1 /* 2 * Percpu refcounts: 3 * (C) 2012 Google, Inc. 4 * Author: Kent Overstreet <koverstreet@google.com> 5 * 6 * This implements a refcount with similar semantics to atomic_t - atomic_inc(), 7 * atomic_dec_and_test() - but percpu. 8 * 9 * There's one important difference between percpu refs and normal atomic_t 10 * refcounts; you have to keep track of your initial refcount, and then when you 11 * start shutting down you call percpu_ref_kill() _before_ dropping the initial 12 * refcount. 13 * 14 * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less 15 * than an atomic_t - this is because of the way shutdown works, see 16 * percpu_ref_kill()/PERCPU_COUNT_BIAS. 17 * 18 * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the 19 * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill() 20 * puts the ref back in single atomic_t mode, collecting the per cpu refs and 21 * issuing the appropriate barriers, and then marks the ref as shutting down so 22 * that percpu_ref_put() will check for the ref hitting 0. After it returns, 23 * it's safe to drop the initial ref. 24 * 25 * USAGE: 26 * 27 * See fs/aio.c for some example usage; it's used there for struct kioctx, which 28 * is created when userspaces calls io_setup(), and destroyed when userspace 29 * calls io_destroy() or the process exits. 30 * 31 * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it 32 * calls percpu_ref_kill(), then hlist_del_rcu() and synchronize_rcu() to remove 33 * the kioctx from the proccess's list of kioctxs - after that, there can't be 34 * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop 35 * the initial ref with percpu_ref_put(). 36 * 37 * Code that does a two stage shutdown like this often needs some kind of 38 * explicit synchronization to ensure the initial refcount can only be dropped 39 * once - percpu_ref_kill() does this for you, it returns true once and false if 40 * someone else already called it. The aio code uses it this way, but it's not 41 * necessary if the code has some other mechanism to synchronize teardown. 42 * around. 43 */ 44 45 #ifndef _LINUX_PERCPU_REFCOUNT_H 46 #define _LINUX_PERCPU_REFCOUNT_H 47 48 #include <linux/atomic.h> 49 #include <linux/kernel.h> 50 #include <linux/percpu.h> 51 #include <linux/rcupdate.h> 52 #include <linux/gfp.h> 53 54 struct percpu_ref; 55 typedef void (percpu_ref_func_t)(struct percpu_ref *); 56 57 /* flags set in the lower bits of percpu_ref->percpu_count_ptr */ 58 enum { 59 __PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */ 60 __PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */ 61 __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD, 62 63 __PERCPU_REF_FLAG_BITS = 2, 64 }; 65 66 /* @flags for percpu_ref_init() */ 67 enum { 68 /* 69 * Start w/ ref == 1 in atomic mode. Can be switched to percpu 70 * operation using percpu_ref_switch_to_percpu(). If initialized 71 * with this flag, the ref will stay in atomic mode until 72 * percpu_ref_switch_to_percpu() is invoked on it. 73 */ 74 PERCPU_REF_INIT_ATOMIC = 1 << 0, 75 76 /* 77 * Start dead w/ ref == 0 in atomic mode. Must be revived with 78 * percpu_ref_reinit() before used. Implies INIT_ATOMIC. 79 */ 80 PERCPU_REF_INIT_DEAD = 1 << 1, 81 }; 82 83 struct percpu_ref { 84 atomic_long_t count; 85 /* 86 * The low bit of the pointer indicates whether the ref is in percpu 87 * mode; if set, then get/put will manipulate the atomic_t. 88 */ 89 unsigned long percpu_count_ptr; 90 percpu_ref_func_t *release; 91 percpu_ref_func_t *confirm_switch; 92 bool force_atomic:1; 93 struct rcu_head rcu; 94 }; 95 96 int __must_check percpu_ref_init(struct percpu_ref *ref, 97 percpu_ref_func_t *release, unsigned int flags, 98 gfp_t gfp); 99 void percpu_ref_exit(struct percpu_ref *ref); 100 void percpu_ref_switch_to_atomic(struct percpu_ref *ref, 101 percpu_ref_func_t *confirm_switch); 102 void percpu_ref_switch_to_percpu(struct percpu_ref *ref); 103 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 104 percpu_ref_func_t *confirm_kill); 105 void percpu_ref_reinit(struct percpu_ref *ref); 106 107 /** 108 * percpu_ref_kill - drop the initial ref 109 * @ref: percpu_ref to kill 110 * 111 * Must be used to drop the initial ref on a percpu refcount; must be called 112 * precisely once before shutdown. 