From: Roman Gushchin <klamm@yandex-team.ru>
To: Eric Dumazet <eric.dumazet@gmail.com>, paulmck@linux.vnet.ibm.com
Cc: Jesper Dangaard Brouer <brouer@redhat.com>,
Dipankar Sarma <dipankar@in.ibm.com>,
zhmurov@yandex-team.ru, linux-kernel@vger.kernel.org,
netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
James Morris <jmorris@namei.org>,
Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
Patrick McHardy <kaber@trash.net>
Subject: Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro
Date: Wed, 29 May 2013 14:09:29 +0400 [thread overview]
Message-ID: <51A5D3D9.1000106@yandex-team.ru> (raw)
In-Reply-To: <1369804097.3301.615.camel@edumazet-glaptop>
On 29.05.2013 09:08, Eric Dumazet wrote:
> On Tue, 2013-05-28 at 18:31 -0700, Paul E. McKenney wrote:
>> On Tue, May 28, 2013 at 05:34:53PM -0700, Eric Dumazet wrote:
>>> On Tue, 2013-05-28 at 13:10 +0400, Roman Gushchin wrote:
>>>> On 28.05.2013 04:12, Eric Dumazet wrote:
> About your earlier question, I really don't know why compiler would
> cache a memory read if we explicitly use barrier() to prevent this from
> happening.
I agree.
> BTW Roman patch generates a double load as in :
>
> 2cb1: 49 8b 07 mov (%r15),%rax
> 2cb4: 49 8b 07 mov (%r15),%rax
>
>
> ...
> 2ea2: e8 f9 dc ff ff callq ba0 <sock_put>
> 2ea7: 8b 0c 24 mov (%rsp),%ecx
> 2eaa: e9 02 fe ff ff jmpq 2cb1 <udp4_lib_lookup2+0x91>
>
> because of ACCESS_ONCE() used twice, once explicitly in
> hlist_nulls_for_each_entry_rcu(), and once in rcu_dereference_raw()
>
> While barrier();ptr = rcu_dereference(X); does generate a single load.
It's true.
Unfortunately, using barrier() can also prevent gcc from making some
(acceptable) code optimizations, because barrier() has a global effect,
and everything we want to reload is the (head->first) pointer.
So, to be absolutely precise, we have to introduce and use
the ACCESS_FIELD_ONCE() macro.
In any case, it doesn't look like a big problem.
In my mind, the best solution is to use the ACCESS_FIELD_ONCE() macro,
but using barrier() is also an acceptable solution.
Regards,
Roman
next prev parent reply other threads:[~2013-05-29 10:09 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-21 9:05 [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro Roman Gushchin
2013-05-21 10:40 ` David Laight
2013-05-21 11:55 ` Roman Gushchin
2013-05-21 13:42 ` David Laight
2013-05-21 12:09 ` Paul E. McKenney
2013-05-21 12:46 ` Roman Gushchin
2013-05-21 12:58 ` Paul E. McKenney
2013-05-21 13:37 ` Eric Dumazet
2013-05-21 13:44 ` Eric Dumazet
2013-05-21 14:47 ` Roman Gushchin
2013-05-21 15:16 ` Eric Dumazet
2013-05-21 15:51 ` Roman Gushchin
2013-05-21 15:38 ` Eric Dumazet
2013-05-21 15:51 ` Roman Gushchin
2013-05-21 18:12 ` [PATCH v2] " Roman Gushchin
2013-05-22 2:01 ` Eric Dumazet
2013-05-22 5:49 ` Eric Dumazet
2013-05-22 11:58 ` Roman Gushchin
2013-05-22 12:30 ` Eric Dumazet
2013-05-22 13:07 ` Roman Gushchin
2013-05-22 17:45 ` Paul E. McKenney
2013-05-22 19:17 ` Roman Gushchin
2013-05-25 11:37 ` Paul E. McKenney
2013-05-27 11:34 ` Roman Gushchin
2013-05-27 17:55 ` Roman Gushchin
2013-05-28 0:12 ` Eric Dumazet
2013-05-28 9:10 ` Roman Gushchin
2013-05-29 0:34 ` Eric Dumazet
2013-05-29 1:31 ` Paul E. McKenney
2013-05-29 5:08 ` Eric Dumazet
2013-05-29 10:09 ` Roman Gushchin [this message]
2013-05-29 19:06 ` Eric Dumazet
2013-05-30 8:25 ` Roman Gushchin
2013-06-02 23:31 ` Eric Dumazet
2013-06-03 2:58 ` David Miller
2013-06-03 3:12 ` Eric Dumazet
2013-06-03 3:27 ` David Miller
2013-06-03 3:42 ` Paul E. McKenney
2013-06-03 3:47 ` Eric Dumazet
2013-06-03 3:49 ` David Miller
2013-06-03 6:05 ` Paul E. McKenney
2013-06-10 18:29 ` Boris B. Zhmurov
2013-06-10 18:51 ` Eric Dumazet
2013-06-03 3:48 ` Paul E. McKenney
2013-06-03 3:42 ` Paul E. McKenney
2013-05-29 9:17 ` Roman Gushchin
2013-05-29 1:19 ` Paul E. McKenney
2013-05-22 13:27 ` David Laight
2013-05-22 13:27 ` David Laight
2013-05-22 13:36 ` Eric Dumazet
2013-05-22 14:23 ` David Laight
2013-05-22 14:23 ` David Laight
2013-05-22 13:55 ` Roman Gushchin
2013-05-22 9:58 ` Paul E. McKenney
2013-05-22 12:28 ` Eric Dumazet
2013-05-22 13:00 ` Paul E. McKenney
2013-05-22 14:16 ` Eric Dumazet
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