Luciano Coelho wrote:
> On Wed, 2010-06-09 at 19:48 +0200, Coelho Luciano (Nokia-D/Helsinki)
> wrote:
>
>> On Wed, 2010-06-09 at 17:18 +0200, ext Jan Engelhardt wrote:
>>
>>>>>> + timer = __idletimer_tg_find_by_label(info->label);
>>>>>> + if (!timer) {
>>>>>> + spin_unlock(&list_lock);
>>>>>> + timer = idletimer_tg_create(info);
>>>>>>
>>>>>>
>>>>> How does this prevent creating the same timer twice?
>>>>>
>>>> The timer will only be created if __idletimer_tg_find_by_label() returns
>>>> NULL, which means that no timer with that label has been found. "info"
>>>> won't be the same if info->label is different, right? Or can it change
>>>> on the fly?
>>>>
>>> One thing to be generally aware about is that things could potentially
>>> be instantiated by another entity between the time a label was looked up
>>> with negative result and the time one tries to add it.
>>> It may thus be required to extend keeping the lock until after
>>> idletimer_tg_create, in other words, lookup and create must be atomic
>>> to the rest of the world.
>>>
>> Ahh, sure! I missed the actual point of Patrick's question. I had the
>> idletimer_tg_create() inside the lock, but when I added the
>> sysfs_create_file() there (which can sleep), I screwed up with the
>> locking.
>>
>> I'll move the sysfs file creation to outside that function so I can keep
>> the lock until after the timer is added to the list. Thanks for
>> clarifying!
>>
>
> Hmmm... after struggling with this for a while, I think it's not really
> possible to simply create the sysfs file outside of the lock, because if
> the sysfs creation fails, we will again risk a race condition.
>
> I think the only way is to delay the sysfs file creation and do it in a
> workqueue.
>
Why don't you simply use a mutex instead of the spinlock? It would be better
to only do the lookup once and store the timer pointer in the target
structure
anyways.
--
To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
