Re: [OpenRISC] [PATCH v6 1/9] locking/qspinlock: Add ARCH_USE_QUEUED_SPINLOCKS_XCHG32

Stafford Horne
2021-04-07T20:13:26Z

On Wed, Apr 07, 2021 at 11:47:49AM +0200, Peter Zijlstra wrote:
> On Wed, Apr 07, 2021 at 08:52:08AM +0900, Stafford Horne wrote:
> > Why doesn't RISC-V add the xchg16 emulation code similar to OpenRISC? For
> > OpenRISC we added xchg16 and xchg8 emulation code to enable qspinlocks. So
> > one thought is with CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32=y, can we remove our
> > xchg16/xchg8 emulation code?
>
> CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32 is guaranteed crap.
>
> All the architectures that have wanted it are RISC style LL/SC archs,
> and for them a cmpxchg loop is a daft thing to do, since it reduces the
> chance of it behaving sanely.
>
> Why would we provide something that's known to be suboptimal? If an
> architecture chooses to not care about determinism and or fwd progress,
> then that's their choice. But not one, I feel, we should encourage.

Thanks, this is the response I was hoping my comment would provoke.

So not enabling CONFIG_ARCH_USE_QUEUED_SPINLOCKS_XCHG32 for architectures
unless they really want it should be the way.

-Stafford