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SubjectRe: Change in functionality of futex() system call.
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On Tue, Jun 7, 2011 at 2:53 PM, Andrew Lutomirski <luto@mit.edu> wrote:
> On Tue, Jun 7, 2011 at 3:33 PM, David Oliver <david@rgmadvisors.com> wrote:
>> On Tue, Jun 7, 2011 at 2:19 PM, Andrew Lutomirski <luto@mit.edu> wrote:
>>> On Tue, Jun 7, 2011 at 3:10 PM, David Oliver <david@rgmadvisors.com> wrote:
>>>> On Tue, Jun 7, 2011 at 1:43 PM, Andrew Lutomirski <luto@mit.edu> wrote:
>>>>> On Tue, Jun 7, 2011 at 11:58 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>>>>> Le mardi 07 juin 2011 à 10:44 -0400, Andy Lutomirski a écrit :
>>>>>>> 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.
>>>>>>
>>>>>> If a group of cooperating processes uses a memory segment to exchange
>>>>>> critical information, do you really think this memory segment will be
>>>>>> readable by other unrelated processes on the machine ?
>>>>>
>>>>> Depends on the design.
>>>>>
>>>>> I have some software I'm working on that uses shared files and could
>>>>> easily use futexes.
>>>>>
>>>> I have software which currently uses shared files for a one way
>>>> transfer of information, which is modeled precisely by the futex (as
>>>> contrasted to the mutex) model. In this case, the number of receivers
>>>> is undetermined, so the number of wakeups is set to maxint.
>>>>
>>>> The receivers are minimally trusted: they have read access to the
>>>> files, so they cannot accidentally affect other processes use of the
>>>> data. Requiring my files to be writeable by all clients would require
>>>> a serious increase in the amount of software needing to be trusted.
>>>
>>> What's wrong with adding a FUTEX_WAIT_NOCONSUME flag then?  Your
>>> program can use it to get exactly the semantics it wants and my
>>> program can use it or not depending on which semantics it wants.
>>>
>> 1. I would prefer not to require my programs have to check for kernel
>> version (code named "working", "regressed", and "altered") to decide
>> which parameters need to be sent to the futex call.
>
> You don't have to check for kernel version.  Just try
> FUTEX_WAIT_NOCONSUME first and retry with FUTEX_WAIT if it returns
> -EINVAL.
>
... and punt if that gives me an EFAULT. Possible but clumsy.
Fortunately, I'm not writing code for general consumption.

> I think you've already lost on regressed kernels regardless :-/
>
>> 2. Doing FUTEX_WAIT_NOCONSUME would change the semantics of
>> futex_wake() between the "working" and "altered" kernels, as it would
>> no longer return the number of processes woken.
>
> True, but that change couldn't affect old code because old code
> wouldn't use FUTEX_WAIT_NOCONSUME.
>
So, how would I find out the number of processes awakened by the
futex_wake() - I only care for statistical purposes.

>>
>> It seems that FUTEX_WAIT_NOCONSUME would be rather like a
>> non-consuming read on a pipe.
>
> More like a nonconsuming read on an eventfd, which sounds very useful.
>  (Actually, I'm porting code from Windows to Linux right now that
> wants that feature...)
>
> The reason I bring this up now is that I've been annoyed that
> FUTEX_WAIT can be used on an R/O mapping to interfere with futexes in
> that mapping.  Under the original semantics this would have been
> pretty much impossible to fix, but the regression has been there for
> long enough that we have the option right now to fix it better instead
> of restoring the original behavior.
>
Not being a kernel developer, the change seems very recent - about
when I started finding my code failing with EFAULTs.

From my perspective, that's a real case of my futexes being interfered with :).
>
> --Andy
>

--
Cheers!

David.


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