Re: [PATCH v4 02/13] riscv: Do not fail to build on byte/halfword operations with Zawrs

Andrew Jones
2024-07-31T14:10:21Z

On Wed, Jul 31, 2024 at 09:23:54AM GMT, Alexandre Ghiti wrote:
> riscv does not have lr instructions on byte and halfword but the
> qspinlock implementation actually uses such atomics provided by the
> Zabha extension, so those sizes are legitimate.

We currently always come to __cmpwait() through smp_cond_load_relaxed()
and queued_spin_lock_slowpath() adds another invocation. However, isn't
the reason we're hitting the BUILD_BUG() because the switch fails to find
a case for 16, not because it fails to find cases for 1 or 2? The new
invocation passes a pointer to a struct mcs_spinlock, which looks like
it has size 16. We need to ensure that when ptr points to a pointer that
we pass the size of uintptr_t.

>
> Then instead of failing to build, just fallback to the !Zawrs path.

No matter what sizes we're failing on, if we do this then
queued_spin_lock_slowpath() won't be able to take advantage of Zawrs.

Thanks,
drew

>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
> arch/riscv/include/asm/cmpxchg.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index ebbce134917c..9ba497ea18a5 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -268,7 +268,8 @@ static __always_inline void __cmpwait(volatile void *ptr,
> break;
> #endif
> default:
> - BUILD_BUG();
> + /* RISC-V doesn't have lr instructions on byte and half-word. */
> + goto no_zawrs;
> }
>
> return;
> --
> 2.39.2
>
>
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