Re: [RFC 0/4] futex2: Add new futex interface

[Andrà Almeida]

Hello Arnd,

On 6/25/20 3:48 AM, Arnd Bergmann wrote:
> On Fri, Jun 12, 2020 at 8:51 PM Andrà Almeida <andrealmeid@xxxxxxxxxxxxx> wrote:
> 
>> - The proposed interface uses ktime_t type for absolute timeout, and I
>>   assumed that it should use values in a nsec resolution. If this is true,
>>   we have some problems with i386 ABI, please check out the
>>   COMPAT_32BIT_TIME implementation in patch 1 for more details. I
>>   haven't added a time64 implementation yet, until this is clarified.
> 
> ktime_t is not part of the uapi headers, and has always been considered
> an implementation detail of the kernel so far. I would argue it should
> stay that way. The most sensible alternatives would be to either use
> a "__u64 *timeout" argument for a relative timeout, or a
> "struct __kernel_timespec *timeout" for an absolute timeout.
> 
> old_time32_t also makes no sense for multiple reasons:
> 
> - It's another kernel internal type and not part of the uapi headers
> - your time32 call has different calling conventions from your time64
>   version, not just a different type.
> - there should be no need to add syscalls that are known to be buggy
>   when there is a replacement type that does not have that bug.
> 

Thanks for the input. As stated by tglx at [1], "supporting relative
timeouts is wrong to begin with", my next patch will use "struct
__kernel_timespec *timeout" for an absolute timeout.

>> - Is expected to have a x32 ABI implementation as well? In the case of
>>   wait and wake, we could use the same as x86_64 ABI. However, for the
>>   waitv (aka wait on multiple futexes) we would need a proper x32 entry
>>   since we are dealing with 32bit pointers.
> 
> For new syscalls, I'd actually recommend not having a separate
> entry point, but just checking 'if (in_compat_syscall())' inside of the
> implementation to pick one behavior vs the other when accessing
> the user pointers. This keeps the implementation simpler and
> avoids assigning a new x32 syscall number that would be different
> from all the other architectures.
> 

Cool, this will make the code cleaner.

>       Arnd
> 


Thanks,
	AndrÃ

[1] https://lkml.org/lkml/2019/7/31/1499