Re: [PATCH] tcm_vhost: Wait for pending requests in vhost_scsi_flush()

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Apr 12, 2013 at 02:25:23PM +0800, Asias He wrote:
> On Thu, Apr 11, 2013 at 01:47:21PM +0300, Michael S. Tsirkin wrote:
> > On Tue, Apr 09, 2013 at 05:39:43PM +0800, Asias He wrote:
> > > This patch makes vhost_scsi_flush() wait for all the pending requests
> > > issued before the flush operation to be finished.
> > > 
> > > Changes in v3:
> > > - Rebase
> > > - Drop 'tcm_vhost: Wait for pending requests in
> > >   vhost_scsi_clear_endpoint()' in this series, we already did that in
> > >   'tcm_vhost: Use vq->private_data to indicate if the endpoint is setup'
> > > 
> > > Changes in v2:
> > > - Increase/Decrease inflight requests in
> > >   vhost_scsi_{allocate,free}_cmd and tcm_vhost_{allocate,free}_evt
> > > 
> > > Signed-off-by: Asias He <asias@redhat.com>
> > 
> > Nack, let's not do this home-grown here.  Please use a kref.
> > 
> > The array of two trick is also too tricky for my taste.
> > 
> > Please replace during_flush in tcm_vhost_cmd and tcm_vhost_evt
> > by a kref pointer, allocate a new kref when you flush.
> > 
> > Access can be done with RCU so we won't need any locks.
> 
> I do not think kref helps and the right place to use here. Also, a
> pointer kref in tcm_vhost_cmd and tcm_vhost_evt is not enough, you need
> a wait queue as well.
> 
> Do you mean something as so:
> 
>    struct vhost_scsi_inflight {
>    	struct kref kref;
>    	wait_queue_head_t wait;
>    }
>    
>    vhost_scsi_allocate_cmd()
>    	rcu_read_lock()
>    	tv_cmd->inflight = rcu_dereference(vs->vs_inflight)
>    	kref_get(&tv_cmd->inflight->kref)
>    	rcu_read_unlock()
>    
>    vhost_scsi_free_cmd()
>    	kref_put(&tv_cmd->inflight.kref, my_release)
>    
>    my_release()
>    	wake_up(&inflight->wait)
>    
>    vhost_scsi_flush()
>    	old_inflight = vs->vs_inflight;
>    	new_inflight = kmalloc(*new_inflight, ...)
>    	rcu_assign_pointer(vs->vs_inflight, new_inflight);
>    	wait_event(old_inflight->wait, atomic_read(&old_inflight->kref->refcount) == 0)
>    	synchronize_rcu();
>    	free(old_inflight)
> 
> 1) The kref need to be accessed in the free cmd/evt function, you can not use
> rcu to protect it.

No, it's vs_inflight pointer that is protected by RCU.
But if you prefer, we can have it per-vq and
protected by vq mutex.


> 2) No need to use synchronize_rcu to wait for the reader of
> vs->vs_inflight to finish. We need to wait on the wait queue anyway. At
> time time, we are safe to free the old_inflight.

RCU is to avoid old vhost_scsi_allocate_cmd from using
the old pointer. But we can use vq flush instead, that's
often done in vhost.

> 3) The kref is not used in a standard way. We are refcounting the evt
> and cmd, not the vhost_scsi_inflight. A single is atomic conter is
> enough.

Looks standard to me.

> Though, I do not like the array trick too. I can change to allocate
> vhost_scsi_inflight when we flush.

That's better but homegrown refcounting is better avoided too.

