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 - try to increment a percpu refcount 214 * @ref: percpu_ref to try-get 215 * 216 * Increment a percpu refcount unless its count already reached zero. 217 * Returns %true on success; %false on failure. 218 * 219 * This function is safe to call as long as @ref is between init and exit. 220 */ 221 static inline bool percpu_ref_tryget(struct percpu_ref *ref) 222 { 223 unsigned long __percpu *percpu_count; 224 bool ret; 225 226 rcu_read_lock(); 227 228 if (__ref_is_percpu(ref, &percpu_count)) { 229 this_cpu_inc(*percpu_count); 230 ret = true; 231 } else { 232 ret = atomic_long_inc_not_zero(&ref->count); 233 } 234 235 rcu_read_unlock(); 236 237 return ret; 238 } 239 240 /** 241 * percpu_ref_tryget_live - try to increment a live percpu refcount 242 * @ref: percpu_ref to try-get 243 * 244 * Increment a percpu refcount unless it has already been killed. Returns 245 * %true on success; %false on failure. 246 * 247 * Completion of percpu_ref_kill() in itself doesn't guarantee that this 248 * function will fail. For such guarantee, percpu_ref_kill_and_confirm() 249 * should be used. After the confirm_kill callback is invoked, it's 250 * guaranteed that no new reference will be given out by 251 * percpu_ref_tryget_live(). 252 * 253 * This function is safe to call as long as @ref is between init and exit. 254 */ 255 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) 256 { 257 unsigned long __percpu *percpu_count; 258 bool ret = false; 259 260 rcu_read_lock(); 261 262 if (__ref_is_percpu(ref, &percpu_count)) { 263 this_cpu_inc(*percpu_count); 264 ret = true; 265 } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) { 266 ret = atomic_long_inc_not_zero(&ref->count); 267 } 268 269 rcu_read_unlock(); 270 271 return ret; 272 } 273 274 /** 275 * percpu_ref_put_many - decrement a percpu refcount 276 * @ref: percpu_ref to put 277 * @nr: number of references to put 278 * 279 * Decrement the refcount, and if 0, call the release function (which was passed 280 * to percpu_ref_init()) 281 * 282 * This function is safe to call as long as @ref is between init and exit. 283 */ 284 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr) 285 { 286 unsigned long __percpu *percpu_count; 287 288 rcu_read_lock(); 289 290 if (__ref_is_percpu(ref, &percpu_count)) 291 this_cpu_sub(*percpu_count, nr); 292 else if (unlikely(atomic_long_sub_and_test(nr, &ref->count))) 293 ref->release(ref); 294 295 rcu_read_unlock(); 296 } 297 298 /** 299 * percpu_ref_put - decrement a percpu refcount 300 * @ref: percpu_ref to put 301 * 302 * Decrement the refcount, and if 0, call the release function (which was passed 303 * to percpu_ref_init()) 304 * 305 * This function is safe to call as long as @ref is between init and exit. 306 */ 307 static inline void percpu_ref_put(struct percpu_ref *ref) 308 { 309 percpu_ref_put_many(ref, 1); 310 } 311 312 /** 313 * percpu_ref_is_dying - test whether a percpu refcount is dying or dead 314 * @ref: percpu_ref to test 315 * 316 * Returns %true if @ref is dying or dead. 317 * 318 * This function is safe to call as long as @ref is between init and exit 319 * and the caller is responsible for synchronizing against state changes. 320 */ 321 static inline bool percpu_ref_is_dying(struct percpu_ref *ref) 322 { 323 return ref->percpu_count_ptr & __PERCPU_REF_DEAD; 324 } 325 326 /** 327 * percpu_ref_is_zero - test whether a percpu refcount reached zero 328 * @ref: percpu_ref to test 329 * 330 * Returns %true if @ref reached zero. 331 * 332 * This function is safe to call as long as @ref is between init and exit. 333 */ 334 static inline bool percpu_ref_is_zero(struct percpu_ref *ref) 335 { 336 unsigned long __percpu *percpu_count; 337 338 if (__ref_is_percpu(ref, &percpu_count)) 339 return false; 340 return !atomic_long_read(&ref->count); 341 } 342 343 #endif 344