netdev - Re: [Cluster-devel] [PATCH 4/6] dlm: use sctp 1-to-1 API

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Message-ID: <55CC6571.70009@redhat.com>
Date:	Thu, 13 Aug 2015 10:37:53 +0100
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	David Laight <David.Laight@...LAB.COM>,
	"cluster-devel@...hat.com" <cluster-devel@...hat.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Vlad Yasevich <vyasevich@...il.com>,
	"tan.hu@....com.cn" <tan.hu@....com.cn>,
	Neil Horman <nhorman@...driver.com>
Subject: Re: [Cluster-devel] [PATCH 4/6] dlm: use sctp 1-to-1 API

Hi,

On 12/08/15 17:42, Marcelo Ricardo Leitner wrote:
> Em 12-08-2015 12:33, David Laight escreveu:
>> From: Marcelo Ricardo Leitner
>>> Sent: 12 August 2015 14:16
>>> Em 12-08-2015 07:23, David Laight escreveu:
>>>> From: Marcelo Ricardo Leitner
>>>>> Sent: 11 August 2015 23:22
>>>>> DLM is using 1-to-many API but in a 1-to-1 fashion. That is, it's not
>>>>> needed but this causes it to use sctp_do_peeloff() to mimic an
>>>>> kernel_accept() and this causes a symbol dependency on sctp module.
>>>>>
>>>>> By switching it to 1-to-1 API we can avoid this dependency and also
>>>>> reduce quite a lot of SCTP-specific code in lowcomms.c.
>>>> ...
>>>>
>>>> You still need to enable sctp notifications (I think the patch deleted
>>>> that code).
>>>> Otherwise you don't get any kind of indication if the remote system
>>>> 'resets' (ie sends an new INIT chunk) on an existing connection.
>>>
>>> Right, it would miss the restart event and could generate a corrupted
>>> tx/rx buffers by glueing parts of old messages with new ones.
>>
>> Except that it is SCTP so you'd expect DATA chunks to contain entire
>> messages and so get unexpected message sequences rather than corrupt
>> messages.
>
> I was thinking on cases where the buf for recvmsg is not enough to 
> hold the chunk, so that the remaining is left for another attempt 
> (sctp_recvmsg, around line 2130), but sounds like we won't purge rx 
> buffer when the reset happens so that doesn't matter. The association 
> is replaced, but the buffers are kept.
>
> Out of order messages aren't a problem for dlm. It can recover from 
> that just fine, as it doesn't have a specific handshake at beginning 
> or something like that and upper layers are agnostic to that state 
> transition (disconnect/reconnect/...), this should be fine.
>
I'm not sure thats true - DLM does rely on message ordering in some 
cases in order to ensure correct functioning. So depending on how SCTP 
is interfaced to DLM, it might potentially be an issue,

Steve.

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