[PATCH 2/2] nvme-rdma: move admin queue cleanup to nvme_rdma_free_ctrl

[Steve Wise]

> > Hey Sagi, here is some lite reading for you. :)
> >
> > Prelude:  As part of disconnecting an iwarp connection, the iwarp provider
needs
> > to post an IW_CM_EVENT_CLOSE event to iw_cm, which is scheduled onto the
> > singlethread workq thread for iw_cm.
> >
> > Here is what happens with Sagi's patch:
> >
> > nvme_rdma_device_unplug() calls nvme_rdma_stop_queue() which calls
> > rdma_disconnect().  This triggers the disconnect.  iw_cxgb4 posts the
> > IW_CM_EVENT_CLOSE to iw_cm, which ends up calling cm_close_handler() in the
> > iw_cm workq thread context.  cm_close_handler() calls the rdma_cm event
> handler
> > for this cm_id, function cm_iw_handler(), which blocks until any currently
> > running event handler for this cm_id finishes.  It does this by calling
> > cm_disable_callback().  However since this whole unplug process is running
in
> > the event handler function for this same cm_id, the iw_cm workq thread is
now
> > stuck in a deadlock.   nvme_rdma_device_unplug() however, continues on and
> > schedules the controller delete worker thread and waits for it to complete.
The
> > delete controller worker thread tries to disconnect and destroy all the
> > remaining IO queues, but gets stuck in the destroy() path on the first IO
queue
> > because the iw_cm workq thread is already stuck, and processing the CLOSE
> event
> > is required to release a reference the iw_cm has on the iwarp providers qp.
So
> > everything comes to a grinding halt....
> >
> > Now: Ming's 2 patches avoid this deadlock because the cm_id that received
the
> > device removal event is disconnected/destroyed _only after_ all the
controller
> > queues are disconnected/destroyed.  So nvme_rdma_device_unplug() doesn't get
> > stuck waiting for the controller to delete the io queues, and only after
that
> > completes, does it delete the cm_id/qp that got the device removal event.
It
> > then returns thus causing the rdma_cm to release the cm_id's callback mutex.
> > This causes the iw_cm workq thread to now unblock and we continue on.  (can
> you
> > say house of cards?)
> >
> > So the net is:  the cm_id that received the device remove event _must_ be
> > disconnect/destroyed _last_.
> 
> Hey Steve, thanks for the detailed description.
> 
> IMHO, this really looks like a buggy design in iWARP connection
> management implementation. The fact that a rdma_disconnect forward
> progress is dependent on other cm_id execution looks wrong and
> backwards to me.
> 

Yea.  I'm not sure how to fix it at this point...

> Given that the device is being removed altogether I don't think that
> calling rdma_disconnect is important at all. Given the fact that
> ib_drain_qp makes sure that the queue-pair is in error state does
> this incremental patch resolve the iwarp-deadlock you are seeing?
>

I'll need to change c4iw_drain_sq/rq() to move the QP to ERROR explicitly.  When
I implemented them I assumed the QP would be disconnected.  Let me try this and
report back.
 
> --
> diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
> index fd90b5c00aae..e7eec7d8c705 100644
> --- a/drivers/nvme/host/rdma.c
> +++ b/drivers/nvme/host/rdma.c
> @@ -1342,8 +1342,12 @@ static int nvme_rdma_device_unplug(struct
> nvme_rdma_queue *queue)
>                  goto queue_delete;
>          }
> 
> +       /*
> +        * iwcm does not handle rdma_disconnect within DEVICE_REMOVAL
> +        * event very well, so we settle with qp drain
> +        */
> +       ib_drain_qp(queue->qp);
>          /* Free this queue ourselves */
> -       nvme_rdma_stop_queue(queue);
>          nvme_rdma_destroy_queue_ib(queue);
> 
>          /* Return non-zero so the cm_id will destroy implicitly */
> --