Re: [PATCH v4 3/4] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

Peter Zijlstra
2021-03-29T17:01:02Z

On Sat, Mar 27, 2021 at 06:06:38PM +0000, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
>
> Some architectures don't have sub-word swap atomic instruction,
> they only have the full word's one.
>
> The sub-word swap only improve the performance when:
> NR_CPUS < 16K
> * 0- 7: locked byte
> * 8: pending
> * 9-15: not used
> * 16-17: tail index
> * 18-31: tail cpu (+1)
>
> The 9-15 bits are wasted to use xchg16 in xchg_tail.
>
> Please let architecture select xchg16/xchg32 to implement
> xchg_tail.

So I really don't like this, this pushes complexity into the generic
code for something that's really not needed.

Lots of RISC already implement sub-word atomics using word ll/sc.
Obviously they're not sharing code like they should be :/ See for
example arch/mips/kernel/cmpxchg.c.

Also, I really do think doing ticket locks first is a far more sensible
step.