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

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

On Sun, 29 Oct 2023 00:40:17 +0800
"wuqiang.matt" <wuqiang.matt@bytedance.com> wrote:

> On 2023/10/28 20:49, Masami Hiramatsu (Google) wrote:
> > 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.
> 
> Ok, it's fine to me. Thank you.
> 
> 
> > But this must be fixed because arch_cmpxchg_local() is required
> > for each arch anyway.
> 
> Yes. I'm working on the new update for arc/openrisc/hexagon. It would
> be better resolve this issue first, then consider the objpool update
> of using arch_cmpxchg_local.
> 
> >>
> >>> 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.
> 
> Right. The arc processor does have the CONFIG_ARC_HAS_LLSC option, but
> I doubt the correctness of arch_cmpxchg_relaxed and arch_cmpxchg:
> 
> arch_cmpxchg_relaxed:
> ...
>          switch(sizeof((_p_))) {
>          case 4:
> ....
> 
> arch_cmpxchg:
> ...
> 	BUILD_BUG_ON(sizeof(_p_) != 4);		
> ...
> 
> _p is the address pointer, so I'm thinking it's a typo but I couldn't
> yet confirm. There is not much about arc processors in the web :(

Hmm, indeed. This seems like a bug but it depends on the 'llock  %0, [%1]'
can take a 32bit address or 32bit data register. Usually it should
check the size of data, but need to check with ISA manual.

Vineet, can you check this suspicious bug?

Thank you,

> 
> 
> > Thank you,
> > 
> >>
> >>>       Arnd
> >>
> >> Regards,
> >> wuqiang
> >>
> > 
> > 
> 


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