Il 14/03/2013 03:07, Asias He ha scritto:
> 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;
Aha, cool! But please add a comment.
>> What you're trying to do is really an rwlock, just use that. :)
>
> Yes, but the downside is that it introduces another lock.
Can't it can replace the existing mutex?
Paolo
>
>> 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;
>>>
>>>
>>
>
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