From: Boqun Feng <boqun.feng@gmail.com>
To: Huacai Chen <chenhuacai@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
Huacai Chen <chenhuacai@loongson.cn>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
Arnd Bergmann <arnd@arndb.de>, Waiman Long <longman@redhat.com>,
Guo Ren <guoren@kernel.org>,
Linux-Arch <linux-arch@vger.kernel.org>,
Rui Wang <wangrui@loongson.cn>,
Xuefeng Li <lixuefeng@loongson.cn>,
Jiaxun Yang <jiaxun.yang@flygoat.com>
Subject: Re: [PATCH RFC 1/2] arch: Introduce ARCH_HAS_HW_XCHG_SMALL
Date: Mon, 26 Jul 2021 18:39:26 +0800 [thread overview]
Message-ID: <YP6Q3s5Kpg2A1NRZ@boqun-archlinux> (raw)
In-Reply-To: <CAAhV-H4aNr2BuG1imx6RcfEQtarjbrUU+-_PbGRg4jX5ygr_iA@mail.gmail.com>
On Mon, Jul 26, 2021 at 04:56:49PM +0800, Huacai Chen wrote:
> Hi, Geert,
>
> On Mon, Jul 26, 2021 at 4:36 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> >
> > Hi Huacai,
> >
> > On Sat, Jul 24, 2021 at 2:36 PM Huacai Chen <chenhuacai@loongson.cn> wrote:
> > > Introduce a new Kconfig option ARCH_HAS_HW_XCHG_SMALL, which means arch
> > > has hardware sub-word xchg/cmpxchg support. This option will be used as
> > > an indicator to select the bit-field definition in the qspinlock data
> > > structure.
> > >
> > > Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> >
> > Thanks for your patch!
> >
> > > --- a/arch/Kconfig
> > > +++ b/arch/Kconfig
> > > @@ -228,6 +228,10 @@ config ARCH_HAS_FORTIFY_SOURCE
> > > An architecture should select this when it can successfully
> > > build and run with CONFIG_FORTIFY_SOURCE.
> > >
> > > +# Select if arch has hardware sub-word xchg/cmpxchg support
> > > +config ARCH_HAS_HW_XCHG_SMALL
> >
> > What do you mean by "hardware"?
> > Does a software fallback count?
> This new option is supposed as an indicator to select bit-field
> definition of qspinlock, software fallback is not helpful in this
> case.
>
I don't think this is true. IIUC, the rationale of the config is that
for some architectures, since the architectural cmpxchg doesn't provide
forward-progress guarantee then using cmpxchg of machine-word to
implement xchg{8,16}() may cause livelock, therefore these architectures
don't want to provide xchg{8,16}(), as a result they cannot work with
qspinlock when _Q_PENDING_BITS is 8.
So as long as an architecture can provide and has already provided an
implementation of xchg{8,16}() which guarantee forward-progress (even
though the implementation is using a machine-word size cmpxchg), the
architecture doesn't need to select ARCH_HAS_HW_XCHG_SMALL.
Regards,
Boqun
> >
> > > --- a/arch/m68k/Kconfig
> > > +++ b/arch/m68k/Kconfig
> > > @@ -5,6 +5,7 @@ config M68K
> > > select ARCH_32BIT_OFF_T
> > > select ARCH_HAS_BINFMT_FLAT
> > > select ARCH_HAS_DMA_PREP_COHERENT if HAS_DMA && MMU && !COLDFIRE
> > > + select ARCH_HAS_HW_XCHG_SMALL
> >
> > M68k CPUs which support the CAS (Compare And Set) instruction do
> > support this on 8-bit, 16-bit, and 32-bit quantities.
> > M68k CPUs which lack CAS use a software implementation, which
> > supports the same quantities.
> >
> > As CAS is used only if CONFIG_RMW_INSNS=y, perhaps this needs
> > a dependency?
> OK, I think this dependency is needed.
>
> Huacai
>
> >
> > select ARCH_HAS_HW_XCHG_SMALL if RMW_INSNS
> >
> > Gr{oetje,eeting}s,
> >
> > Geert
> >
> > --
> > Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
> >
> > In personal conversations with technical people, I call myself a hacker. But
> > when I'm talking to journalists I just say "programmer" or something like that.
> > -- Linus Torvalds
next prev parent reply other threads:[~2021-07-26 10:40 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-24 12:36 [PATCH RFC 1/2] arch: Introduce ARCH_HAS_HW_XCHG_SMALL Huacai Chen
2021-07-24 12:36 ` [PATCH RFC 2/2] qspinlock: Use ARCH_HAS_HW_XCHG_SMALL to select _Q_PENDING_BITS definition Huacai Chen
2021-07-24 19:24 ` [PATCH RFC 1/2] arch: Introduce ARCH_HAS_HW_XCHG_SMALL Arnd Bergmann
2021-07-25 3:06 ` Jiaxun Yang
2021-07-25 10:08 ` Arnd Bergmann
2021-07-26 8:36 ` Geert Uytterhoeven
2021-07-26 8:56 ` Huacai Chen
2021-07-26 10:39 ` Boqun Feng [this message]
2021-07-26 16:41 ` Guo Ren
2021-07-26 17:03 ` Boqun Feng
2021-07-26 21:20 ` Waiman Long
2021-07-27 1:27 ` Guo Ren
2021-07-27 2:29 ` Boqun Feng
2021-07-27 2:46 ` Waiman Long
2021-07-27 11:05 ` Peter Zijlstra
2021-07-28 10:40 ` Huacai Chen
2021-07-28 11:01 ` Peter Zijlstra
2021-07-29 16:38 ` Huacai Chen
2021-07-27 10:17 ` Peter Zijlstra
2021-07-27 1:07 ` Guo Ren
2021-07-27 1:52 ` Wang Rui
2021-07-27 11:03 ` Peter Zijlstra
2021-07-27 2:00 ` Boqun Feng
2021-07-27 10:50 ` Peter Zijlstra
2021-07-27 10:12 ` Peter Zijlstra
2021-07-26 9:00 ` Arnd Bergmann
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