1/* 2 * Sleepable Read-Copy Update mechanism for mutual exclusion. 3 * 4 * This program is free software; you can redistribute it and/or modify 5 * it under the terms of the GNU General Public License as published by 6 * the Free Software Foundation; either version 2 of the License, or 7 * (at your option) any later version. 8 * 9 * This program is distributed in the hope that it will be useful, 10 * but WITHOUT ANY WARRANTY; without even the implied warranty of 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 12 * GNU General Public License for more details. 13 * 14 * You should have received a copy of the GNU General Public License 15 * along with this program; if not, write to the Free Software 16 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. 17 * 18 * Copyright (C) IBM Corporation, 2006 19 * 20 * Author: Paul McKenney <paulmck@us.ibm.com> 21 * 22 * For detailed explanation of Read-Copy Update mechanism see - 23 * Documentation/RCU/ *.txt 24 * 25 */ 26 27#include <linux/module.h> 28#include <linux/mutex.h> 29#include <linux/percpu.h> 30#include <linux/preempt.h> 31#include <linux/rcupdate.h> 32#include <linux/sched.h> 33#include <linux/smp.h> 34#include <linux/srcu.h> 35 36static int init_srcu_struct_fields(struct srcu_struct *sp) 37{ 38 sp->completed = 0; 39 mutex_init(&sp->mutex); 40 sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array); 41 return sp->per_cpu_ref ? 0 : -ENOMEM; 42} 43 44#ifdef CONFIG_DEBUG_LOCK_ALLOC 45 46int __init_srcu_struct(struct srcu_struct *sp, const char *name, 47 struct lock_class_key *key) 48{ 49#ifdef CONFIG_DEBUG_LOCK_ALLOC 50 /* Don't re-initialize a lock while it is held. */ 51 debug_check_no_locks_freed((void *)sp, sizeof(*sp)); 52 lockdep_init_map(&sp->dep_map, name, key, 0); 53#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ 54 return init_srcu_struct_fields(sp); 55} 56EXPORT_SYMBOL_GPL(__init_srcu_struct); 57 58#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ 59 60/** 61 * init_srcu_struct - initialize a sleep-RCU structure 62 * @sp: structure to initialize. 63 * 64 * Must invoke this on a given srcu_struct before passing that srcu_struct 65 * to any other function. Each srcu_struct represents a separate domain 66 * of SRCU protection. 67 */ 68int init_srcu_struct(struct srcu_struct *sp) 69{ 70 return init_srcu_struct_fields(sp); 71} 72EXPORT_SYMBOL_GPL(init_srcu_struct); 73 74#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */ 75 76/* 77 * srcu_readers_active_idx -- returns approximate number of readers 78 * active on the specified rank of per-CPU counters. 79 */ 80 81static int srcu_readers_active_idx(struct srcu_struct *sp, int idx) 82{ 83 int cpu; 84 int sum; 85 86 sum = 0; 87 for_each_possible_cpu(cpu) 88 sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]; 89 return sum; 90} 91 92/** 93 * srcu_readers_active - returns approximate number of readers. 94 * @sp: which srcu_struct to count active readers (holding srcu_read_lock). 95 * 96 * Note that this is not an atomic primitive, and can therefore suffer 97 * severe errors when invoked on an active srcu_struct. That said, it 98 * can be useful as an error check at cleanup time. 99 */ 100static int srcu_readers_active(struct srcu_struct *sp) 101{ 102 return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1); 103} 104 105/** 106 * cleanup_srcu_struct - deconstruct a sleep-RCU structure 107 * @sp: structure to clean up. 108 * 109 * Must invoke this after you are finished using a given srcu_struct that 110 * was initialized via init_srcu_struct(), else you leak memory. 111 */ 112void cleanup_srcu_struct(struct srcu_struct *sp) 113{ 114 int sum; 115 116 sum = srcu_readers_active(sp); 117 WARN_ON(sum); /* Leakage unless caller handles error. */ 118 if (sum != 0) 119 return; 120 free_percpu(sp->per_cpu_ref); 121 sp->per_cpu_ref = NULL; 122} 123EXPORT_SYMBOL_GPL(cleanup_srcu_struct); 124 125/* 126 * Counts the new reader in the appropriate per-CPU element of the 127 * srcu_struct. Must be called from process context. 128 * Returns an index that must be passed to the matching srcu_read_unlock(). 129 */ 130int __srcu_read_lock(struct srcu_struct *sp) 131{ 132 int idx; 133 134 preempt_disable(); 135 idx = sp->completed & 0x1; 136 barrier(); /* ensure compiler looks -once- at sp->completed. */ 137 per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++; 138 srcu_barrier(); /* ensure compiler won't misorder critical section. */ 139 preempt_enable(); 140 return idx; 141} 142EXPORT_SYMBOL_GPL(__srcu_read_lock); 143 144/* 145 * Removes the count for the old reader from the appropriate per-CPU 146 * element of the srcu_struct. Note that this may well be a different 147 * CPU than that which was incremented by the corresponding srcu_read_lock(). 148 * Must be called from process context. 149 */ 150void __srcu_read_unlock(struct srcu_struct *sp, int idx) 151{ 152 preempt_disable(); 153 srcu_barrier(); /* ensure compiler won't misorder critical section. */ 154 per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--; 155 preempt_enable(); 156} 157EXPORT_SYMBOL_GPL(__srcu_read_unlock); 158 159/* 160 * Helper function for synchronize_srcu() and synchronize_srcu_expedited(). 