linux-kernel - Re: seqcount usage in xt_replace

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Message-ID: <20190110144812.mkbokbj2iyryj6lv@breakpoint.cc>
Date:   Thu, 10 Jan 2019 15:48:12 +0100
From:   Florian Westphal <fw@...len.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Florian Westphal <fw@...len.de>,
        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()

Peter Zijlstra <peterz@...radead.org> wrote:
> 	/*
> 	 * Ensure contents of newinfo are visible before assigning to
> 	 * private.
> 	 */
> 	smp_wmb();
> 	table->private = newinfo;
> 
> we have:
> 
> 	smp_store_release(&table->private, newinfo);
> 
> But what store does that second smp_wmb() order against? The comment:
> 
> 	/* make sure all cpus see new ->private value */
> 	smp_wmb();
> 
> makes no sense what so ever, no smp_*() barrier can provide such
> guarantees.

IIRC I added this at the request of a reviewer of an earlier iteration
of that patch.

IIRC the concern was that compiler/hw could re-order

smb_wmb();
table->private = newinfo;
/* wait until all cpus are done with old table */

 into:

smb_wmb();
/* wait until all cpus are done with old table */
...
table->private = newinfo;

and that (obviously) makes the wait-loop useless.

> > Only alternative I see that might work is synchronize_rcu (the
> > _do_table functions are called with rcu read lock held).
> > 
> > I guess current scheme is cheaper though.
> 
> 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.

(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).