netdev - Re: [PATCH] ipv6: sit: Replace rcu_assign_pointer() with RCU_INIT

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Message-Id: <20140827.180703.431737353995283706.davem@davemloft.net>
Date:	Wed, 27 Aug 2014 18:07:03 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	bernat.ada@...il.com
Cc:	kuznet@....inr.ac.ru, jmorris@...ei.org, yoshfuji@...ux-ipv6.org,
	kaber@...sh.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] ipv6: sit: Replace rcu_assign_pointer() with
 RCU_INIT_POINTER()

From: Andreea-Cristina Bernat <bernat.ada@...il.com>
Date: Tue, 26 Aug 2014 16:46:32 +0300

> According to RCU_INIT_POINTER()'s block comment 3.a, it can be used if
> "3.   The referenced data structure has already been exposed to readers either
> at compile time or via rcu_assign_pointer() -and-
>  a.   You have not made -any- reader-visible changes to this structure since
> then".
> 
> This case fulfills the conditions above because between the rtnl_dereference()
> call and the rcu_assign_pointer() call there is no update of that value.
> Therefore, this patch makes the replacement.
> 
> The following Coccinelle semantic patch was used:
> @@
> @@
> 
> - rcu_assign_pointer
> + RCU_INIT_POINTER
>   (...,
> (
>  rtnl_dereference(...)
> |
>  rcu_dereference_protected(...)
> ) )
> 
> Signed-off-by: Andreea-Cristina Bernat <bernat.ada@...il.com>

These changes are not correct.

We need the memory barrier added by rcu_assign_pointer(), which occurs
before the assignment, otherwise the RCU pointer update might be visible
before the stores that initialize the rest of the structure in question.

I'm not applying any of these patches, sorry.
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