From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Florian Westphal <fw@strlen.de>,
Anatol Pomozov <anatol.pomozov@gmail.com>,
Dmitry Vyukov <dvyukov@google.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()
Date: Fri, 11 Jan 2019 06:08:10 -0800 [thread overview]
Message-ID: <20190111140810.GA1215@linux.ibm.com> (raw)
In-Reply-To: <20190111083411.GM1900@hirez.programming.kicks-ass.net>
On Fri, Jan 11, 2019 at 09:34:11AM +0100, Peter Zijlstra wrote:
> On Thu, Jan 10, 2019 at 11:29:20PM +0100, Florian Westphal wrote:
> > Peter Zijlstra <peterz@infradead.org> wrote:
> > > Would using synchronize_rcu() not also mean you can get rid of that
> > > xt_write_recseq*() stuff entirely?
> >
> > No, because those are used to synchronize with cpus that read
> > the ruleset counters, see
> >
> > net/ipv4/netfilter/ip_tables.c:get_counters().
>
> Ah, bummer :/
>
> > > Anyway, synchronize_rcu() can also take a little while, but I don't
> > > think anywere near 30 seconds.
> >
> > Ok, I think in that case it would be best to just replace the
> > recseq value sampling with smp_mb + synchronize_rcu plus a comment
> > that explains why its done.
>
> synchronize_rcu() implies smp_mb() on all CPUs.
Yes, it does, but in the case of idle CPUs, the smp_mb() calls are only
required to follow any pre-existing RCU read-side critical section on
the one hand an precede any RCU read-side critical section completing
after the synchronize_rcu() on the other.
To do more would mean waking up idle CPUs, which does not make the
battery-powered guys happy. ;-)
Thanx, Paul
next prev parent reply other threads:[~2019-01-11 14:08 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-08 19:33 seqcount usage in xt_replace_table() Anatol Pomozov
2019-01-08 22:37 ` Florian Westphal
2019-01-10 12:41 ` Peter Zijlstra
2019-01-10 12:53 ` Dmitry Vyukov
2019-01-10 20:18 ` Peter Zijlstra
2019-01-10 14:48 ` Florian Westphal
2019-01-10 20:20 ` Peter Zijlstra
2019-01-10 20:25 ` Peter Zijlstra
2019-01-10 22:29 ` Florian Westphal
2019-01-11 8:34 ` Peter Zijlstra
2019-01-11 14:08 ` Paul E. McKenney [this message]
2019-01-10 14:52 ` Paul E. McKenney
2019-01-09 0:02 ` Andrea Parri
2019-01-09 0:36 ` Anatol Pomozov
2019-01-09 5:35 ` Dmitry Vyukov
2019-01-09 11:24 ` Andrea Parri
2019-01-09 11:55 ` Dmitry Vyukov
2019-01-09 12:11 ` Andrea Parri
2019-01-09 12:29 ` Dmitry Vyukov
2019-01-09 17:10 ` Paul E. McKenney
2019-01-10 8:49 ` Dmitry Vyukov
2019-01-10 12:30 ` Andrea Parri
2019-01-10 12:38 ` Dmitry Vyukov
2019-01-10 12:46 ` Andrea Parri
2019-01-10 13:25 ` Dmitry Vyukov
2019-01-10 14:50 ` Paul E. McKenney
2019-01-10 12:44 ` Peter Zijlstra
2019-01-10 12:54 ` Dmitry Vyukov
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