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SubjectRe: [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@cn.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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