On Mon, 1 Apr 2024 at 23:39, Paul E. McKenney <paulmck@xxxxxxxxxx> 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 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@xxxxxxxxxx> > Cc: 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: <linux-arch@xxxxxxxxxxxxxxx> Acked-by: Marco Elver <elver@xxxxxxxxxx> > --- > 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 >
