Re: [PATCH v2 cmpxchg 09/13] lib: Add one-byte emulation function

[Boqun Feng <boqun.feng@gmail.com>]

On Wed, May 01, 2024 at 04:01:26PM -0700, Paul E. McKenney wrote:
> 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@kernel.org>
> Acked-by: Marco Elver <elver@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
> Cc: Douglas Anderson <dianders@chromium.org>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: <linux-arch@vger.kernel.org>
> ---
>  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.

Just out of curiosity, why is this `data_race` needed? cmpxchg is atomic
so there should be no chance for a data race?

Regards,
Boqun

> +	} while (ret != old32.w);
> +	return old;
> +}
> +EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
> -- 
> 2.40.1
>