linux-kernel - Re: [PATCH] drivers/net: Use RCU_INIT

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Date:	Mon, 24 Mar 2014 16:47:32 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"Michael S. Tsirkin" <mst@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Monam Agarwal <monamagarwal123@...il.com>, davem@...emloft.net,
	jasowang@...hat.com, xemul@...allels.com, wuzhy@...ux.vnet.ibm.com,
	therbert@...gle.com, yamato@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers/net: Use RCU_INIT_POINTER(x, NULL) in tun.c

On 03/24/2014 01:25 PM, Eric Dumazet wrote:
> On Mon, 2014-03-24 at 07:09 +0200, Michael S. Tsirkin wrote:
> 
>> Seems an incredibly strict requirement for something that just
>> silences a warning.
>> What exactly should I test?
>> I intended to just verify this produces same code as before
>> d322f45ceed525daa under a recent gcc.
> 
> Thats because many rcu_assign_pointer(X, NULL) were already converted to
> RCU_INIT_POINTER(X, NULL)
> 
> Quite frankly I don't know why you bother at all.
> 
> Adding back the lazy test in rcu_assign_pointer() doesn't help to make
> the API cleaner and easier to understand.
> 
> People are usually using RCU API without really understanding
> all the issues. They tend to add superfluous barriers because they feel
> better. 
> 
> Having separate RCU_INIT_POINTER() and rcu_assign_pointer() serve as
> better documentation of the code, I find it more easier to immediately
> check what is going on while reviewing stuff.
> 
> Presumably, checkpatch.pl could be augmented to suggest to use
> RCU_INIT_POINTER(X, NULL) instead of rcu_assign_pointer(X, NULL)


I prefer rcu_assign_pointer(X, NULL) than RCU_INIT_POINTER(X, NULL),
NULL should not be a special pointer value to the users of RCU.

the RCU implements should hide the difference if RCU implements
differentiate the values for optimization.

RCU_INIT_POINTER() sounds as an initialization-stage API. If we need
something different for NULL pointer, I prefer
rcu_assign_*null*_pointer().

rcu_assign_pointer(X, NULL) implies compiler barrier(), but
RCU_INIT_POINTER(X, NULL) doesn't.

> 
> 
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