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1// SPDX-License-Identifier: GPL-2.0+2/*3 * Emulated 1-byte cmpxchg operation for architectures lacking direct4 * support for this size. This is implemented in terms of 4-byte cmpxchg5 * operations.6 *7 * Copyright (C) 2024 Paul E. McKenney.8 */9 10#include <linux/types.h>11#include <linux/export.h>12#include <linux/instrumented.h>13#include <linux/atomic.h>14#include <linux/panic.h>15#include <linux/bug.h>16#include <asm-generic/rwonce.h>17#include <linux/cmpxchg-emu.h>18 19union u8_32 {20 u8 b[4];21 u32 w;22};23 24/* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */25uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)26{27 u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);28 int i = ((uintptr_t)p) & 0x3;29 union u8_32 old32;30 union u8_32 new32;31 u32 ret;32 33 ret = READ_ONCE(*p32);34 do {35 old32.w = ret;36 if (old32.b[i] != old)37 return old32.b[i];38 new32.w = old32.w;39 new32.b[i] = new;40 instrument_atomic_read_write(p, 1);41 ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.42 } while (ret != old32.w);43 return old;44}45EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);46