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 { 96 atomic_long_t count; 97 /* 98 * The low bit of the pointer indicates whether the ref is in percpu 99 * mode; if set, then get/put will manipulate the atomic_t. 100 */ 101 unsigned long percpu_count_ptr; 102 percpu_ref_func_t *release; 103 percpu_ref_func_t *confirm_switch; 104 bool force_atomic:1; 105 bool allow_reinit:1; 106 struct rcu_head rcu; 107 }; 108 109 int __must_check percpu_ref_init(struct percpu_ref *ref, 110 percpu_ref_func_t *release, unsigned int flags, 111 gfp_t gfp); 112 void percpu_ref_exit(struct percpu_ref *ref); 113 void percpu_ref_switch_to_atomic(struct percpu_ref *ref, 114 percpu_ref_func_t *confirm_switch); 115 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref); 116 void percpu_ref_switch_to_percpu(struct percpu_ref *ref); 117 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 118 percpu_ref_func_t *confirm_kill); 119 void percpu_ref_resurrect(struct percpu_ref *ref); 120 void percpu_ref_reinit(struct percpu_ref *ref); 121 122 /** 123 * percpu_ref_kill - drop the initial ref 124 * @ref: percpu_ref to kill 125 * 126 * Must be used to drop the initial ref on a percpu refcount; must be called 127 * precisely once before shutdown. 128 * 129 * Switches @ref into atomic mode before gathering up the percpu counters 130 * and dropping the initial ref. 131 * 132 * There are no implied RCU grace periods between kill and release. 133 */ 134 static inline void percpu_ref_kill(struct percpu_ref *ref) 135 { 136 percpu_ref_kill_and_confirm(ref, NULL); 137 } 138 139 /* 140 * Internal helper. Don't use outside percpu-refcount proper. The 141 * function doesn't return the pointer and let the caller test it for NULL 142 * because doing so forces the compiler to generate two conditional 143 * branches as it can't assume that @ref->percpu_count is not NULL. 144 */ 145 static inline bool __ref_is_percpu(struct percpu_ref *ref, 146 unsigned long __percpu **percpu_countp) 147 { 148 unsigned long percpu_ptr; 149 150 /* 151 * The value of @ref->percpu_count_ptr is tested for 152 * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then 153 * used as a pointer. If the compiler generates a separate fetch 154 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in 155 * between contaminating the pointer value, meaning that 156 * READ_ONCE() is required when fetching it. 157 * 158 * The smp_read_barrier_depends() implied by READ_ONCE() pairs 159 * with smp_store_release() in __percpu_ref_switch_to_percpu(). 160 */ 161 percpu_ptr = READ_ONCE(ref->percpu_count_ptr); 162 163 /* 164 * Theoretically, the following could test just ATOMIC; however, 165 * then we'd have to mask off DEAD separately as DEAD may be 166 * visible without ATOMIC if we race with percpu_ref_kill(). DEAD 167 * implies ATOMIC anyway. Test them together. 168 */ 169 if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD)) 170 return false; 171 172 *percpu_countp = (unsigned long __percpu *)percpu_ptr; 173 return true; 174 } 175 176 /** 177 * percpu_ref_get_many - increment a percpu refcount 178 * @ref: percpu_ref to get 179 * @nr: number of references to get 180 * 181 * Analogous to atomic_long_add(). 182 * 183 * This function is safe to call as long as @ref is between init and exit. 184 */ 185 static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr) 186 { 187 unsigned long __percpu *percpu_count; 188 189 rcu_read_lock(); 190 191 if (__ref_is_percpu(ref, &percpu_count)) 192 this_cpu_add(*percpu_count, nr); 193 else 194 atomic_long_add(nr, &ref->count); 195 196 rcu_read_unlock(); 197 } 198 199 /** 200 * percpu_ref_get - increment a percpu refcount 201 * @ref: percpu_ref to get 202 * 203 * Analagous to atomic_long_inc(). 204 * 205 * This function is safe to call as long as @ref is between init and exit. 206 */ 207 static inline void percpu_ref_get(struct percpu_ref *ref) 208 { 209 percpu_ref_get_many(ref, 1); 210 } 211 212 /** 213 * percpu_ref_tryget_many - try to increment a percpu refcount 214 * @ref: percpu_ref to try-get 215 * @nr: number of references to get 216 * 217 * Increment a percpu refcount by @nr unless its count already reached zero. 218 * Returns %true on success; %false on failure. 