Re: futex(2) man page update help request

[Darren Hart <dvhart-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On 1/16/15, 12:54 PM, "Michael Kerrisk (man-pages)"
<mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

>On 01/16/2015 04:20 PM, Thomas Gleixner wrote:
>> On Fri, 16 Jan 2015, Michael Kerrisk (man-pages) wrote:
>> 
>>> Hello Thomas,
>>>
>>> On 01/15/2015 11:23 PM, Thomas Gleixner wrote:
>>>> On Thu, 15 Jan 2015, Michael Kerrisk (man-pages) wrote:
>>>>>> [EINVAL] uaddr equal uaddr2. Requeue to same futex.
>>>>>
>>>>> ??? I added this, but does this error not occur only for PI requeues?
>>>>
>>>> It's equally wrong for normal futexes. And its actually the same code
>>>> checking for this for all variants.
>>>
>>> I don't understand "equally wrong" in your reply, I'm sorry. Do you
>>> mean:
>>>
>>> a) This error text should be there for both normal and PI requeues
>> 
>> It is there for both. The requeue code has that check independent of
>> the requeue type (normal/pi). It never makes sense to requeue
>> something to itself whether normal or pi futex. We added this for PI,
>> because there it is harmful, but we did not special case it. So normal
>> futexes get the same treatment.
>
>Hello Thomas, 
>
>Color me stupid, but I can't see this in futex_requeue(). Where is that
>check that is "independent of the requeue type (normal/pi)"?
>
>When I look through futex_requeue(), all the likely looking sources
>of EINVAL are governed by a check on the 'requeue_pi' argument.


Right, in the non-PI case, I believe there are valid use cases: move to
the back of the FIFO, for example (OK, maybe the only example?). Both
tests ensuring uaddr1 != uaddr2 are under the requeue_pi conditional
block. The second compares the keys in case they are not FUTEX_PRIVATE
(uaddrs would be different, but still the same backing store).

Thomas, am I missing a test for this someplace else?


-- 
Darren Hart
Intel Open Source Technology Center