1 #define pr_fmt(fmt) "%s: " fmt "\n", __func__ 2 3 #include <linux/kernel.h> 4 #include <linux/sched.h> 5 #include <linux/wait.h> 6 #include <linux/percpu-refcount.h> 7 8 /* 9 * Initially, a percpu refcount is just a set of percpu counters. Initially, we 10 * don't try to detect the ref hitting 0 - which means that get/put can just 11 * increment or decrement the local counter. Note that the counter on a 12 * particular cpu can (and will) wrap - this is fine, when we go to shutdown the 13 * percpu counters will all sum to the correct value 14 * 15 * (More precisely: because modular arithmetic is commutative the sum of all the 16 * percpu_count vars will be equal to what it would have been if all the gets 17 * and puts were done to a single integer, even if some of the percpu integers 18 * overflow or underflow). 19 * 20 * The real trick to implementing percpu refcounts is shutdown. We can't detect 21 * the ref hitting 0 on every put - this would require global synchronization 22 * and defeat the whole purpose of using percpu refs. 23 * 24 * What we do is require the user to keep track of the initial refcount; we know 25 * the ref can't hit 0 before the user drops the initial ref, so as long as we 26 * convert to non percpu mode before the initial ref is dropped everything 27 * works. 28 * 29 * Converting to non percpu mode is done with some RCUish stuff in 30 * percpu_ref_kill. Additionally, we need a bias value so that the 31 * atomic_long_t can't hit 0 before we've added up all the percpu refs. 32 */ 33 34 #define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1)) 35 36 static DEFINE_SPINLOCK(percpu_ref_switch_lock); 37 static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq); 38 39 static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref) 40 { 41 return (unsigned long __percpu *) 42 (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD); 43 } 44 45 /** 46 * percpu_ref_init - initialize a percpu refcount 47 * @ref: percpu_ref to initialize 48 * @release: function which will be called when refcount hits 0 49 * @flags: PERCPU_REF_INIT_* flags 50 * @gfp: allocation mask to use 51 * 52 * Initializes @ref. If @flags is zero, @ref starts in percpu mode with a 53 * refcount of 1; analagous to atomic_long_set(ref, 1). See the 54 * definitions of PERCPU_REF_INIT_* flags for flag behaviors. 55 * 56 * Note that @release must not sleep - it may potentially be called from RCU 57 * callback context by percpu_ref_kill(). 58 */ 59 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release, 60 unsigned int flags, gfp_t gfp) 61 { 62 size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS, 63 __alignof__(unsigned long)); 64 unsigned long start_count = 0; 65 66 ref->percpu_count_ptr = (unsigned long) 67 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp); 68 if (!ref->percpu_count_ptr) 69 return -ENOMEM; 70 71 ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC; 72 73 if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) 74 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC; 75 else 76 start_count += PERCPU_COUNT_BIAS; 77 78 if (flags & PERCPU_REF_INIT_DEAD) 79 ref->percpu_count_ptr |= __PERCPU_REF_DEAD; 80 else 81 start_count++; 82 83 atomic_long_set(&ref->count, start_count); 84 85 ref->release = release; 86 ref->confirm_switch = NULL; 87 return 0; 88 } 89 EXPORT_SYMBOL_GPL(percpu_ref_init); 90 91 /** 92 * percpu_ref_exit - undo percpu_ref_init() 93 * @ref: percpu_ref to exit 94 * 95 * This function exits @ref. The caller is responsible for ensuring that 96 * @ref is no longer in active use. The usual places to invoke this 97 * function from are the @ref->release() callback or in init failure path 98 * where percpu_ref_init() succeeded but other parts of the initialization 99 * of the embedding object failed. 100 */ 101 void percpu_ref_exit(struct percpu_ref *ref) 102 { 103 unsigned long __percpu *percpu_count = percpu_count_ptr(ref); 104 105 if (percpu_count) { 106 /* non-NULL confirm_switch indicates switching in progress */ 107 WARN_ON_ONCE(ref->confirm_switch); 108 free_percpu(percpu_count); 109 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD; 110 } 111 } 112 EXPORT_SYMBOL_GPL(percpu_ref_exit); 113 114 static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu) 115 { 116 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu); 117 118 ref->confirm_switch(ref); 119 ref->confirm_switch = NULL; 120 wake_up_all(&percpu_ref_switch_waitq); 121 122 /* drop ref from percpu_ref_switch_to_atomic() */ 123 percpu_ref_put(ref); 124 } 125 126 static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu) 127 { 128 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu); 129 unsigned long __percpu *percpu_count = percpu_count_ptr(ref); 130 unsigned long count = 0; 131 int cpu; 132 133 for_each_possible_cpu(cpu) 134 count += *per_cpu_ptr(percpu_count, cpu); 135 136 pr_debug("global %ld percpu %ld", 137 atomic_long_read(&ref->count), (long)count); 138 139 /* 140 * It's crucial that we sum the percpu counters _before_ adding the sum 141 * to &ref->count; since gets could be happening on one cpu while puts 142 * happen on another, adding a single cpu's count could cause 143 * @ref->count to hit 0 before we've got a consistent value - but the 144 * sum of all the counts will be consistent and correct. 