Use the new cmpxchg_emu_u8() to emulate one-byte cmpxchg() on ARM systems
with ARCH < ARMv6K.

[ paulmck: Apply Arnd Bergmann and Nathan Chancellor feedback. ]

Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/all/54798f68-48f7-4c65-9cba-47c0bf175143@sirena.org.uk/
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Closes: https://lore.kernel.org/all/CA+G9fYuZ+pf6p8AXMZWtdFtX-gbG8HMaBKp=XbxcdzA_QeLkxQ@mail.gmail.com/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Cc: "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Davis <afd@ti.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Eric DeVolder <eric.devolder@oracle.com>
Cc: Rob Herring <robh@kernel.org>
Cc: <linux-arm-kernel@lists.infradead.org>
---
 arch/arm/Kconfig               | 1 +
 arch/arm/include/asm/cmpxchg.h | 7 ++++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ee5115252aac4..a867a7d967aa5 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -34,6 +34,7 @@ config ARM
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
 	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT if CPU_V7
+	select ARCH_NEED_CMPXCHG_1_EMU if CPU_V6
 	select ARCH_SUPPORTS_ATOMIC_RMW
 	select ARCH_SUPPORTS_CFI_CLANG
 	select ARCH_SUPPORTS_HUGETLBFS if ARM_LPAE
diff --git a/arch/arm/include/asm/cmpxchg.h b/arch/arm/include/asm/cmpxchg.h
index 44667bdb4707a..a428e06fe94ee 100644
--- a/arch/arm/include/asm/cmpxchg.h
+++ b/arch/arm/include/asm/cmpxchg.h
@@ -5,6 +5,7 @@
 #include <linux/irqflags.h>
 #include <linux/prefetch.h>
 #include <asm/barrier.h>
+#include <linux/cmpxchg-emu.h>
 
 #if defined(CONFIG_CPU_SA1100) || defined(CONFIG_CPU_SA110)
 /*
@@ -162,7 +163,11 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 	prefetchw((const void *)ptr);
 
 	switch (size) {
-#ifndef CONFIG_CPU_V6	/* min ARCH >= ARMv6K */
+#ifdef CONFIG_CPU_V6	/* min ARCH < ARMv6K */
+	case 1:
+		oldval = cmpxchg_emu_u8((volatile u8 *)ptr, old, new);
+		break;
+#else /* min ARCH >= ARMv6K */
 	case 1:
 		do {
 			asm volatile("@ __cmpxchg1\n"
-- 
2.40.1

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.

Yours,
Linus Walleij

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"

On Tue, Jun 4, 2024 at 11:14 PM Paul E. McKenney <paulmck@kernel.org> wrote:

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

Yup, also fix the commit message to be == CPU_V6,
with that:
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Yours,
Linus Walleij

On Wed, Jun 05, 2024 at 10:38:07AM +0200, Linus Walleij wrote:
> On Tue, Jun 4, 2024 at 11:14 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> 
> > Thank you for looking this over!  Does the following patch (to be merged
> > into the original) capture it properly?
> 
> Yup, also fix the commit message to be == CPU_V6,
> with that:

Good catch, and will fix.

> Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

Thank you!  I will apply these on my next rebase.

							Thanx, Paul