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 */ 79 PERCPU_REF_INIT_ATOMIC = 1 << 0, 80 81 /* 82 * Start dead w/ ref == 0 in atomic mode. Must be revived with 83 * percpu_ref_reinit() before used. Implies INIT_ATOMIC. 84 */ 85 PERCPU_REF_INIT_DEAD = 1 << 1, 86 }; 87 88 struct percpu_ref { 89 atomic_long_t count; 90 /* 91 * The low bit of the pointer indicates whether the ref is in percpu 92 * mode; if set, then get/put will manipulate the atomic_t. 93 */ 94 unsigned long percpu_count_ptr; 95 percpu_ref_func_t *release; 96 percpu_ref_func_t *confirm_switch; 97 bool force_atomic:1; 98 struct rcu_head rcu; 99 }; 100 101 int __must_check percpu_ref_init(struct percpu_ref *ref, 102 percpu_ref_func_t *release, unsigned int flags, 103 gfp_t gfp); 104 void percpu_ref_exit(struct percpu_ref *ref); 105 void percpu_ref_switch_to_atomic(struct percpu_ref *ref, 106 percpu_ref_func_t *confirm_switch); 107 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref); 108 void percpu_ref_switch_to_percpu(struct percpu_ref *ref); 109 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 110 percpu_ref_func_t *confirm_kill); 111 void percpu_ref_resurrect(struct percpu_ref *ref); 112 void percpu_ref_reinit(struct percpu_ref *ref); 113 114 /** 115 * percpu_ref_kill - drop the initial ref 116 * @ref: percpu_ref to kill 117 * 118 * Must be used to drop the initial ref on a percpu refcount; must be called 119 * precisely once before shutdown. 120 * 121 * Switches @ref into atomic mode before gathering up the percpu counters 122 * and dropping the initial ref. 123 * 124 * There are no implied RCU grace periods between kill and release. 125 */ 126 static inline void percpu_ref_kill(struct percpu_ref *ref) 127 { 128 percpu_ref_kill_and_confirm(ref, NULL); 129 } 130 131 /* 132 * Internal helper. Don't use outside percpu-refcount proper. The 133 * function doesn't return the pointer and let the caller test it for NULL 134 * because doing so forces the compiler to generate two conditional 135 * branches as it can't assume that @ref->percpu_count is not NULL. 136 */ 137 static inline bool __ref_is_percpu(struct percpu_ref *ref, 138 unsigned long __percpu **percpu_countp) 139 { 140 unsigned long percpu_ptr; 141 142 /* 143 * The value of @ref->percpu_count_ptr is tested for 144 * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then 145 * used as a pointer. If the compiler generates a separate fetch 146 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in 147 * between contaminating the pointer value, meaning that 148 * READ_ONCE() is required when fetching it. 149 * 150 * The smp_read_barrier_depends() implied by READ_ONCE() pairs 151 * with smp_store_release() in __percpu_ref_switch_to_percpu(). 152 */ 153 percpu_ptr = READ_ONCE(ref->percpu_count_ptr); 154 155 /* 156 * Theoretically, the following could test just ATOMIC; however, 157 * then we'd have to mask off DEAD separately as DEAD may be 158 * visible without ATOMIC if we race with percpu_ref_kill(). DEAD 159 * implies ATOMIC anyway. Test them together. 160 */ 161 if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD)) 162 return false; 163 164 *percpu_countp = (unsigned long __percpu *)percpu_ptr; 165 return true; 166 } 167 168 /** 169 * percpu_ref_get_many - increment a percpu refcount 170 * @ref: percpu_ref to get 171 * @nr: number of references to get 172 * 173 * Analogous to atomic_long_add(). 174 * 175 * This function is safe to call as long as @ref is between init and exit. 176 */ 177 static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr) 178 { 179 unsigned long __percpu *percpu_count; 180 181 rcu_read_lock_sched(); 182 183 if (__ref_is_percpu(ref, &percpu_count)) 184 this_cpu_add(*percpu_count, nr); 185 else 186 atomic_long_add(nr, &ref->count); 187 188 rcu_read_unlock_sched(); 189 } 190 191 /** 192 * percpu_ref_get - increment a percpu refcount 193 * @ref: percpu_ref to get 194 * 195 * Analagous to atomic_long_inc(). 