Re: futex(2) man page update help request

[Michael Kerrisk (man-pages)]

On 01/19/2015 11:45 AM, Thomas Gleixner wrote:
> On Fri, 16 Jan 2015, Darren Hart wrote:
>> On 1/16/15, 12:54 PM, "Michael Kerrisk (man-pages)"
>> <mtk.manpages@xxxxxxxxx> 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?
> 
> No, I had a short look at the code misread it. So, yes, it's a valid
> operation for the non PI case. Sorry for the confusion.

Thanks for the confirmation, Thomas. Page updated to remove 
FUTEX_CMP_REQUEUE from that error case.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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