Re: [PATCH 3/3] locking/atomic: openrisc: use generic_cmpxchg[64]_local for arch_cmpxchg[64]_local

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

Hi Wuqiang,

On Thu, 26 Oct 2023 19:05:51 +0800
"wuqiang.matt" <wuqiang.matt@bytedance.com> wrote:

> On 2023/10/26 16:46, Arnd Bergmann wrote:
> > On Thu, Oct 26, 2023, at 09:39, wuqiang.matt wrote:
> >> arch_cmpxchg[64]_local() are not defined for openrisc. So implement
> >> them with generci_cmpxchg[64]_local, advised by Masami Hiramatsu.
> >>
> >> Closes:
> >> https://lore.kernel.org/linux-trace-kernel/169824660459.24340.14614817132696360531.stgit@devnote2
> >> Closes:
> >> https://lore.kernel.org/oe-kbuild-all/202310241310.Ir5uukOG-lkp@intel.com
> >>
> >> Signed-off-by: wuqiang.matt <wuqiang.matt@bytedance.com>
> > 
> > I think on architectures that have actual atomics, you
> > generally want to define this to be the same as arch_cmpxchg()
> > rather than the generic version.
> > 
> > It depends on the relative cost of doing one atomic compared
> > to an irq-disable/enable pair, but everyone else went with
> > the former if they could. The exceptions are armv4/armv5,
> > sparc32 and parisc, which don't have a generic cmpxchg()
> > or similar operation.
> 
> Sure, better native than the generic. I'll try to collect more
> insights before next move.

So I will temporally remove the last change (use arch_cmpxchg_local
in objpool) until these series are rewritten with arch native code,
so that the next release will not break the kernel build.

But this must be fixed because arch_cmpxchg_local() is required
for each arch anyway.

> 
> > You could do the thing that sparc64 and xtensa do, which
> > use the native cmpxchg for supported word sizes but the
> > generic version for 1- and 2-byte swaps, but that has its
> > own set of problems if you end up doing operations on both
> > the entire word and a sub-unit of the same thing.
> 
> Thank you for pointing out this. I'll do some research on these
> implementations.

arc also has the LL-SC instruction but depends on the core feature,
so I think we can use it.

Thank you,

> 
> >      Arnd
> 
> Regards,
> wuqiang
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>