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Message-Id: <20240401213950.3910531-1-paulmck@kernel.org>
Date: Mon, 1 Apr 2024 14:39:43 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: linux-kernel@...r.kernel.org
Cc: kernel-team@...a.com,
"Paul E. McKenney" <paulmck@...nel.org>,
Marco Elver <elver@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Douglas Anderson <dianders@...omium.org>,
Petr Mladek <pmladek@...e.com>,
linux-arch@...r.kernel.org
Subject: [PATCH RFC cmpxchg 1/8] lib: Add one-byte and two-byte cmpxchg() emulation functions
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 and two-byte cmpxchg()
would be extremely useful. Therefore, provide cmpxchg_emu_u8() and
cmpxchg_emu_u16() that emulate one-byte and two-byte cmpxchg() in terms
of four-byte cmpxchg().
Note that these emulations are 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. ]
Link: https://lore.kernel.org/all/0733eb10-5e7a-4450-9b8a-527b97c842ff@paulmck-laptop/
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Marco Elver <elver@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc: Douglas Anderson <dianders@...omium.org>
Cc: Petr Mladek <pmladek@...e.com>
Cc: <linux-arch@...r.kernel.org>
---
arch/Kconfig | 3 ++
include/linux/cmpxchg-emu.h | 16 ++++++++
lib/Makefile | 1 +
lib/cmpxchg-emu.c | 74 +++++++++++++++++++++++++++++++++++++
4 files changed, 94 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 ae4a4f37bbf08..01093c60952a5 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_2_EMU
+ bool
+
endmenu
diff --git a/include/linux/cmpxchg-emu.h b/include/linux/cmpxchg-emu.h
new file mode 100644
index 0000000000000..fee8171fa05eb
--- /dev/null
+++ b/include/linux/cmpxchg-emu.h
@@ -0,0 +1,16 @@
+/* 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);
+uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t new);
+
+#endif /* __LINUX_CMPXCHG_EMU_H */
diff --git a/lib/Makefile b/lib/Makefile
index ffc6b2341b45a..1d93b61a7ecbe 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_2_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..a88c4f3c88430
--- /dev/null
+++ b/lib/cmpxchg-emu.c
@@ -0,0 +1,74 @@
+/* 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.
+ */
+
+#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));
+ } while (ret != old32.w);
+ return old;
+}
+EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
+
+union u16_32 {
+ u16 h[2];
+ u32 w;
+};
+
+/* Emulate two-byte cmpxchg() in terms of 4-byte cmpxchg. */
+uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t new)
+{
+ u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
+ int i = (((uintptr_t)p) & 0x2) / 2;
+ union u16_32 old32;
+ union u16_32 new32;
+ u32 ret;
+
+ WARN_ON_ONCE(((uintptr_t)p) & 0x1);
+ ret = READ_ONCE(*p32);
+ do {
+ old32.w = ret;
+ if (old32.h[i] != old)
+ return old32.h[i];
+ new32.w = old32.w;
+ new32.h[i] = new;
+ instrument_atomic_read_write(p, 2);
+ ret = data_race(cmpxchg(p32, old32.w, new32.w));
+ } while (ret != old32.w);
+ return old;
+}
+EXPORT_SYMBOL_GPL(cmpxchg_emu_u16);
--
2.40.1
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