linux-kernel - Re: seqcount usage in xt_replace

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Date:   Thu, 10 Jan 2019 21:25:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Florian Westphal <fw@...len.de>
Cc:     Anatol Pomozov <anatol.pomozov@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, paulmck@...ux.ibm.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()

On Thu, Jan 10, 2019 at 03:48:12PM +0100, Florian Westphal wrote:
> Peter Zijlstra <peterz@...radead.org> wrote:
> > Is performance a concern in this path? There is no comment justifying
> > this 'creative' stuff.
> 
> We have to wait until all cpus are done with current iptables ruleset.
> 
> Before this 'creative' change, this relied on get_counters
> synchronization.  And that caused wait times of 30 seconds or more on
> busy systems.
> 
> I have no objections swapping this with a synchronize_rcu() if that
> makes it easier.

Would using synchronize_rcu() not also mean you can get rid of that
xt_write_recseq*() stuff entirely?

Anyway, synchronize_rcu() can also take a little while, but I don't
think anywere near 30 seconds.

> (synchronize_rcu might be confusing though, as we don't use rcu
>  for table->private), and one 'has to know' that all the netfilter
>  hooks, including the iptables eval loop, run with rcu_read_lock
>  held).

A comment can help there, right?