> > > ---
> > >  drivers/vhost/tcm_vhost.c | 88 ++++++++++++++++++++++++++++++++++++++++++++---
> > >  drivers/vhost/tcm_vhost.h |  4 +++
> > >  2 files changed, 87 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > > index 1f9116c..719ce13 100644
> > > --- a/drivers/vhost/tcm_vhost.c
> > > +++ b/drivers/vhost/tcm_vhost.c
> > > @@ -91,6 +91,15 @@ struct vhost_scsi {
> > >  	struct mutex vs_events_lock; /* protect vs_events_dropped,events_nr */
> > >  	bool vs_events_dropped; /* any missed events */
> > >  	int vs_events_nr; /* num of pending events */
> > > +
> > > +	/*
> > > +	 * vs_inflight[0]/[1] are used to track requests issued
> > > +	 * before/during the flush operation
> > > +	 */
> > > +	u64 vs_inflight[2];
> > > +	wait_queue_head_t vs_flush_wait; /* wait queue for flush operation */
> > > +	spinlock_t vs_flush_lock; /* lock to protect vs_during_flush */
> > > +	int vs_during_flush; /* flag to indicate if we are in flush operation */
> > >  };
> > >  
> > >  /* Local pointer to allocated TCM configfs fabric module */
> > > @@ -108,6 +117,46 @@ static int iov_num_pages(struct iovec *iov)
> > >  	       ((unsigned long)iov->iov_base & PAGE_MASK)) >> PAGE_SHIFT;
> > >  }
> > >  
> > > +static int tcm_vhost_inc_inflight(struct vhost_scsi *vs)
> > > +{
> > > +	int during_flush;
> > > +
> > > +	spin_lock(&vs->vs_flush_lock);
> > > +	during_flush = vs->vs_during_flush;
> > > +	vs->vs_inflight[during_flush]++;
> > > +	spin_unlock(&vs->vs_flush_lock);
> > > +
> > > +	return during_flush;
> > > +}
> > > +
> > > +static void tcm_vhost_dec_inflight(struct vhost_scsi *vs, int during_flush)
> > > +{
> > > +	u64 inflight;
> > > +
> > > +	spin_lock(&vs->vs_flush_lock);
> > > +	inflight = vs->vs_inflight[during_flush]--;
> > > +	/*
> > > +	 * Wakeup the waiter when all the requests issued before the flush
> > > +	 * operation are finished and we are during the flush operation.
> > > +	 */
> > > +	if (!inflight && !during_flush && vs->vs_during_flush)
> > > +		wake_up(&vs->vs_flush_wait);
> > > +	spin_unlock(&vs->vs_flush_lock);
> > > +}
> > > +
> > > +static bool tcm_vhost_done_inflight(struct vhost_scsi *vs)
> > > +{
> > > +	bool ret = false;
> > > +
> > > +	/* The requests issued before the flush operation are finished ? */
> > > +	spin_lock(&vs->vs_flush_lock);
> > > +	if (!vs->vs_inflight[0])
> > > +		ret = true;
> > > +	spin_unlock(&vs->vs_flush_lock);
> > > +
> > > +	return ret;
> > > +}
> > > +
> > >  static bool tcm_vhost_check_feature(struct vhost_scsi *vs, int feature)
> > >  {
> > >  	bool ret = false;
> > > @@ -402,6 +451,7 @@ static int tcm_vhost_queue_tm_rsp(struct se_cmd *se_cmd)
> > >  static void tcm_vhost_free_evt(struct vhost_scsi *vs, struct tcm_vhost_evt *evt)
> > >  {
> > >  	mutex_lock(&vs->vs_events_lock);
> > > +	tcm_vhost_dec_inflight(vs, evt->during_flush);
> > >  	vs->vs_events_nr--;
> > >  	kfree(evt);
> > >  	mutex_unlock(&vs->vs_events_lock);
> > > @@ -423,6 +473,7 @@ static struct tcm_vhost_evt *tcm_vhost_allocate_evt(struct vhost_scsi *vs,
> > >  	if (evt) {
> > >  		evt->event.event = event;
> > >  		evt->event.reason = reason;