113 * 114 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the 115 * percpu counters and dropping the initial ref. 116 */ 117 static inline void percpu_ref_kill(struct percpu_ref *ref) 118 { 119 return percpu_ref_kill_and_confirm(ref, NULL); 120 } 121 122 /* 123 * Internal helper. Don't use outside percpu-refcount proper. The 124 * function doesn't return the pointer and let the caller test it for NULL 125 * because doing so forces the compiler to generate two conditional 126 * branches as it can't assume that @ref->percpu_count is not NULL. 127 */ 128 static inline bool __ref_is_percpu(struct percpu_ref *ref, 129 unsigned long __percpu **percpu_countp) 130 { 131 unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr); 132 133 /* paired with smp_store_release() in percpu_ref_reinit() */ 134 smp_read_barrier_depends(); 135 136 if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC)) 137 return false; 138 139 *percpu_countp = (unsigned long __percpu *)percpu_ptr; 140 return true; 141 } 142 143 /** 144 * percpu_ref_get - increment a percpu refcount 145 * @ref: percpu_ref to get 146 * 147 * Analagous to atomic_long_inc(). 148 * 149 * This function is safe to call as long as @ref is between init and exit. 150 */ 151 static inline void percpu_ref_get(struct percpu_ref *ref) 152 { 153 unsigned long __percpu *percpu_count; 154 155 rcu_read_lock_sched(); 156 157 if (__ref_is_percpu(ref, &percpu_count)) 158 this_cpu_inc(*percpu_count); 159 else 160 atomic_long_inc(&ref->count); 161 162 rcu_read_unlock_sched(); 163 } 164 165 /** 166 * percpu_ref_tryget - try to increment a percpu refcount 167 * @ref: percpu_ref to try-get 168 * 169 * Increment a percpu refcount unless its count already reached zero. 170 * Returns %true on success; %false on failure. 171 * 172 * This function is safe to call as long as @ref is between init and exit. 173 */ 174 static inline bool percpu_ref_tryget(struct percpu_ref *ref) 175 { 176 unsigned long __percpu *percpu_count; 177 int ret; 178 179 rcu_read_lock_sched(); 180 181 if (__ref_is_percpu(ref, &percpu_count)) { 182 this_cpu_inc(*percpu_count); 183 ret = true; 184 } else { 185 ret = atomic_long_inc_not_zero(&ref->count); 186 } 187 188 rcu_read_unlock_sched(); 189 190 return ret; 191 } 192 193 /** 194 * percpu_ref_tryget_live - try to increment a live percpu refcount 195 * @ref: percpu_ref to try-get 196 * 197 * Increment a percpu refcount unless it has already been killed. Returns 198 * %true on success; %false on failure. 199 * 200 * Completion of percpu_ref_kill() in itself doesn't guarantee that this 201 * function will fail. For such guarantee, percpu_ref_kill_and_confirm() 202 * should be used. After the confirm_kill callback is invoked, it's 203 * guaranteed that no new reference will be given out by 204 * percpu_ref_tryget_live(). 205 * 206 * This function is safe to call as long as @ref is between init and exit. 207 */ 208 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) 209 { 210 unsigned long __percpu *percpu_count; 211 int ret = false; 212 213 rcu_read_lock_sched(); 214 215 if (__ref_is_percpu(ref, &percpu_count)) { 216 this_cpu_inc(*percpu_count); 217 ret = true; 218 } else if (!(ACCESS_ONCE(ref->percpu_count_ptr) & __PERCPU_REF_DEAD)) { 219 ret = atomic_long_inc_not_zero(&ref->count); 220 } 221 222 rcu_read_unlock_sched(); 223 224 return ret; 225 } 226 227 /** 228 * percpu_ref_put - decrement a percpu refcount 229 * @ref: percpu_ref to put 230 * 231 * Decrement the refcount, and if 0, call the release function (which was passed 232 * to percpu_ref_init()) 233 * 234 * This function is safe to call as long as @ref is between init and exit. 235 */ 236 static inline void percpu_ref_put(struct percpu_ref *ref) 237 { 238 unsigned long __percpu *percpu_count; 239 240 rcu_read_lock_sched(); 241 242 if (__ref_is_percpu(ref, &percpu_count)) 243 this_cpu_dec(*percpu_count); 244 else if (unlikely(atomic_long_dec_and_test(&ref->count))) 245 ref->release(ref); 246 247 rcu_read_unlock_sched(); 248 } 249 250 /** 251 * percpu_ref_is_zero - test whether a percpu refcount reached zero 252 * @ref: percpu_ref to test 253 * 254 * Returns %true if @ref reached zero. 255 * 256 * This function is safe to call as long as @ref is between init and exit. 257 */ 258 static inline bool percpu_ref_is_zero(struct percpu_ref *ref) 259 { 260 unsigned long __percpu *percpu_count; 261 262 if (__ref_is_percpu(ref, &percpu_count)) 263 return false; 264 return !atomic_long_read(&ref->count); 265 } 266 267 #endif 268