barrier.h (2458B)
1/* SPDX-License-Identifier: GPL-2.0-only */ 2/* 3 * Based on arch/arm/include/asm/barrier.h 4 * 5 * Copyright (C) 2012 ARM Ltd. 6 * Copyright (C) 2013 Regents of the University of California 7 * Copyright (C) 2017 SiFive 8 */ 9 10#ifndef _ASM_RISCV_BARRIER_H 11#define _ASM_RISCV_BARRIER_H 12 13#ifndef __ASSEMBLY__ 14 15#define nop() __asm__ __volatile__ ("nop") 16 17#define RISCV_FENCE(p, s) \ 18 __asm__ __volatile__ ("fence " #p "," #s : : : "memory") 19 20/* These barriers need to enforce ordering on both devices or memory. */ 21#define mb() RISCV_FENCE(iorw,iorw) 22#define rmb() RISCV_FENCE(ir,ir) 23#define wmb() RISCV_FENCE(ow,ow) 24 25/* These barriers do not need to enforce ordering on devices, just memory. */ 26#define __smp_mb() RISCV_FENCE(rw,rw) 27#define __smp_rmb() RISCV_FENCE(r,r) 28#define __smp_wmb() RISCV_FENCE(w,w) 29 30#define __smp_store_release(p, v) \ 31do { \ 32 compiletime_assert_atomic_type(*p); \ 33 RISCV_FENCE(rw,w); \ 34 WRITE_ONCE(*p, v); \ 35} while (0) 36 37#define __smp_load_acquire(p) \ 38({ \ 39 typeof(*p) ___p1 = READ_ONCE(*p); \ 40 compiletime_assert_atomic_type(*p); \ 41 RISCV_FENCE(r,rw); \ 42 ___p1; \ 43}) 44 45/* 46 * This is a very specific barrier: it's currently only used in two places in 47 * the kernel, both in the scheduler. See include/linux/spinlock.h for the two 48 * orderings it guarantees, but the "critical section is RCsc" guarantee 49 * mandates a barrier on RISC-V. The sequence looks like: 50 * 51 * lr.aq lock 52 * sc lock <= LOCKED 53 * smp_mb__after_spinlock() 54 * // critical section 55 * lr lock 56 * sc.rl lock <= UNLOCKED 57 * 58 * The AQ/RL pair provides a RCpc critical section, but there's not really any 59 * way we can take advantage of that here because the ordering is only enforced 60 * on that one lock. Thus, we're just doing a full fence. 61 * 62 * Since we allow writeX to be called from preemptive regions we need at least 63 * an "o" in the predecessor set to ensure device writes are visible before the 64 * task is marked as available for scheduling on a new hart. While I don't see 65 * any concrete reason we need a full IO fence, it seems safer to just upgrade 66 * this in order to avoid any IO crossing a scheduling boundary. In both 67 * instances the scheduler pairs this with an mb(), so nothing is necessary on 68 * the new hart. 69 */ 70#define smp_mb__after_spinlock() RISCV_FENCE(iorw,iorw) 71 72#include <asm-generic/barrier.h> 73 74#endif /* __ASSEMBLY__ */ 75 76#endif /* _ASM_RISCV_BARRIER_H */