On Mon, Apr 15, 2013 at 01:11:54PM +0300, Michael S. Tsirkin wrote:
> On Sun, Apr 14, 2013 at 08:27:14PM +0800, Asias He wrote:
> > On Sun, Apr 14, 2013 at 12:58:03PM +0300, Michael S. Tsirkin wrote:
> > > On Sat, Apr 13, 2013 at 11:29:14AM +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 v4:
> > > > - Introduce vhost_scsi_inflight
> > > > - Drop array to track flush
> > > > - Use RCU to protect vs_inflight explicitly
> > > >
> > > > 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@xxxxxxxxxx>
> > > > ---
> > > > drivers/vhost/tcm_vhost.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++
> > > > drivers/vhost/tcm_vhost.h | 5 ++++
> > > > 2 files changed, 78 insertions(+)
> > > >
> > > > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > > > index e09f0fe..5dde525 100644
> > > > --- a/drivers/vhost/tcm_vhost.c
> > > > +++ b/drivers/vhost/tcm_vhost.c
> > > > @@ -74,6 +74,11 @@ enum {
> > > > #define VHOST_SCSI_MAX_VQ 128
> > > > #define VHOST_SCSI_MAX_EVENT 128
> > > >
> > > > +struct vhost_scsi_inflight {
> > > > + wait_queue_head_t wait;
> > > > + atomic_t count;
> > >
> > > Okay now let's switch to kref + completion, make it more
> > > descriptive.
> >
> > I still do not see why kref is better.
>
> It makes the fact you are doing reference counting, explicit.
See the version I sent yesterday.
> > Completion sounds good.
> >
> > > > +};
> > > > +
> > > > struct vhost_scsi {
> > > > /* Protected by vhost_scsi->dev.mutex */
> > > > struct tcm_vhost_tpg **vs_tpg;
> > > > @@ -91,6 +96,8 @@ 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 */
> > > > +
> > > > + struct vhost_scsi_inflight __rcu *vs_inflight; /* track inflight req */
> > > > };
> > > >
> > > > /* Local pointer to allocated TCM configfs fabric module */
> > > > @@ -108,6 +115,51 @@ static int iov_num_pages(struct iovec *iov)
> > > > ((unsigned long)iov->iov_base & PAGE_MASK)) >> PAGE_SHIFT;
> > > > }
> > > >
> > > > +static struct vhost_scsi_inflight *
> > > > +tcm_vhost_alloc_inflight(struct vhost_scsi *vs)
> > > > +{
> > > > + struct vhost_scsi_inflight *inflight;
> > > > +
> > > > + inflight = kzalloc(sizeof(*inflight), GFP_KERNEL);
> > > > + if (inflight) {
> > >
> > > This is used in set_features, so let's make it int
> > > and return error to user if not. No need to corrupt kernel
> > > memory silently like this.
> >
> > What do you mean by used in set_features? The return value of
> > tcm_vhost_alloc_inflight was used at some point, but now it is not used.
> > So I will return int.
> >
> > Why is it corrupted?
>
> You skip flushes so something can be in flight, our code
> assumes flush actually flushes things.
So what is the best we can do if it fails to allocate memory for inflight
in vhost_scsi_flush. BTW, this is a downside of using dynamic allocation.
The best I can think of:
We set vs->vs_inflight to NULL which means we do not track the new
requests and keep going to do the flush and wait for the old requests to
be done.
> > >
> > >
> > > > + atomic_set(&inflight->count, 0);
> > >
> > >
> > > Ugh. So once all requests finish, refcount is 0
> > > and then inflight is freed, and then the next request will
> > > get a freed inflight value and dereference. Looks pretty bad,
> > > but maybe there's an increment somewhere that fixes it.
> >
> > How can the next request get a freed inflight? It can not happen. The
> > old inflight is freed only after all the requests which reference it are
> > finished. See the last few lines in vhost_scsi_flush.
> >
> > > But let's not go there. That's why I said above we should use kref +
> > > completion. That makes is very clear how to use it correctly.
