1 /* SPDX-License-Identifier: GPL-2.0 */ 2 /* 3 * Percpu refcounts: 4 * (C) 2012 Google, Inc. 5 * Author: Kent Overstreet <koverstreet@google.com> 6 * 7 * This implements a refcount with similar semantics to atomic_t - atomic_inc(), 8 * atomic_dec_and_test() - but percpu. 9 * 10 * There's one important difference between percpu refs and normal atomic_t 11 * refcounts; you have to keep track of your initial refcount, and then when you 12 * start shutting down you call percpu_ref_kill() _before_ dropping the initial 13 * refcount. 14 * 15 * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less 16 * than an atomic_t - this is because of the way shutdown works, see 17 * percpu_ref_kill()/PERCPU_COUNT_BIAS. 18 * 19 * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the 20 * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill() 21 * puts the ref back in single atomic_t mode, collecting the per cpu refs and 22 * issuing the appropriate barriers, and then marks the ref as shutting down so 23 * that percpu_ref_put() will check for the ref hitting 0. After it returns, 24 * it's safe to drop the initial ref. 25 * 26 * USAGE: 27 * 28 * See fs/aio.c for some example usage; it's used there for struct kioctx, which 29 * is created when userspaces calls io_setup(), and destroyed when userspace 30 * calls io_destroy() or the process exits. 31 * 32 * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it 33 * removes the kioctx from the proccess's table of kioctxs and kills percpu_ref. 34 * After that, there can't be any new users of the kioctx (from lookup_ioctx()) 35 * and it's then safe to drop the initial ref with percpu_ref_put(). 36 * 37 * Note that the free path, free_ioctx(), needs to go through explicit call_rcu() 38 * to synchronize with RCU protected lookup_ioctx(). percpu_ref operations don't 39 * imply RCU grace periods of any kind and if a user wants to combine percpu_ref 40 * with RCU protection, it must be done explicitly. 41 * 42 * Code that does a two stage shutdown like this often needs some kind of 43 * explicit synchronization to ensure the initial refcount can only be dropped 44 * once - percpu_ref_kill() does this for you, it returns true once and false if 45 * someone else already called it. The aio code uses it this way, but it's not 46 * necessary if the code has some other mechanism to synchronize teardown. 47 * around. 48 */ 49 50 #ifndef _LINUX_PERCPU_REFCOUNT_H 51 #define _LINUX_PERCPU_REFCOUNT_H 52 53 #include <linux/atomic.h> 54 #include <linux/kernel.h> 55 #include <linux/percpu.h> 56 #include <linux/rcupdate.h> 57 #include <linux/gfp.h> 58 59 struct percpu_ref; 60 typedef void (percpu_ref_func_t)(struct percpu_ref *); 61 62 /* flags set in the lower bits of percpu_ref->percpu_count_ptr */ 63 enum { 64 __PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */ 65 __PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */ 66 __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD, 67 68 __PERCPU_REF_FLAG_BITS = 2, 69 }; 70 71 /* @flags for percpu_ref_init() */ 72 enum { 73 /* 74 * Start w/ ref == 1 in atomic mode. Can be switched to percpu 75 * operation using percpu_ref_switch_to_percpu(). If initialized 76 * with this flag, the ref will stay in atomic mode until 77 * percpu_ref_switch_to_percpu() is invoked on it. 78 * Implies ALLOW_REINIT. 79 */ 80 PERCPU_REF_INIT_ATOMIC = 1 << 0, 81 82 /* 83 * Start dead w/ ref == 0 in atomic mode. Must be revived with 84 * percpu_ref_reinit() before used. Implies INIT_ATOMIC and 85 * ALLOW_REINIT. 