From: Boqun Feng <boqun.feng@gmail.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
elver@google.com, akpm@linux-foundation.org, tglx@linutronix.de,
peterz@infradead.org, dianders@chromium.org, pmladek@suse.com,
arnd@arndb.de, torvalds@linux-foundation.org,
kernel-team@meta.com
Subject: Re: [PATCH v2 cmpxchg 09/13] lib: Add one-byte emulation function
Date: Mon, 13 May 2024 07:44:00 -0700 [thread overview]
Message-ID: <ZkInMNOsLO5XbDj5@boqun-archlinux> (raw)
In-Reply-To: <20240501230130.1111603-9-paulmck@kernel.org>
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
>
next prev parent reply other threads:[~2024-05-13 14:44 UTC|newest]
Thread overview: 109+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-01 21:39 [PATCH RFC cmpxchg 0/8] Provide emulation for one- and two-byte cmpxchg() Paul E. McKenney
2024-04-01 21:39 ` [PATCH RFC cmpxchg 1/8] lib: Add one-byte and two-byte cmpxchg() emulation functions Paul E. McKenney
2024-04-02 13:07 ` Marco Elver
2024-04-02 17:15 ` Paul E. McKenney
2024-04-01 21:39 ` [PATCH RFC cmpxchg 2/8] sparc: Emulate one-byte and two-byte cmpxchg Paul E. McKenney
2024-04-01 22:38 ` Al Viro
2024-04-01 23:58 ` Paul E. McKenney
2024-04-02 0:07 ` Al Viro
2024-04-02 3:37 ` Al Viro
2024-04-02 4:11 ` Al Viro
2024-04-02 4:18 ` Paul E. McKenney
2024-04-02 4:28 ` [PATCH 1/8] sparc32: make __cmpxchg_u32() return u32 Al Viro
2024-04-02 4:28 ` [PATCH 2/8] sparc32: make the first argument of __cmpxchg_u64() volatile u64 * Al Viro
2024-04-02 4:28 ` [PATCH 3/8] sparc32: unify __cmpxchg_u{32,64} Al Viro
2024-04-02 7:28 ` Arnd Bergmann
2024-04-02 20:02 ` Paul E. McKenney
2024-04-02 4:28 ` [PATCH 4/8] sparc32: add __cmpxchg_u{8,16}() and teach __cmpxchg() to handle those sizes Al Viro
2024-04-02 4:28 ` [PATCH 5/8] parisc: __cmpxchg_u32(): lift conversion into the callers Al Viro
2024-04-02 4:28 ` [PATCH 6/8] parisc: unify implementations of __cmpxchg_u{8,32,64} Al Viro
2024-04-02 4:28 ` [PATCH 7/8] parisc: add missing export of __cmpxchg_u8() Al Viro
2024-04-02 4:28 ` [PATCH 8/8] parisc: add u16 support to cmpxchg() Al Viro
2024-04-02 20:03 ` [PATCH 1/8] sparc32: make __cmpxchg_u32() return u32 Paul E. McKenney
2024-04-03 22:20 ` Al Viro
2024-04-04 3:09 ` Paul E. McKenney
2024-04-02 4:17 ` [PATCH RFC cmpxchg 2/8] sparc: Emulate one-byte and two-byte cmpxchg Paul E. McKenney
2024-04-01 21:39 ` [PATCH RFC cmpxchg 3/8] ARC: " Paul E. McKenney
2024-04-02 8:14 ` Arnd Bergmann
2024-04-02 17:06 ` Paul E. McKenney
2024-04-02 20:52 ` Paul E. McKenney
2024-04-04 11:57 ` Arnd Bergmann
2024-04-04 14:44 ` Paul E. McKenney
2024-04-04 15:06 ` Arnd Bergmann
2024-04-01 21:39 ` [PATCH RFC cmpxchg 4/8] csky: " Paul E. McKenney
2024-04-01 21:39 ` [PATCH RFC cmpxchg 5/8] sh: " Paul E. McKenney
2024-04-01 21:39 ` [PATCH RFC cmpxchg 6/8] xtensa: " Paul E. McKenney
2024-04-01 21:39 ` [PATCH RFC cmpxchg 7/8] parisc: Emulate " Paul E. McKenney
2024-04-01 21:39 ` [PATCH RFC cmpxchg 8/8] riscv: Emulate one-byte and " Paul E. McKenney
2024-04-04 14:15 ` Palmer Dabbelt
2024-04-04 14:50 ` Paul E. McKenney
2024-05-11 6:50 ` Guo Ren
2024-05-11 14:54 ` Paul E. McKenney
2024-05-11 20:44 ` Leonardo Bras
2024-04-08 17:47 ` [PATCH RFC cmpxchg 0/8] Provide emulation for one- and two-byte cmpxchg() Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 01/14] sparc32: make __cmpxchg_u32() return u32 Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 02/14] sparc32: make the first argument of __cmpxchg_u64() volatile u64 * Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 03/14] sparc32: unify __cmpxchg_u{32,64} Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 04/14] sparc32: add __cmpxchg_u{8,16}() and teach __cmpxchg() to handle those sizes Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 05/14] parisc: __cmpxchg_u32(): lift conversion into the callers Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 06/14] parisc: unify implementations of __cmpxchg_u{8,32,64} Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 07/14] parisc: add missing export of __cmpxchg_u8() Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 08/14] parisc: add u16 support to cmpxchg() Paul E. McKenney