> > > So:
> > > - initialize to 1
> > > - swap pointer with RCU
> > > - decrement
> > > - wait_for_completion
> >
> > We can not go there.
>
> Right. But it's confusing, and also adds overhead on data path
> (wakeup each time last request is completed).
> Let's do standard ref counting: init to 1, before flush - decrement
> and wait for completion.
Yes, as I mentioned in the other mail, we can optimize the 'wakeup too
much' issue. Certainly, init to 1 and decrement to start flush is one
way to do it.
> > >
> > >
> > > > + init_waitqueue_head(&inflight->wait);
> > > > + }
> > > > + rcu_assign_pointer(vs->vs_inflight, inflight);
> > > > + synchronize_rcu();
> > > > +
> > > > + return inflight;
> > > > +}
> > > > +
> > >
> > > This looks like it will overwrite inflight without
> > > freeing the old one. In fact it won't because caller
> > > has saved the pointer but this interface is
> > > just too tricky. Please just opencode this function.
> > >
> >
> > Did you see the old inflight was freed in vhost_scsi_flush().
> > It was code in a helper function because it is used in too places.
> > One is in vhost_scsi_open and the other is in vhost_scsi_scsi?
>
> The name is still confusing.
> alloc should simply allocate and return pointer.
> Have callers do assign and flush as appropriate.
> In particular open does not need synchronize_rcu,
> and not checking old inflight value.
It's just a name for the helper. We can change to what ever we want.
>
> > >
> > > > +static struct vhost_scsi_inflight *
> > > > +tcm_vhost_inc_inflight(struct vhost_scsi *vs)
> > > > +{
> > > > + struct vhost_scsi_inflight *inflight;
> > > > +
> > > > + rcu_read_lock();
> > > > + inflight = rcu_dereference(vs->vs_inflight);
> > > > + if (inflight)
> > >
> > > How can it be NULL?
> >
> > When tcm_vhost_alloc_inflight failed to allocate inflight.
>
> Then we won't flush which is a wrong way to handle such
> and error. Instead, fail the command.
Okay, we can not track it we fail it. It is a safe way to go.
> > > > + atomic_inc(&inflight->count);
> > > > + rcu_read_unlock();
> > > > +
> > > > + return inflight;
> > > > +}
> > > > +
> > > > +static void tcm_vhost_dec_inflight(struct vhost_scsi_inflight *inflight)
> > > > +{
> > > > + /*
> > > > + * Wakeup the waiter when all the requests issued before the flush
> > > > + * operation are finished.
> > > > + */
> > > > + if (inflight && !atomic_dec_return(&inflight->count))
> > > > + wake_up(&inflight->wait);
> > > > +}
> > > > +
> > > > +static bool tcm_vhost_done_inflight(struct vhost_scsi_inflight *inflight)
> > > > +{
> > > > + return atomic_read(&inflight->count) == 0;
> > > > +}
> > > > +
> > > > static bool tcm_vhost_check_feature(struct vhost_scsi *vs, int feature)
> > > > {
> > > > bool ret = false;
> > > > @@ -402,6 +454,7 @@ 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(evt->inflight);
> > > > vs->vs_events_nr--;
> > > > kfree(evt);
> > > > mutex_unlock(&vs->vs_events_lock);
> > > > @@ -423,6 +476,7 @@ static struct tcm_vhost_evt *tcm_vhost_allocate_evt(struct vhost_scsi *vs,
> > > > if (evt) {
> > > > evt->event.event = event;
> > > > evt->event.reason = reason;
> > > > + evt->inflight = tcm_vhost_inc_inflight(vs);
> > > > vs->vs_events_nr++;
> > > > }
> > > > mutex_unlock(&vs->vs_events_lock);
> > > > @@ -445,6 +499,8 @@ static void vhost_scsi_free_cmd(struct tcm_vhost_cmd *tv_cmd)
> > > > kfree(tv_cmd->tvc_sgl);
> > > > }
> > > >
> > > > + tcm_vhost_dec_inflight(tv_cmd->inflight);
> > > > +
> > > > kfree(tv_cmd);
> > > > }
> > > >
> > > > @@ -595,6 +651,7 @@ static struct tcm_vhost_cmd *vhost_scsi_allocate_cmd(
> > > > tv_cmd->tvc_data_direction = data_direction;
> > > > tv_cmd->tvc_nexus = tv_nexus;
> > > > tv_cmd->tvc_vhost = vs;
> > > > + tv_cmd->inflight = tcm_vhost_inc_inflight(vs);
> > > >
> > > > return tv_cmd;
> > > > }
> > > > @@ -983,10 +1040,22 @@ static void vhost_scsi_flush_vq(struct vhost_scsi *vs, int index)
> > > > static void vhost_scsi_flush(struct vhost_scsi *vs)
> > > > {
> > > > int i;
> > > > + struct vhost_scsi_inflight *inflight;
> > > > +
> > > > + inflight = ACCESS_ONCE(vs->vs_inflight);
> > >
> > > rcu_dereference_protected ? This ACCESS_ONCE looks bogus.
