Re: [PATCH 1/2] [PING] Optimize generic spinlock code and use C11 like atomic macros.

[Torvald Riegel <triegel at redhat dot com>]

On Wed, 2017-05-31 at 10:29 +0200, Stefan Liebler wrote:
> On 05/30/2017 09:18 AM, Torvald Riegel wrote:
> > On Wed, 2017-05-10 at 15:00 +0200, Stefan Liebler wrote:
> >> On 05/03/2017 01:38 PM, Stefan Liebler wrote:
> >>> On 04/25/2017 08:46 AM, Stefan Liebler wrote:
> >>>> On 04/18/2017 03:09 PM, Stefan Liebler wrote:
> >>>>> On 04/10/2017 01:59 PM, Stefan Liebler wrote:
> >>>>>> On 04/09/2017 03:51 PM, Torvald Riegel wrote:
> >>>>>>> On Fri, 2017-04-07 at 18:22 +0200, Stefan Liebler wrote:
> >>>>>>>> @architecture maintainers:
> >>>>>>>> I've added defines of ATOMIC_EXCHANGE_USES_CAS in the architecture
> >>>>>>>> specific atomic-machine.h files.
> >>>>>>>> See comment in include/atomic.h:
> >>>>>>>> /* ATOMIC_EXCHANGE_USES_CAS is equal to 1 if atomic_exchange
> >>>>>>>> operations
> >>>>>>>>      are implemented based on a CAS loop; otherwise, this is 0 and we
> >>>>>>>> assume
> >>>>>>>>      that the atomic_exchange operations could provide better
> >>>>>>>> performance
> >>>>>>>>      than a CAS loop.  */
> >>>>>>>>
> >>>>>>>> Can review the definition to 0 or 1 in the atomic-machine.h file of
> >>>>>>>> your
> >>>>>>>> architecture, please?
> >>>>>
> >>>>>
> >>>>> PING
> >>>>>
> >>>> PING
> >>>
> >>> PING
> >>
> >> PING
> >>
> >> @Torvald:
> >> I'm not sure if we will get answers from everybody.
> >> What do you propose how to proceed with the definitions of
> >> ATOMIC_EXCHANGE_USES_CAS?
> > 
> > Which archs are still missing?  If maintainers don't reply, I suggest we
> > do our best to figure out what's the case for each missing arch, and add
> > a note that it should be checked eventually (eg, /* XXX Is this actually
> > correct?  */) to the code, and then commit.  Please CC: me on such a
> > patch, so I can have a last look at it.
> > 
> 
> We are sure for these archs:
> -aarch64: #define ATOMIC_EXCHANGE_USES_CAS 0
> -i386/x86: #define ATOMIC_EXCHANGE_USES_CAS 0
> -mips: #define ATOMIC_EXCHANGE_USES_CAS 0/1
> -powerpc: #define ATOMIC_EXCHANGE_USES_CAS 1
> -s390: #define ATOMIC_EXCHANGE_USES_CAS 1
> -tile: #define ATOMIC_EXCHANGE_USES_CAS 0
> 
> Nobody answered for these archs:
> -alpha: #define ATOMIC_EXCHANGE_USES_CAS 1
> -arm: #define ATOMIC_EXCHANGE_USES_CAS 1
> -hppa: #define ATOMIC_EXCHANGE_USES_CAS 1
> -ia64: #define ATOMIC_EXCHANGE_USES_CAS 0
> -m68k: #define ATOMIC_EXCHANGE_USES_CAS 1
> -microblaze: #define ATOMIC_EXCHANGE_USES_CAS 1
> -nios2: #define ATOMIC_EXCHANGE_USES_CAS 1
> -sh: #define ATOMIC_EXCHANGE_USES_CAS 1
> -sparc: #define ATOMIC_EXCHANGE_USES_CAS 1

Joseph, can you clarify for arm?

> In the two of three sparc files ...
> ./sysdeps/sparc/sparc64/atomic-machine.h
> ./sysdeps/sparc/sparc32/sparcv9/atomic-machine.h
> ... there is the swap instruction in macro atomic_exchange_acq
> for 4 bytes.  Other sizes are aborted or done by CAS.
> Shall we use #define ATOMIC_EXCHANGE_USES_CAS 0 here?

I suggest we let the sparc maintainer take care of that (though we could
set it to 0 for sparc32 I guess).

> 
> I've attached the current version with
> /* XXX Is this actually correct?  */
> for all architecture for which we didn't get an answer.
> 
> The second patch "S390: Use generic spinlock code." remains the same.

I'd wait for a few more days to give Joseph time to respond, and then
commit if there are no objections or further input.  I think maintainers
had enough time to comment, and you pinged them often enough already.

Thanks for being patient! :)