Re: [PATCH rdma-RC] IB/cm: Fix sleeping while atomic when creating AH from WC

From: Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
To: "Hefty,
	Sean" <sean.hefty-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>,
	Matan Barak
	<matanb-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>
Cc: Matan Barak <matanb-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>,
	"linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Or Gerlitz <ogerlitz-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>,
	Eran Ben Elisha <eranbe-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>,
	Jason Gunthorpe
	<jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>
Subject: Re: [PATCH rdma-RC] IB/cm: Fix sleeping while atomic when creating AH from WC
Date: Wed, 23 Dec 2015 15:04:18 -0500	[thread overview]
Message-ID: <567AFE42.2080107@redhat.com> (raw)
In-Reply-To: <1828884A29C6694DAF28B7E6B8A82373A9735914-P5GAC/sN6hkd3b2yrw5b5LfspsVTdybXVpNB7YpNyf8@public.gmane.org>

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On 10/15/2015 12:58 PM, Hefty, Sean wrote:
>>>> ib_create_ah_from_wc needs to resolve the DMAC in order to create the
>>>> AH (this may result sending an ARP and waiting for response).
>>>> CM uses this function (which is now sleepable).
>>>
>>> This is a significant change to the CM.  The CM calls are invoked
>> assuming that they return relatively quickly.  They're invoked from
>> callbacks and internally.  Having the calls now wait for an ARP response
>> requires that this be re-architected, so the calling thread doesn't go out
>> to lunch for several seconds.
>>
>> Agree - this is a significant change, but it was done a long time ago
>> (at v4.3 if I recall). When we need to send a message we need to
> 
> We're at 4.3-rc5?
> 
>> figure out the destination MAC. Even the passive side needs to do that
>> as some vendors don't report the source MAC of the packet in their wc.
>> Even if they did, since IP based addressing is rout-able by its
>> nature, it should follow the networking stack rules. Some crazy
>> configurations could force sending responses to packets that came from
>> router1 to router2 - so we have no choice than resolving the DMAC at
>> every side.
> 
> Ib_create_ah_from_wc is broken.   It is now an asynchronous operation, only the call itself was left as synchronous.  We can't block kernel threads for a minute, or however long ARP takes to resolve.  The call itself must change to be async, and all users of it updated to allocate some request, queue it, and handle all race conditions that result -- such as state changes or destruction of the work that caused the request to be initiated.
> 

I don't know who had intended to address this, but it got left out of
the 4.4 work.  We need to not let this drop through the cracks (for
another release).  Can someone please put fixing this properly on their
TODO list?

-- 
Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
              GPG KeyID: 0E572FDD

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