netdev - Re: [PATCH v4 09/16] IB/pvrdma: Add support for Completion Queues

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Message-ID: <20160915073611.GA3851@yuval-lap.uk.oracle.com>
Date:   Thu, 15 Sep 2016 10:36:12 +0300
From:   Yuval Shaia <yuval.shaia@...cle.com>
To:     Adit Ranadive <aditr@...are.com>
Cc:     "dledford@...hat.com" <dledford@...hat.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        pv-drivers <pv-drivers@...are.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "Jorgen S. Hansen" <jhansen@...are.com>,
        Aditya Sarwade <asarwade@...are.com>,
        George Zhang <georgezhang@...are.com>,
        Bryan Tan <bryantan@...are.com>
Subject: Re: [PATCH v4 09/16] IB/pvrdma: Add support for Completion Queues

Hi Adit,
Please see my comments inline.

Besides that I have no more comment for this patch.

Reviewed-by: Yuval Shaia <yuval.shaia@...cle.com>

Yuval

On Thu, Sep 15, 2016 at 12:07:29AM +0000, Adit Ranadive wrote:
> On Wed, Sep 14, 2016 at 05:43:37 -0700, Yuval Shaia wrote:
> > On Sun, Sep 11, 2016 at 09:49:19PM -0700, Adit Ranadive wrote:
> > > +
> > > +static int pvrdma_poll_one(struct pvrdma_cq *cq, struct pvrdma_qp
> > **cur_qp,
> > > +			   struct ib_wc *wc)
> > > +{
> > > +	struct pvrdma_dev *dev = to_vdev(cq->ibcq.device);
> > > +	int has_data;
> > > +	unsigned int head;
> > > +	bool tried = false;
> > > +	struct pvrdma_cqe *cqe;
> > > +
> > > +retry:
> > > +	has_data = pvrdma_idx_ring_has_data(&cq->ring_state->rx,
> > > +					    cq->ibcq.cqe, &head);
> > > +	if (has_data == 0) {
> > > +		if (tried)
> > > +			return -EAGAIN;
> > > +
> > > +		/* Pass down POLL to give physical HCA a chance to poll. */
> > > +		pvrdma_write_uar_cq(dev, cq->cq_handle |
> > PVRDMA_UAR_CQ_POLL);
> > > +
> > > +		tried = true;
> > > +		goto retry;
> > > +	} else if (has_data == PVRDMA_INVALID_IDX) {
> > 
> > I didn't went throw the entire life cycle of RX-ring's head and tail but you
> > need to make sure that PVRDMA_INVALID_IDX error is recoverable one, i.e
> > there is probability that in the next call to pvrdma_poll_one it will be fine.
> > Otherwise it is an endless loop.
> 
> We have never run into this issue internally but I don't think we can recover here 

I briefly reviewed the life cycle of RX-ring's head and tail and didn't
caught any suspicious place that might corrupt it.
So glad to see that you never encountered this case.

> in the driver. The only way to recover would be to destroy and recreate the CQ 
> which we shouldn't do since it could be used by multiple QPs. 

Agree.
But don't they hit the same problem too?

> We don't have a way yet to recover in the device. Once we add that this check
> should go away.

To be honest i have no idea how to do that - i was expecting driver's vendors
to come up with an ideas :)
I once came up with an idea to force restart of the driver but it was
rejected.

> 
> The reason I returned an error value from poll_cq in v3 was to break the possible 
> loop so that it might give clients a chance to recover. But since poll_cq is not expected
> to fail I just log the device error here. I can revert to that version if you want to break
> the possible loop.

Clients (ULPs) cannot recover from this case. They even do not check the
reason of the error and treats any error as -EAGAIN.

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