145 * 146 * Subtracting the bias value then has to happen _after_ adding count to 147 * &ref->count; we need the bias value to prevent &ref->count from 148 * reaching 0 before we add the percpu counts. But doing it at the same 149 * time is equivalent and saves us atomic operations: 150 */ 151 atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count); 152 153 WARN_ONCE(atomic_long_read(&ref->count) <= 0, 154 "percpu ref (%pf) <= 0 (%ld) after switching to atomic", 155 ref->release, atomic_long_read(&ref->count)); 156 157 /* @ref is viewed as dead on all CPUs, send out switch confirmation */ 158 percpu_ref_call_confirm_rcu(rcu); 159 } 160 161 static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref) 162 { 163 } 164 165 static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref, 166 percpu_ref_func_t *confirm_switch) 167 { 168 if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) { 169 if (confirm_switch) 170 confirm_switch(ref); 171 return; 172 } 173 174 /* switching from percpu to atomic */ 175 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC; 176 177 /* 178 * Non-NULL ->confirm_switch is used to indicate that switching is 179 * in progress. Use noop one if unspecified. 180 */ 181 ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch; 182 183 percpu_ref_get(ref); /* put after confirmation */ 184 call_rcu(&ref->rcu, percpu_ref_switch_to_atomic_rcu); 185 } 186 187 static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref) 188 { 189 unsigned long __percpu *percpu_count = percpu_count_ptr(ref); 190 int cpu; 191 192 BUG_ON(!percpu_count); 193 194 if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) 195 return; 196 197 atomic_long_add(PERCPU_COUNT_BIAS, &ref->count); 198 199 /* 200 * Restore per-cpu operation. smp_store_release() is paired 201 * with READ_ONCE() in __ref_is_percpu() and guarantees that the 202 * zeroing is visible to all percpu accesses which can see the 203 * following __PERCPU_REF_ATOMIC clearing. 204 */ 205 for_each_possible_cpu(cpu) 206 *per_cpu_ptr(percpu_count, cpu) = 0; 207 208 smp_store_release(&ref->percpu_count_ptr, 209 ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC); 210 } 211 212 static void __percpu_ref_switch_mode(struct percpu_ref *ref, 213 percpu_ref_func_t *confirm_switch) 214 { 215 lockdep_assert_held(&percpu_ref_switch_lock); 216 217 /* 218 * If the previous ATOMIC switching hasn't finished yet, wait for 219 * its completion. If the caller ensures that ATOMIC switching 220 * isn't in progress, this function can be called from any context. 221 */ 222 wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch, 223 percpu_ref_switch_lock); 224 225 if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD)) 226 __percpu_ref_switch_to_atomic(ref, confirm_switch); 227 else 228 __percpu_ref_switch_to_percpu(ref); 229 } 230 231 /** 232 * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode 233 * @ref: percpu_ref to switch to atomic mode 234 * @confirm_switch: optional confirmation callback 235 * 236 * There's no reason to use this function for the usual reference counting. 237 * Use percpu_ref_kill[_and_confirm](). 238 * 239 * Schedule switching of @ref to atomic mode. All its percpu counts will 240 * be collected to the main atomic counter. On completion, when all CPUs 241 * are guaraneed to be in atomic mode, @confirm_switch, which may not 242 * block, is invoked. This function may be invoked concurrently with all 243 * the get/put operations and can safely be mixed with kill and reinit 244 * operations. Note that @ref will stay in atomic mode across kill/reinit 245 * cycles until percpu_ref_switch_to_percpu() is called. 246 * 247 * This function may block if @ref is in the process of switching to atomic 248 * mode. If the caller ensures that @ref is not in the process of 249 * switching to atomic mode, this function can be called from any context. 