> > > +		evt->during_flush = tcm_vhost_inc_inflight(vs);
> > >  		vs->vs_events_nr++;
> > >  	}
> > >  	mutex_unlock(&vs->vs_events_lock);
> > > @@ -433,6 +484,7 @@ static struct tcm_vhost_evt *tcm_vhost_allocate_evt(struct vhost_scsi *vs,
> > >  static void vhost_scsi_free_cmd(struct tcm_vhost_cmd *tv_cmd)
> > >  {
> > >  	struct se_cmd *se_cmd = &tv_cmd->tvc_se_cmd;
> > > +	struct vhost_scsi *vs = tv_cmd->tvc_vhost;
> > >  
> > >  	/* TODO locking against target/backend threads? */
> > >  	transport_generic_free_cmd(se_cmd, 1);
> > > @@ -445,13 +497,16 @@ static void vhost_scsi_free_cmd(struct tcm_vhost_cmd *tv_cmd)
> > >  		kfree(tv_cmd->tvc_sgl);
> > >  	}
> > >  
> > > +	tcm_vhost_dec_inflight(vs, tv_cmd->during_flush);
> > > +
> > >  	kfree(tv_cmd);
> > >  }
> > >  
> > >  static void tcm_vhost_do_evt_work(struct vhost_scsi *vs,
> > > -	struct virtio_scsi_event *event)
> > > +	struct tcm_vhost_evt *evt)
> > >  {
> > >  	struct vhost_virtqueue *vq = &vs->vqs[VHOST_SCSI_VQ_EVT];
> > > +	struct virtio_scsi_event *event = &evt->event;
> > >  	struct virtio_scsi_event __user *eventp;
> > >  	unsigned out, in;
> > >  	int head, ret;
> > > @@ -511,7 +566,7 @@ static void tcm_vhost_evt_work(struct vhost_work *work)
> > >  	while (llnode) {
> > >  		evt = llist_entry(llnode, struct tcm_vhost_evt, list);
> > >  		llnode = llist_next(llnode);
> > > -		tcm_vhost_do_evt_work(vs, &evt->event);
> > > +		tcm_vhost_do_evt_work(vs, evt);
> > >  		tcm_vhost_free_evt(vs, evt);
> > >  	}
> > >  }
> > > @@ -529,8 +584,8 @@ static void vhost_scsi_complete_cmd_work(struct vhost_work *work)
> > >  	struct virtio_scsi_cmd_resp v_rsp;
> > >  	struct tcm_vhost_cmd *tv_cmd;
> > >  	struct llist_node *llnode;
> > > -	struct se_cmd *se_cmd;
> > >  	int ret, vq;
> > > +	struct se_cmd *se_cmd;
> > >  
> > >  	bitmap_zero(signal, VHOST_SCSI_MAX_VQ);
> > >  	llnode = llist_del_all(&vs->vs_completion_list);
> > > @@ -568,6 +623,7 @@ static void vhost_scsi_complete_cmd_work(struct vhost_work *work)
> > >  }
> > >  
> > >  static struct tcm_vhost_cmd *vhost_scsi_allocate_cmd(
> > > +	struct vhost_scsi *vs,
> > >  	struct tcm_vhost_tpg *tv_tpg,
> > >  	struct virtio_scsi_cmd_req *v_req,
> > >  	u32 exp_data_len,
> > > @@ -592,6 +648,8 @@ static struct tcm_vhost_cmd *vhost_scsi_allocate_cmd(
> > >  	tv_cmd->tvc_exp_data_len = exp_data_len;
> > >  	tv_cmd->tvc_data_direction = data_direction;
> > >  	tv_cmd->tvc_nexus = tv_nexus;
> > > +	tv_cmd->tvc_vhost = vs;
> > > +	tv_cmd->during_flush = tcm_vhost_inc_inflight(vs);
> > >  
> > >  	return tv_cmd;
> > >  }
> > > @@ -842,7 +900,7 @@ static void vhost_scsi_handle_vq(struct vhost_scsi *vs,
> > >  		for (i = 0; i < data_num; i++)
> > >  			exp_data_len += vq->iov[data_first + i].iov_len;
> > >  
> > > -		tv_cmd = vhost_scsi_allocate_cmd(tv_tpg, &v_req,
> > > +		tv_cmd = vhost_scsi_allocate_cmd(vs, tv_tpg, &v_req,
> > >  					exp_data_len, data_direction);
> > >  		if (IS_ERR(tv_cmd)) {
> > >  			vq_err(vq, "vhost_scsi_allocate_cmd failed %ld\n",
> > > @@ -852,7 +910,6 @@ static void vhost_scsi_handle_vq(struct vhost_scsi *vs,
> > >  		pr_debug("Allocated tv_cmd: %p exp_data_len: %d, data_direction"
