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[103.168.172.201]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-43df56d0396sm56146551cf.86.2024.05.13.07.44.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 May 2024 07:44:10 -0700 (PDT) Received: from compute7.internal (compute7.nyi.internal [10.202.2.48]) by mailfauth.nyi.internal (Postfix) with ESMTP id 17EB11200066; Mon, 13 May 2024 10:44:10 -0400 (EDT) Received: from mailfrontend1 ([10.202.2.162]) by compute7.internal (MEProxy); Mon, 13 May 2024 10:44:10 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgedvledrvdeggedgjeelucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfqfgfvpdfurfetoffkrfgpnffqhgen uceurghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmne cujfgurhepfffhvfevuffkfhggtggujgesthdtredttddtvdenucfhrhhomhepuehoqhhu nhcuhfgvnhhguceosghoqhhunhdrfhgvnhhgsehgmhgrihhlrdgtohhmqeenucggtffrrg htthgvrhhnpefhtedvgfdtueekvdekieetieetjeeihedvteehuddujedvkedtkeefgedv vdehtdenucffohhmrghinhepkhgvrhhnvghlrdhorhhgnecuvehluhhsthgvrhfuihiivg eptdenucfrrghrrghmpehmrghilhhfrhhomhepsghoqhhunhdomhgvshhmthhprghuthhh phgvrhhsohhnrghlihhthidqieelvdeghedtieegqddujeejkeehheehvddqsghoqhhunh drfhgvnhhgpeepghhmrghilhdrtghomhesfhhigihmvgdrnhgrmhgv X-ME-Proxy: Feedback-ID: iad51458e:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Mon, 13 May 2024 10:44:09 -0400 (EDT) Date: Mon, 13 May 2024 07:44:00 -0700 From: Boqun Feng To: "Paul E. McKenney" 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 Message-ID: References: <20240501230130.1111603-9-paulmck@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 > Acked-by: Marco Elver > Cc: Andrew Morton > Cc: Thomas Gleixner > Cc: "Peter Zijlstra (Intel)" > Cc: Douglas Anderson > Cc: Petr Mladek > Cc: Arnd Bergmann > Cc: > --- > 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 > +#include > +#include > +#include > +#include > +#include > +#include > +#include > + > +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 >