250 */ 251 void percpu_ref_switch_to_atomic(struct percpu_ref *ref, 252 percpu_ref_func_t *confirm_switch) 253 { 254 unsigned long flags; 255 256 spin_lock_irqsave(&percpu_ref_switch_lock, flags); 257 258 ref->force_atomic = true; 259 __percpu_ref_switch_mode(ref, confirm_switch); 260 261 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags); 262 } 263 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic); 264 265 /** 266 * percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode 267 * @ref: percpu_ref to switch to atomic mode 268 * 269 * Schedule switching the ref to atomic mode, and wait for the 270 * switch to complete. Caller must ensure that no other thread 271 * will switch back to percpu mode. 272 */ 273 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref) 274 { 275 percpu_ref_switch_to_atomic(ref, NULL); 276 wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); 277 } 278 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync); 279 280 /** 281 * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode 282 * @ref: percpu_ref to switch to percpu mode 283 * 284 * There's no reason to use this function for the usual reference counting. 285 * To re-use an expired ref, use percpu_ref_reinit(). 286 * 287 * Switch @ref to percpu mode. This function may be invoked concurrently 288 * with all the get/put operations and can safely be mixed with kill and 289 * reinit operations. This function reverses the sticky atomic state set 290 * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is 291 * dying or dead, the actual switching takes place on the following 292 * percpu_ref_reinit(). 293 * 294 * This function may block if @ref is in the process of switching to atomic 295 * mode. If the caller ensures that @ref is not in the process of 296 * switching to atomic mode, this function can be called from any context. 297 */ 298 void percpu_ref_switch_to_percpu(struct percpu_ref *ref) 299 { 300 unsigned long flags; 301 302 spin_lock_irqsave(&percpu_ref_switch_lock, flags); 303 304 ref->force_atomic = false; 305 __percpu_ref_switch_mode(ref, NULL); 306 307 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags); 308 } 309 EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu); 310 311 /** 312 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation 313 * @ref: percpu_ref to kill 314 * @confirm_kill: optional confirmation callback 315 * 316 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if 317 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be 318 * called after @ref is seen as dead from all CPUs at which point all 319 * further invocations of percpu_ref_tryget_live() will fail. See 320 * percpu_ref_tryget_live() for details. 321 * 322 * This function normally doesn't block and can be called from any context 323 * but it may block if @confirm_kill is specified and @ref is in the 324 * process of switching to atomic mode by percpu_ref_switch_to_atomic(). 325 * 326 * There are no implied RCU grace periods between kill and release. 327 */ 328 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 329 percpu_ref_func_t *confirm_kill) 330 { 331 unsigned long flags; 332 333 spin_lock_irqsave(&percpu_ref_switch_lock, flags); 334 335 WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD, 336 "%s called more than once on %pf!", __func__, ref->release); 337 338 ref->percpu_count_ptr |= __PERCPU_REF_DEAD; 339 __percpu_ref_switch_mode(ref, confirm_kill); 340 percpu_ref_put(ref); 341 342 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags); 343 } 344 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm); 345 346 /** 347 * percpu_ref_reinit - re-initialize a percpu refcount 348 * @ref: perpcu_ref to re-initialize 349 * 350 * Re-initialize @ref so that it's in the same state as when it finished 351 * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been 352 * initialized successfully and reached 0 but not exited. 353 * 354 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while 355 * this function is in progress. 356 */ 357 void percpu_ref_reinit(struct percpu_ref *ref) 358 { 359 WARN_ON_ONCE(!percpu_ref_is_zero(ref)); 360 361 percpu_ref_resurrect(ref); 362 } 363 EXPORT_SYMBOL_GPL(percpu_ref_reinit); 364 365 /** 366 * percpu_ref_resurrect - modify a percpu refcount from dead to live 367 * @ref: perpcu_ref to resurrect 368 * 369 * Modify @ref so that it's in the same state as before percpu_ref_kill() was 370 * called. @ref must be dead but must not yet have exited. 371 * 372 * If @ref->release() frees @ref then the caller is responsible for 373 * guaranteeing that @ref->release() does not get called while this 374 * function is in progress. 375 * 376 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while 377 * this function is in progress. 378 */ 379 void percpu_ref_resurrect(struct percpu_ref *ref) 380 { 381 unsigned long __percpu *percpu_count; 382 unsigned long flags; 383 384 spin_lock_irqsave(&percpu_ref_switch_lock, flags); 385 386 WARN_ON_ONCE(!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)); 387 WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count)); 388 389 ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD; 390 percpu_ref_get(ref); 391 __percpu_ref_switch_mode(ref, NULL); 392 393 spin_unlock_irqrestore(&percpu_ref_switch_lock, flags); 394 } 395 EXPORT_SYMBOL_GPL(percpu_ref_resurrect); 396