linux-kernel - Re: [PATCH] markers: bit-field is not thread-safe nor smp-safe

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Date:	Fri, 10 Oct 2008 13:30:19 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH] markers: bit-field is not thread-safe nor smp-safe

KOSAKI Motohiro wrote:
> Hi Lai-san,
> 
>> bit-field is not thread-safe nor smp-safe.
>>
>> struct marker_entry.rcu_pending is not protected by any lock
>> in rcu-callback free_old_closure().
>> so we must turn it into a safe type.
> 
> hmmm
> however, char also doesn't smp-safe because some architecture doesn't have
> any byte load/store instruction.
> 
> It seems bogus solution to me ;)

Hi, KOSAKI-san,

	Thank you very much!

	char also doesn't smp-safe if the architecture doesn't have
any byte load/store instruction.

	We must use int, is it right?

Lai

> 
> 
>> detail:
>>
>> I suppose rcu_pending and ptype are store in struct marker_entry.tmp1
>>
>> free_old_closure() side:           change ptype side:
>>
>>                                 |  load struct marker_entry.tmp1
>> --------------------------------|--------------------------------
>>                                 |  change ptype bit in tmp1
>> load struct marker_entry.tmp1   |
>> change rcu_pending bit in tmp1  |
>> store tmp1                      |
>> --------------------------------|--------------------------------
>>                                 |  store tmp1
>>
>> now this result equals that free_old_closure() do not change rcu_pending bit, bug.
>>
>> see also: http://en.wikipedia.org/wiki/Bit_field
>>
>> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
>> ---
>> diff --git a/kernel/marker.c b/kernel/marker.c
>> index 7d1faec..4777218 100644
>> --- a/kernel/marker.c
>> +++ b/kernel/marker.c
>> @@ -62,7 +62,7 @@ struct marker_entry {
>>  	int refcount;	/* Number of times armed. 0 if disarmed. */
>>  	struct rcu_head rcu;
>>  	void *oldptr;
>> -	unsigned char rcu_pending:1;
>> +	unsigned char rcu_pending;
>>  	unsigned char ptype:1;
>>  	char name[0];	/* Contains name'\0'format'\0' */
>>  };
> 
> 
> 
> 
> 
> 


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