Architectures are required to provide four-byte cmpxchg() and 64-bit architectures are additionally required to provide eight-byte cmpxchg(). However, there are cases where one-byte cmpxchg() would be extremely useful. Therefore, provide cmpxchg_emu_u8() that emulates one-byte cmpxchg() in terms of four-byte cmpxchg(). Note that this emulations is fully ordered, and can (for example) cause one-byte cmpxchg_relaxed() to incur the overhead of full ordering. If this causes problems for a given architecture, that architecture is free to provide its own lighter-weight primitives. [ paulmck: Apply Marco Elver feedback. ] [ paulmck: Apply kernel test robot feedback. ] [ paulmck: Drop two-byte support per Arnd Bergmann feedback. ] Link: https://lore.kernel.org/all/0733eb10-5e7a-4450-9b8a-527b97c842ff@paulmck-laptop/ Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Acked-by: Marco Elver <elver@xxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx> Cc: Douglas Anderson <dianders@xxxxxxxxxxxx> Cc: Petr Mladek <pmladek@xxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: <linux-arch@xxxxxxxxxxxxxxx> --- arch/Kconfig | 3 +++ include/linux/cmpxchg-emu.h | 15 +++++++++++++ lib/Makefile | 1 + lib/cmpxchg-emu.c | 45 +++++++++++++++++++++++++++++++++++++ 4 files changed, 64 insertions(+) create mode 100644 include/linux/cmpxchg-emu.h create mode 100644 lib/cmpxchg-emu.c diff --git a/arch/Kconfig b/arch/Kconfig index 9f066785bb71d..284663392eef8 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -1609,4 +1609,7 @@ config CC_HAS_SANE_FUNCTION_ALIGNMENT # strict alignment always, even with -falign-functions. def_bool CC_HAS_MIN_FUNCTION_ALIGNMENT || CC_IS_CLANG +config ARCH_NEED_CMPXCHG_1_EMU + bool + endmenu diff --git a/include/linux/cmpxchg-emu.h b/include/linux/cmpxchg-emu.h new file mode 100644 index 0000000000000..998deec67740a --- /dev/null +++ b/include/linux/cmpxchg-emu.h @@ -0,0 +1,15 @@ +/* SPDX-License-Identifier: GPL-2.0+ */ +/* + * Emulated 1-byte and 2-byte cmpxchg operations for architectures + * lacking direct support for these sizes. These are implemented in terms + * of 4-byte cmpxchg operations. + * + * Copyright (C) 2024 Paul E. McKenney. + */ + +#ifndef __LINUX_CMPXCHG_EMU_H +#define __LINUX_CMPXCHG_EMU_H + +uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new); + +#endif /* __LINUX_CMPXCHG_EMU_H */ diff --git a/lib/Makefile b/lib/Makefile index ffc6b2341b45a..cc3d52fdb477d 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -236,6 +236,7 @@ obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o lib-$(CONFIG_GENERIC_BUG) += bug.o obj-$(CONFIG_HAVE_ARCH_TRACEHOOK) += syscall.o +obj-$(CONFIG_ARCH_NEED_CMPXCHG_1_EMU) += cmpxchg-emu.o obj-$(CONFIG_DYNAMIC_DEBUG_CORE) += dynamic_debug.o #ensure exported functions have prototypes diff --git a/lib/cmpxchg-emu.c b/lib/cmpxchg-emu.c new file mode 100644 index 0000000000000..27f6f97cb60dd --- /dev/null +++ b/lib/cmpxchg-emu.c @@ -0,0 +1,45 @@ +// SPDX-License-Identifier: GPL-2.0+ +/* + * Emulated 1-byte cmpxchg operation for architectures lacking direct + * support for this size. This is implemented in terms of 4-byte cmpxchg + * operations. + * + * Copyright (C) 2024 Paul E. McKenney. + */ + +#include <linux/types.h> +#include <linux/export.h> +#include <linux/instrumented.h> +#include <linux/atomic.h> +#include <linux/panic.h> +#include <linux/bug.h> +#include <asm-generic/rwonce.h> +#include <linux/cmpxchg-emu.h> + +union u8_32 { + u8 b[4]; + u32 w; +}; + +/* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */ +uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new) +{ + u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3); + int i = ((uintptr_t)p) & 0x3; + union u8_32 old32; + union u8_32 new32; + u32 ret; + + ret = READ_ONCE(*p32); + do { + old32.w = ret; + if (old32.b[i] != old) + return old32.b[i]; + new32.w = old32.w; + new32.b[i] = new; + instrument_atomic_read_write(p, 1); + ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above. + } while (ret != old32.w); + return old; +} +EXPORT_SYMBOL_GPL(cmpxchg_emu_u8); -- 2.40.1