161 */ 162static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void)) 163{ 164 int idx; 165 166 idx = sp->completed; 167 mutex_lock(&sp->mutex); 168 169 /* 170 * Check to see if someone else did the work for us while we were 171 * waiting to acquire the lock. We need -two- advances of 172 * the counter, not just one. If there was but one, we might have 173 * shown up -after- our helper's first synchronize_sched(), thus 174 * having failed to prevent CPU-reordering races with concurrent 175 * srcu_read_unlock()s on other CPUs (see comment below). So we 176 * either (1) wait for two or (2) supply the second ourselves. 177 */ 178 179 if ((sp->completed - idx) >= 2) { 180 mutex_unlock(&sp->mutex); 181 return; 182 } 183 184 sync_func(); /* Force memory barrier on all CPUs. */ 185 186 /* 187 * The preceding synchronize_sched() ensures that any CPU that 188 * sees the new value of sp->completed will also see any preceding 189 * changes to data structures made by this CPU. This prevents 190 * some other CPU from reordering the accesses in its SRCU 191 * read-side critical section to precede the corresponding 192 * srcu_read_lock() -- ensuring that such references will in 193 * fact be protected. 194 * 195 * So it is now safe to do the flip. 196 */ 197 198 idx = sp->completed & 0x1; 199 sp->completed++; 200 201 sync_func(); /* Force memory barrier on all CPUs. */ 202 203 /* 204 * At this point, because of the preceding synchronize_sched(), 205 * all srcu_read_lock() calls using the old counters have completed. 206 * Their corresponding critical sections might well be still 207 * executing, but the srcu_read_lock() primitives themselves 208 * will have finished executing. 209 */ 210 211 while (srcu_readers_active_idx(sp, idx)) 212 schedule_timeout_interruptible(1); 213 214 sync_func(); /* Force memory barrier on all CPUs. */ 215 216 /* 217 * The preceding synchronize_sched() forces all srcu_read_unlock() 218 * primitives that were executing concurrently with the preceding 219 * for_each_possible_cpu() loop to have completed by this point. 220 * More importantly, it also forces the corresponding SRCU read-side 221 * critical sections to have also completed, and the corresponding 222 * references to SRCU-protected data items to be dropped. 223 * 224 * Note: 225 * 226 * Despite what you might think at first glance, the 227 * preceding synchronize_sched() -must- be within the 228 * critical section ended by the following mutex_unlock(). 229 * Otherwise, a task taking the early exit can race 230 * with a srcu_read_unlock(), which might have executed 231 * just before the preceding srcu_readers_active() check, 232 * and whose CPU might have reordered the srcu_read_unlock() 233 * with the preceding critical section. In this case, there 234 * is nothing preventing the synchronize_sched() task that is 235 * taking the early exit from freeing a data structure that 236 * is still being referenced (out of order) by the task 237 * doing the srcu_read_unlock(). 238 * 239 * Alternatively, the comparison with "2" on the early exit 240 * could be changed to "3", but this increases synchronize_srcu() 241 * latency for bulk loads. So the current code is preferred. 242 */ 243 244 mutex_unlock(&sp->mutex); 245} 246 247/** 248 * synchronize_srcu - wait for prior SRCU read-side critical-section completion 249 * @sp: srcu_struct with which to synchronize. 250 * 251 * Flip the completed counter, and wait for the old count to drain to zero. 252 * As with classic RCU, the updater must use some separate means of 253 * synchronizing concurrent updates. Can block; must be called from 254 * process context. 255 * 256 * Note that it is illegal to call synchronize_srcu() from the corresponding 257 * SRCU read-side critical section; doing so will result in deadlock. 258 * However, it is perfectly legal to call synchronize_srcu() on one 259 * srcu_struct from some other srcu_struct's read-side critical section. 260 */ 261void synchronize_srcu(struct srcu_struct *sp) 262{ 263 __synchronize_srcu(sp, synchronize_sched); 264} 265EXPORT_SYMBOL_GPL(synchronize_srcu); 266 267/** 268 * synchronize_srcu_expedited - like synchronize_srcu, but less patient 269 * @sp: srcu_struct with which to synchronize. 270 * 271 * Flip the completed counter, and wait for the old count to drain to zero. 272 * As with classic RCU, the updater must use some separate means of 273 * synchronizing concurrent updates. Can block; must be called from 274 * process context. 275 * 276 * Note that it is illegal to call synchronize_srcu_expedited() 277 * from the corresponding SRCU read-side critical section; doing so 278 * will result in deadlock. However, it is perfectly legal to call 279 * synchronize_srcu_expedited() on one srcu_struct from some other 280 * srcu_struct's read-side critical section. 281 */ 282void synchronize_srcu_expedited(struct srcu_struct *sp) 283{ 284 __synchronize_srcu(sp, synchronize_sched_expedited); 285} 286EXPORT_SYMBOL_GPL(synchronize_srcu_expedited); 287 288/** 289 * srcu_batches_completed - return batches completed. 290 * @sp: srcu_struct on which to report batch completion. 291 * 292 * Report the number of batches, correlated with, but not necessarily 293 * precisely the same as, the number of grace periods that have elapsed. 294 */ 295 296long srcu_batches_completed(struct srcu_struct *sp) 297{ 298 return sp->completed; 299} 300EXPORT_SYMBOL_GPL(srcu_batches_completed); 301