linux-kernel - Re: [PATCH] fib_trie: rcu_assign

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Date:	Tue, 12 Feb 2008 08:57:14 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	shemminger@...tta.com, paulmck@...ibm.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

On 12-02-2008 02:16, David Miller wrote:
> From: Stephen Hemminger <shemminger@...tta.com>
> Date: Mon, 11 Feb 2008 16:59:54 -0800
> 
> linux-kernel added to CC:, any change to generic kernel infrastructure
> should be posted there
> 
>> Eliminate warnings when rcu_assign_pointer is used with unsigned long.
>> It is reasonable to use RCU with non-pointer values so allow it for general
>> use.  Add a comment to explain the if test.
>>
>> Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
>> ---
>>  include/linux/rcupdate.h |   13 +++++++------
>>  1 files changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>> index 37a642c..c44ac87 100644
>> --- a/include/linux/rcupdate.h
>> +++ b/include/linux/rcupdate.h
>> @@ -172,14 +172,15 @@ struct rcu_head {
>>   * structure after the pointer assignment.  More importantly, this
>>   * call documents which pointers will be dereferenced by RCU read-side
>>   * code.
>> + *
>> + * If value is the NULL (constant 0), then no barrier is needed.
>>   */
>>  
>> -#define rcu_assign_pointer(p, v) \
>> -	({ \
>> -		if (!__builtin_constant_p(v) || \
>> -		    ((v) != NULL)) \
>> -			smp_wmb(); \
>> -		(p) = (v); \
>> +#define rcu_assign_pointer(p, v)			\
>> +	({						\
>> +		if (!(__builtin_constant_p(v) && v))	\

...But, "If value is the NULL (constant 0)" we have:

if (!(1 && NULL != 0)) ==> if (!(0)) and the barrier is used?!

>> +			smp_wmb();			\
>> +		(p) = (v);				\
>>  	})
>>  
>>  /**


Regards,
Jarek P.
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