arc_usr_cmpxchg and preemption

[Max Filippov]

> On Thu, Mar 15, 2018 at 12:03 PM, Alexey Brodkin <Alexey.Brodkin at synopsys.com> wrote:
> Here's a brief analysis:
> ARM:  Looks like they got rid of that stuff in v4.4, see
>       commit db695c0509d6 ("ARM: remove user cmpxchg syscall").
>
> M68K: That's even uglier implementation which is really asking for
>       a facelift, look at sys_atomic_cmpxchg_32() here:
>       https://elixir.bootlin.com/linux/latest/source/arch/m68k/kernel/sys_m68k.c#L461
>
> MIPS: They do it via special sysmips syscall which among other things
>       might handle MIPS_ATOMIC_SET with mips_atomic_set()
>
> I don't immediately see if there're others but really I'm not sure if it even worth trying to
> clean-up all that since efforts might be spent pointlessly.

xtensa is another one. We used to have a buggy implementation in
arch/xtensa/kernel/entry.S:fast_syscall_xtensa which we still keep
disabled by default, just in case somebody wanted backwards
compatibility. I don't think it's worth fixing.

-- 
Thanks.
-- Max