> >
> > okay.
> >
> > > > + if (!tcm_vhost_alloc_inflight(vs))
> > > > + return;
> > > >
> > > > 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 */
> > >
> > > s/until/for/
> >
> > okay.
> >
> > > > + if (inflight) {
> > >
> > > How can this be NULL?
> >
> > When tcm_vhost_alloc_inflight failed to allocate inflight.
>
> Again, wrong way to handle it.
>
> > > > + wait_event(inflight->wait, tcm_vhost_done_inflight(inflight));
> > > > + kfree(inflight);
> > > > + }
> > > > }
> > > >
> > > > /*
> > > > @@ -1195,6 +1264,9 @@ static int vhost_scsi_open(struct inode *inode, struct file *f)
> > > > s->vs_events_dropped = false;
> > > > mutex_init(&s->vs_events_lock);
> > > >
> > > > + if (!tcm_vhost_alloc_inflight(s))
> > > > + return -ENOMEM;
> > > > +
> > > > 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++)
> > > > @@ -1220,6 +1292,7 @@ static int vhost_scsi_release(struct inode *inode, struct file *f)
> > > > vhost_scsi_clear_endpoint(s, &t);
> > > > vhost_dev_stop(&s->dev);
> > > > vhost_dev_cleanup(&s->dev, false);
> > > > + kfree(s->vs_inflight);
> > > > kfree(s);
> > > > return 0;
> > > > }
> > > > diff --git a/drivers/vhost/tcm_vhost.h b/drivers/vhost/tcm_vhost.h
> > > > index 94e9ee53..7567767 100644
> > > > --- a/drivers/vhost/tcm_vhost.h
> > > > +++ b/drivers/vhost/tcm_vhost.h
> > > > @@ -2,6 +2,7 @@
> > > > #define TCM_VHOST_NAMELEN 256
> > > > #define TCM_VHOST_MAX_CDB_SIZE 32
> > > >
> > > > +struct vhost_scsi_inflight;
> > > > struct tcm_vhost_cmd {
> > > > /* Descriptor from vhost_get_vq_desc() for virt_queue segment */
> > > > int tvc_vq_desc;
> > > > @@ -37,6 +38,8 @@ 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;
> > > > + /* Used to track inflight req */
> > > > + struct vhost_scsi_inflight *inflight;
> > > > };
> > > >
> > > > struct tcm_vhost_nexus {
> > > > @@ -91,6 +94,8 @@ struct tcm_vhost_evt {
> > > > struct virtio_scsi_event event;
> > > > /* virtio_scsi event list, serviced from vhost worker thread */
> > > > struct llist_node list;
> > > > + /* Used to track inflight req */
> > > > + struct vhost_scsi_inflight *inflight;
> > > > };
> > > >
> > > > /*
> > > > --
> > > > 1.8.1.4
> >
> > --
> > Asias
--
Asias
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