> > >  			": %d\n", tv_cmd, exp_data_len, data_direction);
> > >  
> > > -		tv_cmd->tvc_vhost = vs;
> > >  		tv_cmd->tvc_vq = vq;
> > >  
> > >  		if (unlikely(vq->iov[out].iov_len !=
> > > @@ -905,6 +962,7 @@ static void vhost_scsi_handle_vq(struct vhost_scsi *vs,
> > >  		 * tcm_vhost_queue_data_in() and tcm_vhost_queue_status()
> > >  		 */
> > >  		tv_cmd->tvc_vq_desc = head;
> > > +
> > >  		/*
> > >  		 * Dispatch tv_cmd descriptor for cmwq execution in process
> > >  		 * context provided by tcm_vhost_workqueue.  This also ensures
> > > @@ -984,9 +1042,23 @@ static void vhost_scsi_flush(struct vhost_scsi *vs)
> > >  {
> > >  	int i;
> > >  
> > > +	/* Flush operation is started */
> > > +	spin_lock(&vs->vs_flush_lock);
> > > +	vs->vs_during_flush = 1;
> > > +	spin_unlock(&vs->vs_flush_lock);
> > > +
> > >  	for (i = 0; i < VHOST_SCSI_MAX_VQ; i++)
> > >  		vhost_scsi_flush_vq(vs, i);
> > >  	vhost_work_flush(&vs->dev, &vs->vs_completion_work);
> > > +	vhost_work_flush(&vs->dev, &vs->vs_event_work);
> > > +
> > > +	/* Wait until all requests issued before the flush to be finished */
> > > +	wait_event(vs->vs_flush_wait, tcm_vhost_done_inflight(vs));
> > > +
> > > +	/* Flush operation is finished */
> > > +	spin_lock(&vs->vs_flush_lock);
> > > +	vs->vs_during_flush = 0;
> > > +	spin_unlock(&vs->vs_flush_lock);
> > >  }
> > >  
> > >  /*
> > > @@ -1094,6 +1166,7 @@ static int vhost_scsi_clear_endpoint(
> > >  	u8 target;
> > >  
> > >  	mutex_lock(&vs->dev.mutex);
> > > +
> > >  	/* Verify that ring has been setup correctly. */
> > >  	for (index = 0; index < vs->dev.nvqs; ++index) {
> > >  		if (!vhost_vq_access_ok(&vs->vqs[index])) {
> > > @@ -1195,6 +1268,11 @@ static int vhost_scsi_open(struct inode *inode, struct file *f)
> > >  	s->vs_events_dropped = false;
> > >  	mutex_init(&s->vs_events_lock);
> > >  
> > > +	s->vs_inflight[0] = 0;
> > > +	s->vs_inflight[1] = 0;
> > > +	spin_lock_init(&s->vs_flush_lock);
> > > +	init_waitqueue_head(&s->vs_flush_wait);
> > > +
> > >  	s->vqs[VHOST_SCSI_VQ_CTL].handle_kick = vhost_scsi_ctl_handle_kick;
> > >  	s->vqs[VHOST_SCSI_VQ_EVT].handle_kick = vhost_scsi_evt_handle_kick;
> > >  	for (i = VHOST_SCSI_VQ_IO; i < VHOST_SCSI_MAX_VQ; i++)
> > > diff --git a/drivers/vhost/tcm_vhost.h b/drivers/vhost/tcm_vhost.h
> > > index 94e9ee53..dd84622 100644
> > > --- a/drivers/vhost/tcm_vhost.h
> > > +++ b/drivers/vhost/tcm_vhost.h
> > > @@ -37,6 +37,8 @@ struct tcm_vhost_cmd {
> > >  	unsigned char tvc_sense_buf[TRANSPORT_SENSE_BUFFER];
> > >  	/* Completed commands list, serviced from vhost worker thread */
> > >  	struct llist_node tvc_completion_list;
> > > +	/* Indicate this command is issued during the flush operaton */
> > > +	int during_flush;
> > >  };
> > >  
> > >  struct tcm_vhost_nexus {
> > > @@ -91,6 +93,8 @@ struct tcm_vhost_evt {
> > >  	struct virtio_scsi_event event;
> > >  	/* virtio_scsi event list, serviced from vhost worker thread */
> > >  	struct llist_node list;
> > > +	/* Indicate this event is issued during the flush operaton */
> > > +	int during_flush;
> > >  };
> > >  
> > >  /*
> > > -- 
> > > 1.8.1.4
> 
> -- 
> Asias