[PATCH] glibc: Remove CPU set size checking from affinity functions [BZ #19143]

[Michael Kerrisk (man-pages)]

Hello Florian.

On 03/10/2016 12:20 PM, Florian Weimer wrote:
> On 03/08/2016 08:42 PM, Michael Kerrisk (man-pages) wrote:
> 
>>> One caveat is that sched_getaffinity can set bits beyond the requested
>>> allocation size (in bits) because the kernel gets a padded CPU vector
>>> and sees a few additional bits.  
>>
>> I'm not quite clear on this point. Does it get a padded CPU vector
>> because CPU_ALLOC() might allocate a vector of size larger than the
>> user requested?
> 
> Yes, this is the problem, combined with CPU_ALLOC_SIZE returning the
> larger size (which is unavoidable).

Thanks for the clarification. I added this paragraph:

       Be aware that CPU_ALLOC(3) may allocate a slightly  larger  CPU
       set  than  requested  (because  CPU sets are implemented as bit
       masks  allocated  in  units  of  sizeof(long)).   Consequently,
       sched_getaffinity()  can  set bits beyond the requested alloca‐
       tion size, because the  kernel  sees  a  few  additional  bits.
       Therefore,  the  caller  should  iterate  over  the bits in the
       returned set, counting those  which  are  set,  and  stop  upon
       reaching  the value returned by CPU_COUNT(3) (rather than iter‐
       ating over the number of bits requested  to  be  allocated).

> This whole interface could have been designed much better (compare
> select to epoll, for instance).

Indeed!

Cheers,

Michael

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