2024-04-08 20:10 ` Linus Torvalds
2024-04-08 20:53 ` Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 09/14] lib: Add one-byte emulation function Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 10/14] ARC: Emulate one-byte cmpxchg Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 11/14] csky: " Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 12/14] sh: " Paul E. McKenney
2024-04-18 8:04 ` Geert Uytterhoeven
2024-04-08 17:49 ` [PATCH cmpxchg 13/14] xtensa: " Paul E. McKenney
2024-04-18 8:06 ` Geert Uytterhoeven
2024-04-18 23:21 ` Paul E. McKenney
2024-04-19 5:07 ` Yujie Liu
2024-04-19 8:02 ` Geert Uytterhoeven
2024-04-20 14:03 ` Paul E. McKenney
2024-04-08 17:49 ` [PATCH cmpxchg 14/14] riscv: " Paul E. McKenney
2024-04-09 17:35 ` Andrea Parri
2024-04-09 18:08 ` Paul E. McKenney
2024-05-01 22:58 ` [PATCH v2 cmpxchg 0/8] Provide emulation for one--byte cmpxchg() Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 01/13] sparc32: make __cmpxchg_u32() return u32 Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 02/13] sparc32: make the first argument of __cmpxchg_u64() volatile u64 * Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 03/13] sparc32: unify __cmpxchg_u{32,64} Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 04/13] sparc32: add __cmpxchg_u{8,16}() and teach __cmpxchg() to handle those sizes Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 05/13] parisc: __cmpxchg_u32(): lift conversion into the callers Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 06/13] parisc: unify implementations of __cmpxchg_u{8,32,64} Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 07/13] parisc: add missing export of __cmpxchg_u8() Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 08/13] parisc: add u16 support to cmpxchg() Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 09/13] lib: Add one-byte emulation function Paul E. McKenney
2024-05-13 14:44 ` Boqun Feng [this message]
2024-05-13 15:41 ` Paul E. McKenney
2024-05-13 15:57 ` Boqun Feng
2024-05-13 21:19 ` Boqun Feng
2024-05-14 14:22 ` Paul E. McKenney
2024-05-14 14:53 ` Boqun Feng
2024-05-14 15:02 ` Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 10/13] ARC: Emulate one-byte cmpxchg Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 11/13] csky: " Paul E. McKenney
2024-05-11 6:42 ` Guo Ren
2024-05-11 14:49 ` Paul E. McKenney
2024-05-01 23:01 ` [PATCH v2 cmpxchg 12/13] sh: " Paul E. McKenney
2024-05-02 4:52 ` John Paul Adrian Glaubitz
2024-05-02 5:06 ` Paul E. McKenney
2024-05-02 5:11 ` John Paul Adrian Glaubitz
2024-05-02 13:33 ` Paul E. McKenney
2024-05-02 20:53 ` Al Viro
2024-05-02 21:01 ` alpha cmpxchg.h (was Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg) Al Viro
2024-05-02 22:16 ` Linus Torvalds
2024-05-02 21:18 ` [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg Paul E. McKenney
2024-05-02 22:07 ` Al Viro
2024-05-02 23:12 ` Paul E. McKenney
2024-05-02 23:24 ` Al Viro
2024-05-02 23:45 ` Paul E. McKenney
2024-05-02 23:32 ` Linus Torvalds
2024-05-03 0:16 ` Paul E. McKenney
2024-05-02 21:50 ` Arnd Bergmann
2024-05-02 5:42 ` D. Jeff Dionne
2024-05-02 11:30 ` Arnd Bergmann
2024-05-01 23:01 ` [PATCH v2 cmpxchg 13/13] xtensa: " Paul E. McKenney
2024-05-02 20:01 ` [PATCH v2 cmpxchg 0/8] Provide emulation for one--byte cmpxchg() Al Viro
2024-05-02 21:20 ` Paul E. McKenney
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