netdev - Re: [PATCH 0/9] Replace rcu_assign_pointer(x, NULL) with RCU_INIT

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Date:	Thu, 20 Mar 2014 08:06:01 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arend van Spriel <arend@...adcom.com>
Cc:	Monam Agarwal <monamagarwal123@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, eddie.wai@...adcom.com
Subject: Re: [PATCH 0/9] Replace rcu_assign_pointer(x, NULL) with
 RCU_INIT_POINTER(x,NULL)

On Thu, Mar 20, 2014 at 03:58:52PM +0100, Arend van Spriel wrote:
> On 20/03/14 15:37, Monam Agarwal wrote:
> >This patchset uses following coccinelle script to replace
> >rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x,NULL)
> >
> >@@
> >expression E;
> >@@
> >- rcu_assign_pointer(
> >+ RCU_INIT_POINTER(
> >E, NULL)
> 
> I am not very familiar with rcu functionality nor coccinelle, but it
> looks too generic. I would think only NULL assignments during
> initialization should use RCU_INIT_POINTER(). The first patch in the
> series does it in a function called unregister_...(). Is it really
> ok to do there?

Hello, Arend,

The thing that rcu_assign_pointer() is doing is ensuring that the
initialization of a structure is carried out before storing a pointer
to that structure.  In the case of the NULL pointer, there is no
structure to initialize, so nothing need be ordered.

So, yes, rcu_assign_pointer(p, NULL) can always safely be converted to
RCU_INIT_POINTER(p, NULL).  ;-)

							Thanx, Paul

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