear one target you
> > also set private_data to NULL ...
>
> vhost_scsi supports multi target. Currently, We can not disable specific target
> under the wwpn. When we clear or set the endpoint, we disable or enable all the
> targets under the wwpn.
>
> >
> > > mutex_unlock(&vs->dev.mutex);
> > > return 0;
> > >
> > > --
> > > 1.8.1.4
>
> --
> Asias
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