1 // SPDX-License-Identifier: GPL-2.0+ 2 /* 3 * Emulated 1-byte and 2-byte cmpxchg operations for architectures lacking 4 * direct support for these sizes. These are implemented in terms of 5 * 4-byte cmpxchg operations. 6 * 7 * Copyright (C) 2024 Paul E. McKenney <paulmck@kernel.org> 8 * Copyright (C) 2026 Bradley Morgan <brads@mainlining.org> 9 */ 10 11 #include <linux/types.h> 12 #include <linux/export.h> 13 #include <linux/instrumented.h> 14 #include <linux/atomic.h> 15 #include <linux/panic.h> 16 #include <linux/bug.h> 17 #include <asm-generic/rwonce.h> 18 #include <linux/cmpxchg-emu.h> 19 20 union u8_32 { 21 u8 b[4]; 22 u32 w; 23 }; 24 25 /* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */ 26 uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new) 27 { 28 u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3); 29 int i = ((uintptr_t)p) & 0x3; 30 union u8_32 old32; 31 union u8_32 new32; 32 u32 ret; 33 34 ret = READ_ONCE(*p32); 35 do { 36 old32.w = ret; 37 if (old32.b[i] != old) 38 return old32.b[i]; 39 new32.w = old32.w; 40 new32.b[i] = new; 41 instrument_atomic_read_write(p, 1); 42 ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above. 43 } while (ret != old32.w); 44 return old; 45 } 46 EXPORT_SYMBOL_GPL(cmpxchg_emu_u8); 47 48 union u16_32 { 49 u16 h[2]; 50 u32 w; 51 }; 52 53 /* Emulate two-byte cmpxchg() in terms of 4-byte cmpxchg. */ 54 unsigned long cmpxchg_emu_u16(volatile u16 *p, unsigned long old, unsigned long new) 55 { 56 u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3); 57 int i = (((uintptr_t)p) & 0x2) / 2; 58 union u16_32 old32; 59 union u16_32 new32; 60 u32 ret; 61 62 WARN_ON_ONCE(((uintptr_t)p) & 0x1); 63 ret = READ_ONCE(*p32); 64 do { 65 old32.w = ret; 66 if (old32.h[i] != (u16)old) 67 return (unsigned long)old32.h[i]; 68 new32.w = old32.w; 69 new32.h[i] = (u16)new; 70 instrument_atomic_read_write(p, 2); 71 ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above. 72 } while (ret != old32.w); 73 return (u16)old; 74 } 75 EXPORT_SYMBOL_GPL(cmpxchg_emu_u16); 76