219 * 220 * This function is safe to call as long as @ref is between init and exit. 221 */ 222 static inline bool percpu_ref_tryget_many(struct percpu_ref *ref, 223 unsigned long nr) 224 { 225 unsigned long __percpu *percpu_count; 226 bool ret; 227 228 rcu_read_lock(); 229 230 if (__ref_is_percpu(ref, &percpu_count)) { 231 this_cpu_add(*percpu_count, nr); 232 ret = true; 233 } else { 234 ret = atomic_long_add_unless(&ref->count, nr, 0); 235 } 236 237 rcu_read_unlock(); 238 239 return ret; 240 } 241 242 /** 243 * percpu_ref_tryget - try to increment a percpu refcount 244 * @ref: percpu_ref to try-get 245 * 246 * Increment a percpu refcount unless its count already reached zero. 247 * Returns %true on success; %false on failure. 248 * 249 * This function is safe to call as long as @ref is between init and exit. 250 */ 251 static inline bool percpu_ref_tryget(struct percpu_ref *ref) 252 { 253 return percpu_ref_tryget_many(ref, 1); 254 } 255 256 /** 257 * percpu_ref_tryget_live - try to increment a live percpu refcount 258 * @ref: percpu_ref to try-get 259 * 260 * Increment a percpu refcount unless it has already been killed. Returns 261 * %true on success; %false on failure. 262 * 263 * Completion of percpu_ref_kill() in itself doesn't guarantee that this 264 * function will fail. For such guarantee, percpu_ref_kill_and_confirm() 265 * should be used. After the confirm_kill callback is invoked, it's 266 * guaranteed that no new reference will be given out by 267 * percpu_ref_tryget_live(). 268 * 269 * This function is safe to call as long as @ref is between init and exit. 270 */ 271 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) 272 { 273 unsigned long __percpu *percpu_count; 274 bool ret = false; 275 276 rcu_read_lock(); 277 278 if (__ref_is_percpu(ref, &percpu_count)) { 279 this_cpu_inc(*percpu_count); 280 ret = true; 281 } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) { 282 ret = atomic_long_inc_not_zero(&ref->count); 283 } 284 285 rcu_read_unlock(); 286 287 return ret; 288 } 289 290 /** 291 * percpu_ref_put_many - decrement a percpu refcount 292 * @ref: percpu_ref to put 293 * @nr: number of references to put 294 * 295 * Decrement the refcount, and if 0, call the release function (which was passed 296 * to percpu_ref_init()) 297 * 298 * This function is safe to call as long as @ref is between init and exit. 299 */ 300 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr) 301 { 302 unsigned long __percpu *percpu_count; 303 304 rcu_read_lock(); 305 306 if (__ref_is_percpu(ref, &percpu_count)) 307 this_cpu_sub(*percpu_count, nr); 308 else if (unlikely(atomic_long_sub_and_test(nr, &ref->count))) 309 ref->release(ref); 310 311 rcu_read_unlock(); 312 } 313 314 /** 315 * percpu_ref_put - decrement a percpu refcount 316 * @ref: percpu_ref to put 317 * 318 * Decrement the refcount, and if 0, call the release function (which was passed 319 * to percpu_ref_init()) 320 * 321 * This function is safe to call as long as @ref is between init and exit. 322 */ 323 static inline void percpu_ref_put(struct percpu_ref *ref) 324 { 325 percpu_ref_put_many(ref, 1); 326 } 327 328 /** 329 * percpu_ref_is_dying - test whether a percpu refcount is dying or dead 330 * @ref: percpu_ref to test 331 * 332 * Returns %true if @ref is dying or dead. 333 * 334 * This function is safe to call as long as @ref is between init and exit 335 * and the caller is responsible for synchronizing against state changes. 336 */ 337 static inline bool percpu_ref_is_dying(struct percpu_ref *ref) 338 { 339 return ref->percpu_count_ptr & __PERCPU_REF_DEAD; 340 } 341 342 /** 343 * percpu_ref_is_zero - test whether a percpu refcount reached zero 344 * @ref: percpu_ref to test 345 * 346 * Returns %true if @ref reached zero. 347 * 348 * This function is safe to call as long as @ref is between init and exit. 349 */ 350 static inline bool percpu_ref_is_zero(struct percpu_ref *ref) 351 { 352 unsigned long __percpu *percpu_count; 353 354 if (__ref_is_percpu(ref, &percpu_count)) 355 return false; 356 return !atomic_long_read(&ref->count); 357 } 358 359 #endif 360