[PATCH 1/5] arch: add CONFIG_ARCH_USE_CUSTOM_READ_ONCE for arm64/alpha

[Alice Ryhl <aliceryhl@google.com>]

By exposing this variable as a config option, conditional compilation in
Rust code may rely on it to determine whether it should use a volatile
read or call a C helper function to perform a READ_ONCE operation.

This config option is also added on alpha for consistency, even if Rust
does not support alpha right now.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 arch/Kconfig                    | 11 +++++++++++
 arch/alpha/Kconfig              |  1 +
 arch/alpha/include/asm/rwonce.h |  4 ++--
 arch/arm64/Kconfig              |  1 +
 arch/arm64/include/asm/rwonce.h |  4 ++--
 5 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 31220f512b16d5cfbc259935c2d3675b60c1e25c..683176bb09e50e31f398bb92678283e5de66b282 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -229,6 +229,17 @@ config HAVE_EFFICIENT_UNALIGNED_ACCESS
 	  See Documentation/core-api/unaligned-memory-access.rst for more
 	  information on the topic of unaligned memory accesses.
 
+config ARCH_USE_CUSTOM_READ_ONCE
+	bool
+	help
+	  Some architectures reorder address-dependent volatile loads,
+	  which means that the default implementation of READ_ONCE that
+	  relies on a volatile load is not appropriate.
+
+	  This symbol should be selected by an architecture if it
+	  redefines READ_ONCE to use a different implementation than a
+	  volatile load.
+
 config ARCH_USE_BUILTIN_BSWAP
 	bool
 	help
diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 80367f2cf821ceb4fc29485b7b21b37d5c310765..1d5d48153ba0087554221e9412a6af0c672d3f5c 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -11,6 +11,7 @@ config ALPHA
 	select ARCH_NO_PREEMPT
 	select ARCH_NO_SG_CHAIN
 	select ARCH_USE_CMPXCHG_LOCKREF
+	select ARCH_USE_CUSTOM_READ_ONCE if SMP
 	select FORCE_PCI
 	select PCI_DOMAINS if PCI
 	select PCI_SYSCALL if PCI
diff --git a/arch/alpha/include/asm/rwonce.h b/arch/alpha/include/asm/rwonce.h
index 35542bcf92b3a883df353784bcb2d243475ccd91..c9f21aa0764625b24e8957923926d09a2eb97e7c 100644
--- a/arch/alpha/include/asm/rwonce.h
+++ b/arch/alpha/include/asm/rwonce.h
@@ -5,7 +5,7 @@
 #ifndef __ASM_RWONCE_H
 #define __ASM_RWONCE_H
 
-#ifdef CONFIG_SMP
+#ifdef CONFIG_ARCH_USE_CUSTOM_READ_ONCE
 
 #include <asm/barrier.h>
 
@@ -28,7 +28,7 @@
 	(typeof(x))__x;							\
 })
 
-#endif /* CONFIG_SMP */
+#endif /* CONFIG_ARCH_USE_CUSTOM_READ_ONCE */
 
 #include <asm-generic/rwonce.h>
 
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 93173f0a09c7deb07b46ab4f16a1a0e4320dfbf1..cd16053c8302479458a05c23ba9cfb73ee50232c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -89,6 +89,7 @@ config ARM64
 	select ARCH_KEEP_MEMBLOCK
 	select ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
 	select ARCH_USE_CMPXCHG_LOCKREF
+	select ARCH_USE_CUSTOM_READ_ONCE if LTO
 	select ARCH_USE_GNU_PROPERTY
 	select ARCH_USE_MEMTEST
 	select ARCH_USE_QUEUED_RWLOCKS
diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
index 78beceec10cda47b319db29d9f79d2a5df35e92d..5da6b2d6a12399a520f7a3310014de723baa278a 100644
--- a/arch/arm64/include/asm/rwonce.h
+++ b/arch/arm64/include/asm/rwonce.h
@@ -5,7 +5,7 @@
 #ifndef __ASM_RWONCE_H
 #define __ASM_RWONCE_H
 
-#if defined(CONFIG_LTO) && !defined(__ASSEMBLER__)
+#if defined(CONFIG_ARCH_USE_CUSTOM_READ_ONCE) && !defined(__ASSEMBLER__)
 
 #include <linux/compiler_types.h>
 #include <asm/alternative-macros.h>
@@ -62,7 +62,7 @@
 })
 
 #endif	/* !BUILD_VDSO */
-#endif	/* CONFIG_LTO && !__ASSEMBLER__ */
+#endif	/* CONFIG_ARCH_USE_CUSTOM_READ_ONCE && !__ASSEMBLER__ */
 
 #include <asm-generic/rwonce.h>
 

-- 
2.52.0.351.gbe84eed79e-goog