86 */ 87 PERCPU_REF_INIT_DEAD = 1 << 1, 88 89 /* 90 * Allow switching from atomic mode to percpu mode. 91 */ 92 PERCPU_REF_ALLOW_REINIT = 1 << 2, 93 }; 94 95 struct percpu_ref_data { 96 atomic_long_t count; 97 percpu_ref_func_t *release; 98 percpu_ref_func_t *confirm_switch; 99 bool force_atomic:1; 100 bool allow_reinit:1; 101 struct rcu_head rcu; 102 struct percpu_ref *ref; 103 }; 104 105 struct percpu_ref { 106 /* 107 * The low bit of the pointer indicates whether the ref is in percpu 108 * mode; if set, then get/put will manipulate the atomic_t. 109 */ 110 unsigned long percpu_count_ptr; 111 112 /* 113 * 'percpu_ref' is often embedded into user structure, and only 114 * 'percpu_count_ptr' is required in fast path, move other fields 115 * into 'percpu_ref_data', so we can reduce memory footprint in 116 * fast path. 117 */ 118 struct percpu_ref_data *data; 119 }; 120 121 int __must_check percpu_ref_init(struct percpu_ref *ref, 122 percpu_ref_func_t *release, unsigned int flags, 123 gfp_t gfp); 124 void percpu_ref_exit(struct percpu_ref *ref); 125 void percpu_ref_switch_to_atomic(struct percpu_ref *ref, 126 percpu_ref_func_t *confirm_switch); 127 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref); 128 void percpu_ref_switch_to_percpu(struct percpu_ref *ref); 129 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 130 percpu_ref_func_t *confirm_kill); 131 void percpu_ref_resurrect(struct percpu_ref *ref); 132 void percpu_ref_reinit(struct percpu_ref *ref); 133 bool percpu_ref_is_zero(struct percpu_ref *ref); 134 135 /** 136 * percpu_ref_kill - drop the initial ref 137 * @ref: percpu_ref to kill 138 * 139 * Must be used to drop the initial ref on a percpu refcount; must be called 140 * precisely once before shutdown. 141 * 142 * Switches @ref into atomic mode before gathering up the percpu counters 143 * and dropping the initial ref. 144 * 145 * There are no implied RCU grace periods between kill and release. 146 */ 147 static inline void percpu_ref_kill(struct percpu_ref *ref) 148 { 149 percpu_ref_kill_and_confirm(ref, NULL); 150 } 151 152 /* 153 * Internal helper. Don't use outside percpu-refcount proper. The 154 * function doesn't return the pointer and let the caller test it for NULL 155 * because doing so forces the compiler to generate two conditional 156 * branches as it can't assume that @ref->percpu_count is not NULL. 157 */ 158 static inline bool __ref_is_percpu(struct percpu_ref *ref, 159 unsigned long __percpu **percpu_countp) 160 { 161 unsigned long percpu_ptr; 162 163 /* 164 * The value of @ref->percpu_count_ptr is tested for 165 * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then 166 * used as a pointer. If the compiler generates a separate fetch 167 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in 168 * between contaminating the pointer value, meaning that 169 * READ_ONCE() is required when fetching it. 170 * 171 * The dependency ordering from the READ_ONCE() pairs 172 * with smp_store_release() in __percpu_ref_switch_to_percpu(). 173 */ 174 percpu_ptr = READ_ONCE(ref->percpu_count_ptr); 175 176 /* 177 * Theoretically, the following could test just ATOMIC; however, 178 * then we'd have to mask off DEAD separately as DEAD may be 179 * visible without ATOMIC if we race with percpu_ref_kill(). DEAD 180 * implies ATOMIC anyway. Test them together. 181 */ 182 if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD)) 183 return false; 184 185 *percpu_countp = (unsigned long __percpu *)percpu_ptr; 186 return true; 187 } 188 189 /** 190 * percpu_ref_get_many - increment a percpu refcount 191 * @ref: percpu_ref to get 192 * @nr: number of references to get 193 * 194 * Analogous to atomic_long_add(). 195 * 196 * This function is safe to call as long as @ref is between init and exit. 197 */ 198 static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr) 199 { 200 unsigned long __percpu *percpu_count; 201 202 rcu_read_lock(); 203 204 if (__ref_is_percpu(ref, &percpu_count)) 205 this_cpu_add(*percpu_count, nr); 206 else 207 atomic_long_add(nr, &ref->data->count); 208 209 rcu_read_unlock(); 210 } 211 212 /** 213 * percpu_ref_get - increment a percpu refcount 214 * @ref: percpu_ref to get 215 * 216 * Analogous to atomic_long_inc(). 217 * 218 * This function is safe to call as long as @ref is between init and exit. 219 */ 220 static inline void percpu_ref_get(struct percpu_ref *ref) 221 { 222 percpu_ref_get_many(ref, 1); 223 } 224 225 /** 226 * percpu_ref_tryget_many - try to increment a percpu refcount 227 * @ref: percpu_ref to try-get 228 * @nr: number of references to get 229 * 230 * Increment a percpu refcount by @nr unless its count already reached zero. 231 * Returns %true on success; %false on failure. 232 * 233 * This function is safe to call as long as @ref is between init and exit. 234 */ 235 static inline bool percpu_ref_tryget_many(struct percpu_ref *ref, 236 unsigned long nr) 237 { 238 unsigned long __percpu *percpu_count; 239 bool ret; 240 241 rcu_read_lock(); 242 243 if (__ref_is_percpu(ref, &percpu_count)) { 244 this_cpu_add(*percpu_count, nr); 245 ret = true; 246 } else { 247 ret = atomic_long_add_unless(&ref->data->count, nr, 0); 248 } 249 250 rcu_read_unlock(); 251 252 return ret; 253 } 254 255 /** 256 * percpu_ref_tryget - try to increment a percpu refcount 257 * @ref: percpu_ref to try-get 258 * 259 * Increment a percpu refcount unless its count already reached zero. 260 * Returns %true on success; %false on failure. 261 * 262 * This function is safe to call as long as @ref is between init and exit. 263 */ 264 static inline bool percpu_ref_tryget(struct percpu_ref *ref) 265 { 266 return percpu_ref_tryget_many(ref, 1); 267 } 268 269 /** 270 * percpu_ref_tryget_live_rcu - same as percpu_ref_tryget_live() but the 271 * caller is responsible for taking RCU. 272 * 273 * This function is safe to call as long as @ref is between init and exit. 274 */ 275 static inline bool percpu_ref_tryget_live_rcu(struct percpu_ref *ref) 276 { 277 unsigned long __percpu *percpu_count; 278 bool ret = false; 279 280 WARN_ON_ONCE(!rcu_read_lock_held()); 281 282 if (likely(__ref_is_percpu(ref, &percpu_count))) { 283 this_cpu_inc(*percpu_count); 284 ret = true; 285 } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) { 286 ret = atomic_long_inc_not_zero(&ref->data->count); 287 } 288 return ret; 289 } 290 291 /** 292 * percpu_ref_tryget_live - try to increment a live percpu refcount 293 * @ref: percpu_ref to try-get 294 * 295 * Increment a percpu refcount unless it has already been killed. Returns 296 * %true on success; %false on failure. 297 * 298 * Completion of percpu_ref_kill() in itself doesn't guarantee that this 299 * function will fail. For such guarantee, percpu_ref_kill_and_confirm() 300 * should be used. After the confirm_kill callback is invoked, it's 301 * guaranteed that no new reference will be given out by 302 * percpu_ref_tryget_live(). 303 * 304 * This function is safe to call as long as @ref is between init and exit. 305 */ 306 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) 307 { 308 bool ret = false; 309 310 rcu_read_lock(); 311 ret = percpu_ref_tryget_live_rcu(ref); 312 rcu_read_unlock(); 313 return ret; 314 } 315 316 /** 317 * percpu_ref_put_many - decrement a percpu refcount 318 * @ref: percpu_ref to put 319 * @nr: number of references to put 320 * 321 * Decrement the refcount, and if 0, call the release function (which was passed 322 * to percpu_ref_init()) 323 * 324 * This function is safe to call as long as @ref is between init and exit. 325 */ 326 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr) 327 { 328 unsigned long __percpu *percpu_count; 329 330 rcu_read_lock(); 331 332 if (__ref_is_percpu(ref, &percpu_count)) 333 this_cpu_sub(*percpu_count, nr); 334 else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count))) 335 ref->data->release(ref); 336 337 rcu_read_unlock(); 338 } 339 340 /** 341 * percpu_ref_put - decrement a percpu refcount 342 * @ref: percpu_ref to put 343 * 344 * Decrement the refcount, and if 0, call the release function (which was passed 345 * to percpu_ref_init()) 346 * 347 * This function is safe to call as long as @ref is between init and exit. 348 */ 349 static inline void percpu_ref_put(struct percpu_ref *ref) 350 { 351 percpu_ref_put_many(ref, 1); 352 } 353 354 /** 355 * percpu_ref_is_dying - test whether a percpu refcount is dying or dead 356 * @ref: percpu_ref to test 357 * 358 * Returns %true if @ref is dying or dead. 359 * 360 * This function is safe to call as long as @ref is between init and exit 361 * and the caller is responsible for synchronizing against state changes. 362 */ 363 static inline bool percpu_ref_is_dying(struct percpu_ref *ref) 364 { 365 return ref->percpu_count_ptr & __PERCPU_REF_DEAD; 366 } 367 368 #endif 369