Re: Change in functionality of futex() system call.

[Darren Hart <dvhart@linux.intel.com>]

On 06/07/2011 07:44 AM, Andy Lutomirski wrote:
> On 06/06/2011 11:13 PM, Darren Hart wrote:
>>
>>
>> On 06/06/2011 11:11 AM, Eric Dumazet wrote:
>>> Le lundi 06 juin 2011 à 10:53 -0700, Darren Hart a écrit :
>>>>
>>>
>>>> If I understand the problem correctly, RO private mapping really doesn't
>>>> make any sense and we should probably explicitly not support it, while
>>>> special casing the RO shared mapping in support of David's scenario.
>>>>
>>>
>>> We supported them in 2.6.18 kernels, apparently. This might sounds
>>> stupid but who knows ?
>>
>>
>> I guess this is actually the key point we need to agree on to provide a
>> solution. This particular case "worked" in 2.6.18 kernels, but that
>> doesn't necessarily mean it was supported, or even intentional.
>>
>> It sounds to me that we agree that we should support RO shared mappings.
>> The question remains about whether we should introduce deliberate
>> support of RO private mappings, and if so, if the forced COW approach is
>> appropriate or not.
>>
> 
> I disagree.
> 
> FUTEX_WAIT has side-effects.  Specifically, it eats one wakeup sent by 
> FUTEX_WAKE.  So if something uses futexes on a file mapping, then a 
> process with only read access could (if the semantics were changed) DoS 
> the other processes by spawning a bunch of threads and FUTEX_WAITing 
> from each of them.
> 
> If there were a FUTEX_WAIT_NOCONSUME that did not consume a wakeup and 
> worked on RO mappings, I would drop my objection.


This sounds like an argument for properly managing file permissions and
carefully selecting the mapping backing your futex word - but I don't
see this as compelling rationale to disable RO support entirely and
certainly not to add yet another futex op code.


-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel