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 moduler arithmatic 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 36static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq); 37 38static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref) 39{ 40 return (unsigned long __percpu *) 41 (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD); 42} 43 44/** 45 * percpu_ref_init - initialize a percpu refcount 46 * @ref: percpu_ref to initialize 47 * @release: function which will be called when refcount hits 0 48 * @flags: PERCPU_REF_INIT_* flags 49 * @gfp: allocation mask to use 50 * 51 * Initializes @ref. If @flags is zero, @ref starts in percpu mode with a 52 * refcount of 1; analagous to atomic_long_set(ref, 1). See the 53 * definitions of PERCPU_REF_INIT_* flags for flag behaviors. 54 * 55 * Note that @release must not sleep - it may potentially be called from RCU 56 * callback context by percpu_ref_kill(). 57 */ 58int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release, 59 unsigned int flags, gfp_t gfp) 60{ 61 size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS, 62 __alignof__(unsigned long)); 63 unsigned long start_count = 0; 64 65 ref->percpu_count_ptr = (unsigned long) 66 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp); 67 if (!ref->percpu_count_ptr) 68 return -ENOMEM; 69 70 ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC; 71 72 if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) 73 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC; 74 else 75 start_count += PERCPU_COUNT_BIAS; 76 77 if (flags & PERCPU_REF_INIT_DEAD) 78 ref->percpu_count_ptr |= __PERCPU_REF_DEAD; 79 else 80 start_count++; 81 82 atomic_long_set(&ref->count, start_count); 83 84 ref->release = release; 85 return 0; 86} 87EXPORT_SYMBOL_GPL(percpu_ref_init); 88 89/** 90 * percpu_ref_exit - undo percpu_ref_init() 91 * @ref: percpu_ref to exit 92 * 93 * This function exits @ref. The caller is responsible for ensuring that 94 * @ref is no longer in active use. The usual places to invoke this 95 * function from are the @ref->release() callback or in init failure path 96 * where percpu_ref_init() succeeded but other parts of the initialization 97 * of the embedding object failed. 98 */ 99void percpu_ref_exit(struct percpu_ref *ref) 100{ 101 unsigned long __percpu *percpu_count = percpu_count_ptr(ref); 102 103 if (percpu_count) { 104 free_percpu(percpu_count); 105 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD; 106 } 107} 108EXPORT_SYMBOL_GPL(percpu_ref_exit); 109 110static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu) 111{ 112 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu); 113 114 ref->confirm_switch(ref); 115 ref->confirm_switch = NULL; 116 wake_up_all(&percpu_ref_switch_waitq); 117 118 /* drop ref from percpu_ref_switch_to_atomic() */ 119 percpu_ref_put(ref); 120} 121 122static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu) 123{ 124 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu); 125 unsigned long __percpu *percpu_count = percpu_count_ptr(ref); 126 unsigned long count = 0; 127 int cpu; 128 129 for_each_possible_cpu(cpu) 130 count += *per_cpu_ptr(percpu_count, cpu); 131 132 pr_debug("global %ld percpu %ld", 133 atomic_long_read(&ref->count), (long)count); 134 135 /* 136 * It's crucial that we sum the percpu counters _before_ adding the sum 137 * to &ref->count; since gets could be happening on one cpu while puts 138 * happen on another, adding a single cpu's count could cause 139 * @ref->count to hit 0 before we've got a consistent value - but the 140 * sum of all the counts will be consistent and correct. 141 * 142 * Subtracting the bias value then has to happen _after_ adding count to 143 * &ref->count; we need the bias value to prevent &ref->count from 144 * reaching 0 before we add the percpu counts. But doing it at the same 145 * time is equivalent and saves us atomic operations: 146 */ 147 atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count); 148 149 WARN_ONCE(atomic_long_read(&ref->count) <= 0, 150 "percpu ref (%pf) <= 0 (%ld) after switching to atomic", 151 ref->release, atomic_long_read(&ref->count)); 152 153 /* @ref is viewed as dead on all CPUs, send out switch confirmation */ 154 percpu_ref_call_confirm_rcu(rcu); 155} 156 157static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref) 158{ 159} 160 161static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref, 162 percpu_ref_func_t *confirm_switch) 163{ 164 if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) { 165 /* switching from percpu to atomic */ 166 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC; 167 168 /* 169 * Non-NULL ->confirm_switch is used to indicate that 170 * switching is in progress. Use noop one if unspecified. 171 */ 172 WARN_ON_ONCE(ref->confirm_switch); 173 ref->confirm_switch = 174 confirm_switch ?: percpu_ref_noop_confirm_switch; 175 176 percpu_ref_get(ref); /* put after confirmation */ 177 call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu); 178 } else if (confirm_switch) { 179 /* 180 * Somebody already set ATOMIC. Switching may still be in 181 * progress. @confirm_switch must be invoked after the 182 * switching is complete and a full sched RCU grace period 183 * has passed. Wait synchronously for the previous 184 * switching and schedule @confirm_switch invocation. 185 */ 186 wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); 187 ref->confirm_switch = confirm_switch; 188 189 percpu_ref_get(ref); /* put after confirmation */ 190 call_rcu_sched(&ref->rcu, percpu_ref_call_confirm_rcu); 191 } 192} 193 194/** 195 * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode 196 * @ref: percpu_ref to switch to atomic mode 197 * @confirm_switch: optional confirmation callback 198 * 199 * There's no reason to use this function for the usual reference counting. 200 * Use percpu_ref_kill[_and_confirm](). 201 * 202 * Schedule switching of @ref to atomic mode. All its percpu counts will 203 * be collected to the main atomic counter. On completion, when all CPUs 204 * are guaraneed to be in atomic mode, @confirm_switch, which may not 205 * block, is invoked. This function may be invoked concurrently with all 206 * the get/put operations and can safely be mixed with kill and reinit 207 * operations. Note that @ref will stay in atomic mode across kill/reinit 208 * cycles until percpu_ref_switch_to_percpu() is called. 209 * 210 * This function normally doesn't block and can be called from any context 211 * but it may block if @confirm_kill is specified and @ref is already in 212 * the process of switching to atomic mode. In such cases, @confirm_switch 213 * will be invoked after the switching is complete. 214 * 215 * Due to the way percpu_ref is implemented, @confirm_switch will be called 216 * after at least one full sched RCU grace period has passed but this is an 217 * implementation detail and must not be depended upon. 218 */ 219void percpu_ref_switch_to_atomic(struct percpu_ref *ref, 220 percpu_ref_func_t *confirm_switch) 221{ 222 ref->force_atomic = true; 223 __percpu_ref_switch_to_atomic(ref, confirm_switch); 224} 225 226static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref) 227{ 228 unsigned long __percpu *percpu_count = percpu_count_ptr(ref); 229 int cpu; 230 231 BUG_ON(!percpu_count); 232 233 if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) 234 return; 235 236 wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); 237 238 atomic_long_add(PERCPU_COUNT_BIAS, &ref->count); 239 240 /* 241 * Restore per-cpu operation. smp_store_release() is paired with 242 * smp_read_barrier_depends() in __ref_is_percpu() and guarantees 243 * that the zeroing is visible to all percpu accesses which can see 244 * the following __PERCPU_REF_ATOMIC clearing. 245 */ 246 for_each_possible_cpu(cpu) 247 *per_cpu_ptr(percpu_count, cpu) = 0; 248 249 smp_store_release(&ref->percpu_count_ptr, 250 ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC); 251} 252 253/** 254 * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode 255 * @ref: percpu_ref to switch to percpu mode 256 * 257 * There's no reason to use this function for the usual reference counting. 258 * To re-use an expired ref, use percpu_ref_reinit(). 259 * 260 * Switch @ref to percpu mode. This function may be invoked concurrently 261 * with all the get/put operations and can safely be mixed with kill and 262 * reinit operations. This function reverses the sticky atomic state set 263 * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is 264 * dying or dead, the actual switching takes place on the following 265 * percpu_ref_reinit(). 266 * 267 * This function normally doesn't block and can be called from any context 268 * but it may block if @ref is in the process of switching to atomic mode 269 * by percpu_ref_switch_atomic(). 270 */ 271void percpu_ref_switch_to_percpu(struct percpu_ref *ref) 272{ 273 ref->force_atomic = false; 274 275 /* a dying or dead ref can't be switched to percpu mode w/o reinit */ 276 if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) 277 __percpu_ref_switch_to_percpu(ref); 278} 279 280/** 281 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation 282 * @ref: percpu_ref to kill 283 * @confirm_kill: optional confirmation callback 284 * 285 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if 286 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be 287 * called after @ref is seen as dead from all CPUs at which point all 288 * further invocations of percpu_ref_tryget_live() will fail. See 289 * percpu_ref_tryget_live() for details. 290 * 291 * This function normally doesn't block and can be called from any context 292 * but it may block if @confirm_kill is specified and @ref is in the 293 * process of switching to atomic mode by percpu_ref_switch_atomic(). 294 * 295 * Due to the way percpu_ref is implemented, @confirm_switch will be called 296 * after at least one full sched RCU grace period has passed but this is an 297 * implementation detail and must not be depended upon. 298 */ 299void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 300 percpu_ref_func_t *confirm_kill) 301{ 302 WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD, 303 "%s called more than once on %pf!", __func__, ref->release); 304 305 ref->percpu_count_ptr |= __PERCPU_REF_DEAD; 306 __percpu_ref_switch_to_atomic(ref, confirm_kill); 307 percpu_ref_put(ref); 308} 309EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm); 310 311/** 312 * percpu_ref_reinit - re-initialize a percpu refcount 313 * @ref: perpcu_ref to re-initialize 314 * 315 * Re-initialize @ref so that it's in the same state as when it finished 316 * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been 317 * initialized successfully and reached 0 but not exited. 318 * 319 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while 320 * this function is in progress. 321 */ 322void percpu_ref_reinit(struct percpu_ref *ref) 323{ 324 WARN_ON_ONCE(!percpu_ref_is_zero(ref)); 325 326 ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD; 327 percpu_ref_get(ref); 328 if (!ref->force_atomic) 329 __percpu_ref_switch_to_percpu(ref); 330} 331EXPORT_SYMBOL_GPL(percpu_ref_reinit); 332