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

Peter Zijlstra
2021-04-07T16:01:09Z

On Wed, Apr 07, 2021 at 04:29:12PM +0200, Christoph Müllner wrote:
> RISC-V defines LR/SC loops consisting of up to 16 instructions as
> constrained LR/SC loops. Such constrained LR/SC loops provide the
> required forward guarantees, that are expected (similar to what other
> architectures, like AArch64, have).

The text quoted by others didn't seem to say such a thing, but whatever.

> What RISC-V does not have is sub-word atomics and if required, we
> would have to implement them as LL/SC sequences. And yes, using atomic
> instructions is preferred over using LL/SC,

(psudo asm, can't be bothered to figure out the actual syntax)

# setup r_and_mask, r_or_mask

.L1
LL r, [word]
AND r, r, r_and_mask
OR r, r, r_or_mask
SC r, [word]
JNE .L1

is what you need for LL/SC based xchg16, that's less than 16
instructions. If RISC-V guarantees fwd progress on that, good, write it
like that and lets end this thread.

The fact that this is apparently hard, is not good.