On Tue, Jul 27, 2021 at 09:07:44AM +0800, Guo Ren wrote:

On Tue, Jul 27, 2021 at 1:03 AM Boqun Feng [off-list ref] wrote:

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On Tue, Jul 27, 2021 at 12:41:34AM +0800, Guo Ren wrote:

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On Mon, Jul 26, 2021 at 6:39 PM Boqun Feng [off-list ref] wrote:

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On Mon, Jul 26, 2021 at 04:56:49PM +0800, Huacai Chen wrote:

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Hi, Geert,

On Mon, Jul 26, 2021 at 4:36 PM Geert Uytterhoeven [off-list ref] wrote:

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Hi Huacai,

On Sat, Jul 24, 2021 at 2:36 PM Huacai Chen [off-list ref] wrote:

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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 <redacted>

Thanks for your patch!

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--- 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.

Seems only atomic could provide forward progress, isn't it? And lr/sc
couldn't implement xchg/cmpxchg primitive properly.

I'm missing you point here, a) ll/sc can provide forward progress and b)
ll/sc instructions are used to implement xchg/cmpxchg (see ARM64 and
PPC).

I don't think arm64 could provide fwd guarantee with ll/sc, otherwise,
they wouldn't add ARM64_HAS_LSE_ATOMICS for large systems.

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How to make CPU ç  "load + cmpxchg" forward-progress? Fusion
these instructions and lock the snoop channel?
Maybe hardware guys would think that it's easier to implement cas +
dcas + amo(short & byte).

Please note that if _Q_PENDING_BITS == 1, then the xchg_tail() is
implemented as a "load + cmpxchg", so if "load + cmpxchg" implementation
of xchg16() doesn't provide forward-progress in an architecture, neither
does xchg_tail().

That's the problem of "_Q_PENDING_BITS == 1", no hardware could
provide "load + ALU + cas" fwd guarantee!

You lost me again... in this patchset, archs that don't have hardware
xchg16 will use _Q_PENDING_BITS == 1, and now you tell me that
"_Q_PENDING_BITS == 1" has a problem, so what's the point of this
patchset? Does the problem of "_Q_PENDING_BITS == 1" need fixes? If so,
when do you plan to send the fix?

Regards,
Boqun

A simple example, atomic a++:
c = READ_ONCE(g_value);
new = c + 1;
while ((old = cmpxchg(&g_value, c, new)) != c) {
    c = old;
    new = c + 1;
}

Q: When it runs on CPU0(500Mhz) & CPU1(2Ghz) in one SMP, how do we
prevent CPU1 from starving CPU0?
A: I think the answer is using AMO-add instruction:
atomic_add(1, &g_value);
(If the arch hasn't atomic instructions and using cmpxchg or lr/sc
implement atomic, it's the same problem.)

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Regards,
Boqun

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Regards,
Boqun

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--- 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

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   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

--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/

-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/