1*a88d970cSPaul E. McKenney // SPDX-License-Identifier: GPL-2.0+ 2*a88d970cSPaul E. McKenney /* 3*a88d970cSPaul E. McKenney * Emulated 1-byte cmpxchg operation for architectures lacking direct 4*a88d970cSPaul E. McKenney * support for this size. This is implemented in terms of 4-byte cmpxchg 5*a88d970cSPaul E. McKenney * operations. 6*a88d970cSPaul E. McKenney * 7*a88d970cSPaul E. McKenney * Copyright (C) 2024 Paul E. McKenney. 8*a88d970cSPaul E. McKenney */ 9*a88d970cSPaul E. McKenney 10*a88d970cSPaul E. McKenney #include <linux/types.h> 11*a88d970cSPaul E. McKenney #include <linux/export.h> 12*a88d970cSPaul E. McKenney #include <linux/instrumented.h> 13*a88d970cSPaul E. McKenney #include <linux/atomic.h> 14*a88d970cSPaul E. McKenney #include <linux/panic.h> 15*a88d970cSPaul E. McKenney #include <linux/bug.h> 16*a88d970cSPaul E. McKenney #include <asm-generic/rwonce.h> 17*a88d970cSPaul E. McKenney #include <linux/cmpxchg-emu.h> 18*a88d970cSPaul E. McKenney 19*a88d970cSPaul E. McKenney union u8_32 { 20*a88d970cSPaul E. McKenney u8 b[4]; 21*a88d970cSPaul E. McKenney u32 w; 22*a88d970cSPaul E. McKenney }; 23*a88d970cSPaul E. McKenney 24*a88d970cSPaul E. McKenney /* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */ cmpxchg_emu_u8(volatile u8 * p,uintptr_t old,uintptr_t new)25*a88d970cSPaul E. McKenneyuintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new) 26*a88d970cSPaul E. McKenney { 27*a88d970cSPaul E. McKenney u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3); 28*a88d970cSPaul E. McKenney int i = ((uintptr_t)p) & 0x3; 29*a88d970cSPaul E. McKenney union u8_32 old32; 30*a88d970cSPaul E. McKenney union u8_32 new32; 31*a88d970cSPaul E. McKenney u32 ret; 32*a88d970cSPaul E. McKenney 33*a88d970cSPaul E. McKenney ret = READ_ONCE(*p32); 34*a88d970cSPaul E. McKenney do { 35*a88d970cSPaul E. McKenney old32.w = ret; 36*a88d970cSPaul E. McKenney if (old32.b[i] != old) 37*a88d970cSPaul E. McKenney return old32.b[i]; 38*a88d970cSPaul E. McKenney new32.w = old32.w; 39*a88d970cSPaul E. McKenney new32.b[i] = new; 40*a88d970cSPaul E. McKenney instrument_atomic_read_write(p, 1); 41*a88d970cSPaul E. McKenney ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above. 42*a88d970cSPaul E. McKenney } while (ret != old32.w); 43*a88d970cSPaul E. McKenney return old; 44*a88d970cSPaul E. McKenney } 45*a88d970cSPaul E. McKenney EXPORT_SYMBOL_GPL(cmpxchg_emu_u8); 46