On Wed, Mar 13, 2013 at 09:56:41AM +0100, Paolo Bonzini wrote:
> Il 13/03/2013 08:34, Asias He ha scritto:
> > 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.
> >
> > 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 <as...@redhat.com>
> > ---
> > drivers/vhost/tcm_vhost.c | 43 +++++++++++++++++++++++++++++++++++++------
> > 1 file changed, 37 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > index 43fb11e..094fb10 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 +90,22 @@ 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?
> > + */
> > + 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 +596,7 @@ 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;
>
> You would still need at least a rcu_read_lock/unlock (actually srcu,
> since vhost_scsi_handle_vq can sleep)...
See handle_rx() and handle_rx() in drivers/vhost/net.c
/* Expects to be always run from workqueue - which acts as
* read-size critical section for our kind of RCU. */
This is how vhost works, no?
But, personally, I would prefer to use explicit locking instead of this
trick.
> > mutex_lock(&vq->mutex);
> > @@ -781,8 +795,9 @@ 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 +841,13 @@ 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];
> > + mutex_lock(&vq->mutex);
> > + rcu_assign_pointer(vq->private_data, vs);
> > + vhost_init_used(vq);
> > + mutex_unlock(&vq->mutex);
>
> ... and a synchronize_srcu here. But this is not correct use of RCU.
> To use RCU correctly, you need to _copy_ (that's the "C" in RCU) the
> whole vs structure on every set_endpoint or clear_endpoint operation,
> and free it after synchronize_srcu returns.
See the comments in struct vhost_virtqueue in drivers/vhost/vhost.h
/* We use a kind of RCU to access private pointer.
* All readers access it from worker, which makes it possible to
* flush the vhost_work instead of synchronize_rcu. Therefore readers do
* not need to call rcu_read_lock/rcu_read_unlock: the beginning of
* vhost_work execution acts instead of rcu_read_lock() and the end of
* vhost_work execution acts instead of rcu_read_unlock().
* Writers use virtqueue mutex. */
void __rcu *private_data;
> What you're trying to do is really an rwlock, just use that. :)
Yes, but the downside is that it introduces another lock.
> Paolo
>
> > + }
> > ret = 0;
> > } else {
> > ret = -EEXIST;
> > @@ -842,6 +863,8 @@ 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 +900,17 @@ 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];
> > + mutex_lock(&vq->mutex);
> > + rcu_assign_pointer(vq->private_data, NULL);
> > + mutex_unlock(&vq->mutex);
> > + }
> > + }
> > mutex_unlock(&vs->dev.mutex);
> > return 0;
> >
> >
>
--
Asias
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