netdev - Re: [PATCH net-next 16/17] net: sched: conditionally take rtnl lock on rules update path

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Date:   Tue, 13 Nov 2018 13:25:52 +0000
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Stefano Brivio <sbrivio@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "jhs@...atatu.com" <jhs@...atatu.com>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH net-next 16/17] net: sched: conditionally take rtnl lock
 on rules update path


On Tue 13 Nov 2018 at 09:40, Stefano Brivio <sbrivio@...hat.com> wrote:
> Hi Vlad,
>
> On Mon, 12 Nov 2018 09:55:45 +0200
> Vlad Buslov <vladbu@...lanox.com> wrote:
>
>> @@ -179,9 +179,25 @@ static void tcf_proto_destroy_work(struct work_struct *work)
>>  	rtnl_unlock();
>>  }
>>  
>> +/* Helper function to lock rtnl mutex when specified condition is true and mutex
>> + * hasn't been locked yet. Will set rtnl_held to 'true' before taking rtnl lock.
>> + * Note that this function does nothing if rtnl is already held. This is
>> + * intended to be used by cls API rules update API when multiple conditions
>> + * could require rtnl lock and its state needs to be tracked to prevent trying
>> + * to obtain lock multiple times.
>> + */
>> +
>> +static void tcf_require_rtnl(bool cond, bool *rtnl_held)
>> +{
>> +	if (!*rtnl_held && cond) {
>> +		*rtnl_held = true;
>> +		rtnl_lock();
>> +	}
>> +}
>
> I guess calls to this function are supposed to be serialised. If that's
> the case (which is my tentative understanding so far), I would indicate
> that in the comment.
>
> If that's not the case, you would be introducing a race I guess.
>
> Same applies to tcf_block_release() from 17/17.

Hi Stefano,

Thank you for reviewing my code!

I did not intend for this function to be serialized. First argument to
tcf_require_rtnl() is passed by value, and second argument is always a
pointer to local stack-allocated value of the caller. Same applies to
tcf_block_release() - its arguments are Qdisc and block which support
concurrency-safe reference counting, and pointer to local variable
rtnl_held, which is not accessible to concurrent users.

What is the race in these cases? Am I missing something?

Vlad