// 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);