1 /* 2 * Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> 3 * 4 * membarrier system call 5 * 6 * This program is free software; you can redistribute it and/or modify 7 * it under the terms of the GNU General Public License as published by 8 * the Free Software Foundation; either version 2 of the License, or 9 * (at your option) any later version. 10 * 11 * This program is distributed in the hope that it will be useful, 12 * but WITHOUT ANY WARRANTY; without even the implied warranty of 13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 14 * GNU General Public License for more details. 15 */ 16 17 #include <linux/syscalls.h> 18 #include <linux/membarrier.h> 19 #include <linux/tick.h> 20 #include <linux/cpumask.h> 21 #include <linux/atomic.h> 22 23 #include "sched.h" /* for cpu_rq(). */ 24 25 /* 26 * Bitmask made from a "or" of all commands within enum membarrier_cmd, 27 * except MEMBARRIER_CMD_QUERY. 28 */ 29 #define MEMBARRIER_CMD_BITMASK \ 30 (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED \ 31 | MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED) 32 33 static void ipi_mb(void *info) 34 { 35 smp_mb(); /* IPIs should be serializing but paranoid. */ 36 } 37 38 static int membarrier_private_expedited(void) 39 { 40 int cpu; 41 bool fallback = false; 42 cpumask_var_t tmpmask; 43 44 if (!(atomic_read(¤t->mm->membarrier_state) 45 & MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)) 46 return -EPERM; 47 48 if (num_online_cpus() == 1) 49 return 0; 50 51 /* 52 * Matches memory barriers around rq->curr modification in 53 * scheduler. 54 */ 55 smp_mb(); /* system call entry is not a mb. */ 56 57 /* 58 * Expedited membarrier commands guarantee that they won't 59 * block, hence the GFP_NOWAIT allocation flag and fallback 60 * implementation. 61 */ 62 if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) { 63 /* Fallback for OOM. */ 64 fallback = true; 65 } 66 67 cpus_read_lock(); 68 for_each_online_cpu(cpu) { 69 struct task_struct *p; 70 71 /* 72 * Skipping the current CPU is OK even through we can be 73 * migrated at any point. The current CPU, at the point 74 * where we read raw_smp_processor_id(), is ensured to 75 * be in program order with respect to the caller 76 * thread. Therefore, we can skip this CPU from the 77 * iteration. 78 */ 79 if (cpu == raw_smp_processor_id()) 80 continue; 81 rcu_read_lock(); 82 p = task_rcu_dereference(&cpu_rq(cpu)->curr); 83 if (p && p->mm == current->mm) { 84 if (!fallback) 85 __cpumask_set_cpu(cpu, tmpmask); 86 else 87 smp_call_function_single(cpu, ipi_mb, NULL, 1); 88 } 89 rcu_read_unlock(); 90 } 91 if (!fallback) { 92 smp_call_function_many(tmpmask, ipi_mb, NULL, 1); 93 free_cpumask_var(tmpmask); 94 } 95 cpus_read_unlock(); 96 97 /* 98 * Memory barrier on the caller thread _after_ we finished 99 * waiting for the last IPI. Matches memory barriers around 100 * rq->curr modification in scheduler. 101 */ 102 smp_mb(); /* exit from system call is not a mb */ 103 return 0; 104 } 105 106 static void membarrier_register_private_expedited(void) 107 { 108 struct task_struct *p = current; 109 struct mm_struct *mm = p->mm; 110 111 /* 112 * We need to consider threads belonging to different thread 113 * groups, which use the same mm. (CLONE_VM but not 114 * CLONE_THREAD). 115 */ 116 if (atomic_read(&mm->membarrier_state) 117 & MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY) 118 return; 119 atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY, 120 &mm->membarrier_state); 121 } 122 123 /** 124 * sys_membarrier - issue memory barriers on a set of threads 125 * @cmd: Takes command values defined in enum membarrier_cmd. 126 * @flags: Currently needs to be 0. For future extensions. 127 * 128 * If this system call is not implemented, -ENOSYS is returned. If the 129 * command specified does not exist, not available on the running 130 * kernel, or if the command argument is invalid, this system call 131 * returns -EINVAL. For a given command, with flags argument set to 0, 132 * this system call is guaranteed to always return the same value until 133 * reboot. 134 * 135 * All memory accesses performed in program order from each targeted thread 136 * is guaranteed to be ordered with respect to sys_membarrier(). If we use 137 * the semantic "barrier()" to represent a compiler barrier forcing memory 138 * accesses to be performed in program order across the barrier, and 139 * smp_mb() to represent explicit memory barriers forcing full memory 140 * ordering across the barrier, we have the following ordering table for 141 * each pair of barrier(), sys_membarrier() and smp_mb(): 142 * 143 * The pair ordering is detailed as (O: ordered, X: not ordered): 144 * 145 * barrier() smp_mb() sys_membarrier() 146 * barrier() X X O 147 * smp_mb() X O O 148 * sys_membarrier() O O O 149 */ 150 SYSCALL_DEFINE2(membarrier, int, cmd, int, flags) 151 { 152 if (unlikely(flags)) 153 return -EINVAL; 154 switch (cmd) { 155 case MEMBARRIER_CMD_QUERY: 156 { 157 int cmd_mask = MEMBARRIER_CMD_BITMASK; 158 159 if (tick_nohz_full_enabled()) 160 cmd_mask &= ~MEMBARRIER_CMD_SHARED; 161 return cmd_mask; 162 } 163 case MEMBARRIER_CMD_SHARED: 164 /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */ 165 if (tick_nohz_full_enabled()) 166 return -EINVAL; 167 if (num_online_cpus() > 1) 168 synchronize_sched(); 169 return 0; 170 case MEMBARRIER_CMD_PRIVATE_EXPEDITED: 171 return membarrier_private_expedited(); 172 case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED: 173 membarrier_register_private_expedited(); 174 return 0; 175 default: 176 return -EINVAL; 177 } 178 } 179