Commit a88d970c authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

lib: Add one-byte emulation function 

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: default avatarPaul E. McKenney <paulmck@kernel.org>
Acked-by: default avatarMarco 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>
parent d428032b

arch/Kconfig

+3 −0
Original line number Diff line number Diff line
@@ -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

include/linux/cmpxchg-emu.h

0 → 100644
+15 −0
Original line number Diff line number Diff line 
/* 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 */

lib/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -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

lib/cmpxchg-emu.c

0 → 100644
+45 −0
Original line number Diff line number Diff line 
// 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.

	} while (ret != old32.w);

	return old;

}

EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);