On 7/26/21 1:03 PM, Boqun Feng wrote:

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

quoted

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

quoted

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

quoted

Hi, Geert,

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

quoted

Hi Huacai,

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

quoted

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!

quoted

--- 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).

quoted

How to make CPU guarantee  "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().

Agreed. The xchg_tail() for the "_Q_PENDING_BITS == 1" case is a 
software emulation of xchg16(). Pure software emulation like that does 
not provide forward progress guarantee. This is usually not a big 
problem for non-RT kernel for which occasional long latency is 
acceptable, but it is not good for RT kernel.

Cheers,
Longman