netdev - Re: [Patch net 00/15] net_sched: remove RCU callbacks from TC

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Message-ID: <CAM_iQpVLHLx9eVgO0S--iq-XkhZE82Y30Z0REwU6qx8OGgm+mA@mail.gmail.com>
Date:   Wed, 25 Oct 2017 15:37:40 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        John Fastabend <john.fastabend@...il.com>,
        Chris Mi <chrism@...lanox.com>
Subject: Re: [Patch net 00/15] net_sched: remove RCU callbacks from TC

On Tue, Oct 24, 2017 at 6:43 PM, David Miller <davem@...emloft.net> wrote:
> From: Cong Wang <xiyou.wangcong@...il.com>
> Date: Mon, 23 Oct 2017 15:02:49 -0700
>
>> Recently, the RCU callbacks used in TC filters and TC actions keep
>> drawing my attention, they introduce at least 4 race condition bugs:
>
> Like Eric, I think doing a full RCU sync on every delete is too big
> a pill to swallow.  This is a major control plane performance
> regression.
>
> Please find another reasonable way to fix this.
>

Alright... I finally find a way to make everyone happy.

My solution is introducing a workqueue for tc filters
and let each RCU callback defer the work to this
workqueue. I solve the flush_workqueue() deadlock
by queuing another work in the same workqueue
at the end, so the execution order should be as same
as it is now. The ugly part is now tcf_block_put() which
looks like below:


static void tcf_block_put_final(struct work_struct *work)
{
        struct tcf_block *block = container_of(work, struct tcf_block, work);
        struct tcf_chain *chain, *tmp;

        /* At this point, all the chains should have refcnt == 1. */
        rtnl_lock();
        list_for_each_entry_safe(chain, tmp, &block->chain_list, list)
                tcf_chain_put(chain);
        rtnl_unlock();
        kfree(block);
}

static void tcf_block_put_deferred(struct work_struct *work)
{
        struct tcf_block *block = container_of(work, struct tcf_block, work);
        struct tcf_chain *chain;

        rtnl_lock();
        /* Hold a refcnt for all chains, except 0, in case they are gone. */
        list_for_each_entry(chain, &block->chain_list, list)
                if (chain->index)
                        tcf_chain_hold(chain);

        /* No race on the list, because no chain could be destroyed. */
        list_for_each_entry(chain, &block->chain_list, list)
                tcf_chain_flush(chain);

        INIT_WORK(&block->work, tcf_block_put_final);
        /* Wait for RCU callbacks to release the reference count and make
         * sure their works have been queued before this.
         */
        rcu_barrier();
        tcf_queue_work(&block->work);
        rtnl_unlock();
}

void tcf_block_put(struct tcf_block *block)
{
        if (!block)
                return;

        INIT_WORK(&block->work, tcf_block_put_deferred);
        /* Wait for existing RCU callbacks to cool down, make sure their works
         * have been queued before this. We can not flush pending works here
         * because we are holding the RTNL lock.
         */
        rcu_barrier();
        tcf_queue_work(&block->work);
}


Paul, does this make any sense to you? ;)