1 // SPDX-License-Identifier: GPL-2.0+ 2 /* 3 * Emulated 1-byte cmpxchg operation for architectures lacking direct 4 * support for this size. This is implemented in terms of 4-byte cmpxchg 5 * 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 19 union u8_32 { 20 u8 b[4]; 21 u32 w; 22 }; 23 24 /* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */ 25 uintptr_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 } 45 EXPORT_SYMBOL_GPL(cmpxchg_emu_u8); 46