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

[Andy Lutomirski <luto@MIT.EDU>]

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.

--Andy