linux-kernel - Re: [PATCH] tipc: ensure skb->lock is initialised

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Message-ID: <4d2ac0ce7f974184ac43b71f19aee7a3@svr-chch-ex1.atlnz.lc>
Date:   Wed, 10 Jul 2019 20:58:21 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     Jon Maloy <jon.maloy@...csson.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "ying.xue@...driver.com" <ying.xue@...driver.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "tipc-discussion@...ts.sourceforge.net" 
        <tipc-discussion@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tipc: ensure skb->lock is initialised


On 11/07/19 1:10 AM, Jon Maloy wrote:
>> -----Original Message-----
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Sent: 10-Jul-19 04:00
>> To: Jon Maloy <jon.maloy@...csson.com>; Eric Dumazet
>> <eric.dumazet@...il.com>; Chris Packham
>> <Chris.Packham@...iedtelesis.co.nz>; ying.xue@...driver.com;
>> davem@...emloft.net
>> Cc: netdev@...r.kernel.org; tipc-discussion@...ts.sourceforge.net; linux-
>> kernel@...r.kernel.org
>> Subject: Re: [PATCH] tipc: ensure skb->lock is initialised
>>
>>
>>
>> On 7/9/19 10:15 PM, Jon Maloy wrote:
>>>
>>> It is not only for lockdep purposes, -it is essential.  But please provide details
>> about where you see that more fixes are needed.
>>>
>>
>> Simple fact that you detect a problem only when skb_queue_purge() is called
>> should talk by itself.
>>
>> As I stated, there are many places where the list is manipulated _without_ its
>> spinlock being held.
> 
> Yes, and that is the way it should be on the send path.
> 
>>
>> You want consistency, then
>>
>> - grab the spinlock all the time.
>> - Or do not ever use it.
> 
> That is exactly what we are doing.
> - The send path doesn't need the spinlock, and never grabs it.
> - The receive path does need it, and always grabs it.
> 
> However, since we don't know from the beginning which path a created
> message will follow, we initialize the queue spinlock "just in case"
> when it is created, even though it may never be used later.
> You can see this as a violation of the principle you are stating
> above, but it is a prize that is worth paying, given savings in code
> volume, complexity and performance.
> 
>>
>> Do not initialize the spinlock just in case a path will use skb_queue_purge()
>> (instead of using __skb_queue_purge())
> 
> I am ok with that. I think we can agree that Chris goes for that
> solution, so we can get this bug fixed.

So would you like a v2 with an improved commit message? I note that I 
said skb->lock instead of head->lock in the subject line so at least 
that should be corrected.