Re: [PATCH 1/3] IB/core: use IB_CQ_REPORT_MISSED_EVENTS to avoid missed CQ callbacks

[Roland Dreier <rdreier-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>]

 > ib_req_notify_cq(IB_CQ_NEXT_COMP) is not guaranteed to generate
 > a callback for the next completion entered since there is a race
 > between arming the callback and another CQE being added to the queue.
 > The IB_CQ_REPORT_MISSED_EVENTS flag was added to detect this
 > race and allow the verbs consumer to call ib_poll_cq() and
 > ib_req_notify_cq() again to avoid delays in processing the CQE.

I'm not sure I understand the race you're fixing here... the existing
code does the rearm before polling:

 > +	int ret;
 >  
 >  	port_priv = container_of(work, struct ib_mad_port_private, work);
 > -	ib_req_notify_cq(port_priv->cq, IB_CQ_NEXT_COMP);
 >  
 > +again:
 >  	while (ib_poll_cq(port_priv->cq, 1, &wc) == 1) {
 >  		if (wc.status == IB_WC_SUCCESS) {
 >  			switch (wc.opcode) {
 > @@ -2246,6 +2247,10 @@ static void ib_mad_completion_handler(struct work_struct *work)
 >  		} else
 >  			mad_error_handler(port_priv, &wc);
 >  	}
 > +	ret = ib_req_notify_cq(port_priv->cq, IB_CQ_NEXT_COMP |
 > +					      IB_CQ_REPORT_MISSED_EVENTS);
 > +	if (ret > 0)
 > +		goto again;

The only issue with the existing code is that it may trigger extra
events that will find the CQ empty when polling.

So this may be a valid optimization but I don't see it fixing any missed
events.  Am I missing something?

Also for all these fixes, I think you only want to rearm the CQ once and
go back and poll if you get a missed events warning; the next time the
CQ is empty, then you know another event will happen.

 - R.
-- 
Roland Dreier <rolandd-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org> || For corporate legal information go to:
http://www.cisco.com/web/about/doing_business/legal/cri/index.html
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