linux-kernel - Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry

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Date:	Wed, 22 May 2013 05:30:37 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Roman Gushchin <klamm@...dex-team.ru>
Cc:	paulmck@...ux.vnet.ibm.com, Dipankar Sarma <dipankar@...ibm.com>,
	zhmurov@...dex-team.ru, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu
 macro

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.



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