196 * 197 * This function is safe to call as long as @ref is between init and exit. 198 */ 199 static inline void percpu_ref_get(struct percpu_ref *ref) 200 { 201 percpu_ref_get_many(ref, 1); 202 } 203 204 /** 205 * percpu_ref_tryget - try to increment a percpu refcount 206 * @ref: percpu_ref to try-get 207 * 208 * Increment a percpu refcount unless its count already reached zero. 209 * Returns %true on success; %false on failure. 210 * 211 * This function is safe to call as long as @ref is between init and exit. 212 */ 213 static inline bool percpu_ref_tryget(struct percpu_ref *ref) 214 { 215 unsigned long __percpu *percpu_count; 216 bool ret; 217 218 rcu_read_lock_sched(); 219 220 if (__ref_is_percpu(ref, &percpu_count)) { 221 this_cpu_inc(*percpu_count); 222 ret = true; 223 } else { 224 ret = atomic_long_inc_not_zero(&ref->count); 225 } 226 227 rcu_read_unlock_sched(); 228 229 return ret; 230 } 231 232 /** 233 * percpu_ref_tryget_live - try to increment a live percpu refcount 234 * @ref: percpu_ref to try-get 235 * 236 * Increment a percpu refcount unless it has already been killed. Returns 237 * %true on success; %false on failure. 238 * 239 * Completion of percpu_ref_kill() in itself doesn't guarantee that this 240 * function will fail. For such guarantee, percpu_ref_kill_and_confirm() 241 * should be used. After the confirm_kill callback is invoked, it's 242 * guaranteed that no new reference will be given out by 243 * percpu_ref_tryget_live(). 244 * 245 * This function is safe to call as long as @ref is between init and exit. 246 */ 247 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) 248 { 249 unsigned long __percpu *percpu_count; 250 bool ret = false; 251 252 rcu_read_lock_sched(); 253 254 if (__ref_is_percpu(ref, &percpu_count)) { 255 this_cpu_inc(*percpu_count); 256 ret = true; 257 } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) { 258 ret = atomic_long_inc_not_zero(&ref->count); 259 } 260 261 rcu_read_unlock_sched(); 262 263 return ret; 264 } 265 266 /** 267 * percpu_ref_put_many - decrement a percpu refcount 268 * @ref: percpu_ref to put 269 * @nr: number of references to put 270 * 271 * Decrement the refcount, and if 0, call the release function (which was passed 272 * to percpu_ref_init()) 273 * 274 * This function is safe to call as long as @ref is between init and exit. 275 */ 276 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr) 277 { 278 unsigned long __percpu *percpu_count; 279 280 rcu_read_lock_sched(); 281 282 if (__ref_is_percpu(ref, &percpu_count)) 283 this_cpu_sub(*percpu_count, nr); 284 else if (unlikely(atomic_long_sub_and_test(nr, &ref->count))) 285 ref->release(ref); 286 287 rcu_read_unlock_sched(); 288 } 289 290 /** 291 * percpu_ref_put - decrement a percpu refcount 292 * @ref: percpu_ref to put 293 * 294 * Decrement the refcount, and if 0, call the release function (which was passed 295 * to percpu_ref_init()) 296 * 297 * This function is safe to call as long as @ref is between init and exit. 298 */ 299 static inline void percpu_ref_put(struct percpu_ref *ref) 300 { 301 percpu_ref_put_many(ref, 1); 302 } 303 304 /** 305 * percpu_ref_is_dying - test whether a percpu refcount is dying or dead 306 * @ref: percpu_ref to test 307 * 308 * Returns %true if @ref is dying or dead. 309 * 310 * This function is safe to call as long as @ref is between init and exit 311 * and the caller is responsible for synchronizing against state changes. 312 */ 313 static inline bool percpu_ref_is_dying(struct percpu_ref *ref) 314 { 315 return ref->percpu_count_ptr & __PERCPU_REF_DEAD; 316 } 317 318 /** 319 * percpu_ref_is_zero - test whether a percpu refcount reached zero 320 * @ref: percpu_ref to test 321 * 322 * Returns %true if @ref reached zero. 323 * 324 * This function is safe to call as long as @ref is between init and exit. 325 */ 326 static inline bool percpu_ref_is_zero(struct percpu_ref *ref) 327 { 328 unsigned long __percpu *percpu_count; 329 330 if (__ref_is_percpu(ref, &percpu_count)) 331 return false; 332 return !atomic_long_read(&ref->count); 333 } 334 335 #endif 336