Re: [PATCH v3 cmpxchg 4/4] ARM: Emulate one-byte cmpxchg

Paul E. McKenney
2024-06-04T21:14:06Z

On Tue, Jun 04, 2024 at 10:52:23PM +0200, Linus Walleij wrote:
> Hi Paul,
>
> thanks for your patch! This caught my eye:
>
> On Tue, Jun 4, 2024 at 7:04 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> > Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on ARM systems
> > with ARCH < ARMv6K.
>
> ARCH == ARMv6.
>
> This ARCH < ARMv6K comes from inversion of the the a bit terse
> comment for ifndef CONFIG_CPU_V6, which means "out of the
> post-v6 CPUs, the following applies to those > V6".
>
> The code in the patch, IIUC make use of cmpxchg_emu_u8()
> if and only if the CPU is V6.
>
> > -#ifndef CONFIG_CPU_V6 /* min ARCH >= ARMv6K */
> > +#ifdef CONFIG_CPU_V6 /* min ARCH < ARMv6K */
>
> This is now a set with one member so this comment should say:
>
> /* ARCH == ARMv6 */
>
> After this change.

Thank you for looking this over! Does the following patch (to be merged
into the original) capture it properly?

Thanx, Paul

------------------------------------------------------------------------

diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
index a428e06fe94ee..9beb64d305866 100644
--- a/arch/arm/include/asm/cmpxchg.h
+++ b/arch/arm/include/asm/cmpxchg.h
@@ -163,11 +163,11 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
prefetchw((const void *)ptr);

switch (size) {
-#ifdef CONFIG_CPU_V6 /* min ARCH < ARMv6K */
+#ifdef CONFIG_CPU_V6 /* ARCH == ARMv6 */
case 1:
oldval = cmpxchg_emu_u8((volatile u8 *)ptr, old, new);
break;
-#else /* min ARCH >= ARMv6K */
+#else /* min ARCH > ARMv6 */
case 1:
do {
asm volatile("@ __cmpxchg1\n"