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

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Date:	Wed, 22 May 2013 17:07:07 +0400
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	Eric Dumazet <eric.dumazet@...il.com>
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 22.05.2013 16:30, Eric Dumazet wrote:
> 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)

Actually, we need to mark an "address of a place" where the field value is
located as volatile before dereferencing. I have no idea how to do it in another way,
except using multiple casts and offsetof's, but, IMHO, it will be even more complex:
	ACCESS_ONCE(typeof(&ptr->field)((char*)ptr + offsetof(typeof(*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.
>

May be we can enhance it by providing better/more detailed comments here?
Have you any suggestions?

Thanks,
Roman
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