From: "Paul E. McKenney" <paulmck@kernel.org>
To: Boqun Feng <boqun.feng@gmail.com>
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 08:41:27 -0700 [thread overview]
Message-ID: <9f0ff126-2806-488e-97cc-7258eff0c574@paulmck-laptop> (raw)
In-Reply-To: <ZkInMNOsLO5XbDj5@boqun-archlinux>
On Mon, May 13, 2024 at 07:44:00AM -0700, Boqun Feng wrote:
> 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?
That is what I thought, too. ;-)
The problem is that the cmpxchg() covers 32 bits, and so without that
data_race(), KCSAN would complain about data races with perfectly
legitimate concurrent accesses to the other three bytes.
The instrument_atomic_read_write(p, 1) beforehand tells KCSAN to complain
about concurrent accesses, but only to that one byte.
Thanx, Paul
> 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 15:41 UTC|newest]
Thread overview: 84+ 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
2024-04-08 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
2024-05-13 15:41 ` Paul E. McKenney [this message]
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
2024-06-04 17:02 ` [PATCH v3 cmpxchg 0/4] " Paul E. McKenney
2024-06-04 17:04 ` [PATCH v3 cmpxchg 1/4] ARC: Emulate one-byte cmpxchg Paul E. McKenney
2024-06-04 17:04 ` [PATCH v3 cmpxchg 2/4] sh: " Paul E. McKenney
2024-06-04 17:09 ` John Paul Adrian Glaubitz
2024-06-04 17:50 ` Paul E. McKenney
2024-06-04 17:56 ` John Paul Adrian Glaubitz
2024-06-04 21:14 ` Paul E. McKenney
2024-06-04 17:04 ` [PATCH v3 cmpxchg 3/4] xtensa: " Paul E. McKenney
2024-06-04 17:04 ` [PATCH v3 cmpxchg 4/4] ARM: " Paul E. McKenney
2024-06-04 20:52 ` Linus Walleij
2024-06-04 21:14 ` Paul E. McKenney
2024-06-05 8:38 ` Linus Walleij
2024-06-05 18:05 ` Paul E. McKenney
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