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Message-ID: <1369225837.3301.324.camel@edumazet-glaptop>
Subject: Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu
 macro
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Roman Gushchin <klamm@yandex-team.ru>
Cc: paulmck@linux.vnet.ibm.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>
Date: Wed, 22 May 2013 05:30:37 -0700
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On Wed, 2013-05-22 at 15:58 +0400, Roman Gushchin wrote:

> +/*
> + * Same as ACCESS_ONCE(), but used for accessing field of a structure.
> + * The main goal is preventing compiler to store &ptr->field in a register.

But &ptr->field is a constant during the whole duration of
udp4_lib_lookup2() and could be in a register, in my case field is at
offset 0, and ptr is a parameter (so could be in a 'register')

The bug you found is that compiler caches the indirection  (ptr->field)
into a register, not that compiler stores &ptr->field into a register.

> + */
> +#define ACCESS_FIELD_ONCE(PTR, FIELD) (((volatile typeof(*PTR) *)PTR)->FIELD)
> +

Here we force the compiler to consider ptr as volatile, but semantically
it is not required in rcu_dereference(ptr->field)  

We want field to be reloaded, not ptr.

So yes, the patch appears to fix the bug, but it sounds not logical to
me.