1 /* 2 * Percpu refcounts: 3 * (C) 2012 Google, Inc. 4 * Author: Kent Overstreet <[email protected]> 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 sychronize_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(). 71 */ 72 PERCPU_REF_INIT_ATOMIC = 1 << 0, 73 74 /* 75 * Start dead w/ ref == 0 in atomic mode. Must be revived with 76 * percpu_ref_reinit() before used. Implies INIT_ATOMIC. 77 */ 78 PERCPU_REF_INIT_DEAD = 1 << 1, 79 }; 80 81 struct percpu_ref { 82 atomic_long_t count; 83 /* 84 * The low bit of the pointer indicates whether the ref is in percpu 85 * mode; if set, then get/put will manipulate the atomic_t. 86 */ 87 unsigned long percpu_count_ptr; 88 percpu_ref_func_t *release; 89 percpu_ref_func_t *confirm_switch; 90 struct rcu_head rcu; 91 }; 92 93 int __must_check percpu_ref_init(struct percpu_ref *ref, 94 percpu_ref_func_t *release, unsigned int flags, 95 gfp_t gfp); 96 void percpu_ref_exit(struct percpu_ref *ref); 97 void percpu_ref_switch_to_atomic(struct percpu_ref *ref, 98 percpu_ref_func_t *confirm_switch); 99 void percpu_ref_switch_to_percpu(struct percpu_ref *ref); 100 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 101 percpu_ref_func_t *confirm_kill); 102 void percpu_ref_reinit(struct percpu_ref *ref); 103 104 /** 105 * percpu_ref_kill - drop the initial ref 106 * @ref: percpu_ref to kill 107 * 108 * Must be used to drop the initial ref on a percpu refcount; must be called 109 * precisely once before shutdown. 110 * 111 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the 112 * percpu counters and dropping the initial ref. 113 */ 114 static inline void percpu_ref_kill(struct percpu_ref *ref) 115 { 116 return percpu_ref_kill_and_confirm(ref, NULL); 117 } 118 119 /* 120 * Internal helper. Don't use outside percpu-refcount proper. The 121 * function doesn't return the pointer and let the caller test it for NULL 122 * because doing so forces the compiler to generate two conditional 123 * branches as it can't assume that @ref->percpu_count is not NULL. 124 */ 125 static inline bool __ref_is_percpu(struct percpu_ref *ref, 126 unsigned long __percpu **percpu_countp) 127 { 128 unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr); 129 130 /* paired with smp_store_release() in percpu_ref_reinit() */ 131 smp_read_barrier_depends(); 132 133 if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC)) 134 return false; 135 136 *percpu_countp = (unsigned long __percpu *)percpu_ptr; 137 return true; 138 } 139 140 /** 141 * percpu_ref_get - increment a percpu refcount 142 * @ref: percpu_ref to get 143 * 144 * Analagous to atomic_long_inc(). 145 * 146 * This function is safe to call as long as @ref is between init and exit. 147 */ 148 static inline void percpu_ref_get(struct percpu_ref *ref) 149 { 150 unsigned long __percpu *percpu_count; 151 152 rcu_read_lock_sched(); 153 154 if (__ref_is_percpu(ref, &percpu_count)) 155 this_cpu_inc(*percpu_count); 156 else 157 atomic_long_inc(&ref->count); 158 159 rcu_read_unlock_sched(); 160 } 161 162 /** 163 * percpu_ref_tryget - try to increment a percpu refcount 164 * @ref: percpu_ref to try-get 165 * 166 * Increment a percpu refcount unless its count already reached zero. 167 * Returns %true on success; %false on failure. 168 * 169 * This function is safe to call as long as @ref is between init and exit. 170 */ 171 static inline bool percpu_ref_tryget(struct percpu_ref *ref) 172 { 173 unsigned long __percpu *percpu_count; 174 int ret; 175 176 rcu_read_lock_sched(); 177 178 if (__ref_is_percpu(ref, &percpu_count)) { 179 this_cpu_inc(*percpu_count); 180 ret = true; 181 } else { 182 ret = atomic_long_inc_not_zero(&ref->count); 183 } 184 185 rcu_read_unlock_sched(); 186 187 return ret; 188 } 189 190 /** 191 * percpu_ref_tryget_live - try to increment a live percpu refcount 192 * @ref: percpu_ref to try-get 193 * 194 * Increment a percpu refcount unless it has already been killed. Returns 195 * %true on success; %false on failure. 196 * 197 * Completion of percpu_ref_kill() in itself doesn't guarantee that this 198 * function will fail. For such guarantee, percpu_ref_kill_and_confirm() 199 * should be used. After the confirm_kill callback is invoked, it's 200 * guaranteed that no new reference will be given out by 201 * percpu_ref_tryget_live(). 202 * 203 * This function is safe to call as long as @ref is between init and exit. 204 */ 205 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) 206 { 207 unsigned long __percpu *percpu_count; 208 int ret = false; 209 210 rcu_read_lock_sched(); 211 212 if (__ref_is_percpu(ref, &percpu_count)) { 213 this_cpu_inc(*percpu_count); 214 ret = true; 215 } else if (!(ACCESS_ONCE(ref->percpu_count_ptr) & __PERCPU_REF_DEAD)) { 216 ret = atomic_long_inc_not_zero(&ref->count); 217 } 218 219 rcu_read_unlock_sched(); 220 221 return ret; 222 } 223 224 /** 225 * percpu_ref_put - decrement a percpu refcount 226 * @ref: percpu_ref to put 227 * 228 * Decrement the refcount, and if 0, call the release function (which was passed 229 * to percpu_ref_init()) 230 * 231 * This function is safe to call as long as @ref is between init and exit. 232 */ 233 static inline void percpu_ref_put(struct percpu_ref *ref) 234 { 235 unsigned long __percpu *percpu_count; 236 237 rcu_read_lock_sched(); 238 239 if (__ref_is_percpu(ref, &percpu_count)) 240 this_cpu_dec(*percpu_count); 241 else if (unlikely(atomic_long_dec_and_test(&ref->count))) 242 ref->release(ref); 243 244 rcu_read_unlock_sched(); 245 } 246 247 /** 248 * percpu_ref_is_zero - test whether a percpu refcount reached zero 249 * @ref: percpu_ref to test 250 * 251 * Returns %true if @ref reached zero. 252 * 253 * This function is safe to call as long as @ref is between init and exit. 254 */ 255 static inline bool percpu_ref_is_zero(struct percpu_ref *ref) 256 { 257 unsigned long __percpu *percpu_count; 258 259 if (__ref_is_percpu(ref, &percpu_count)) 260 return false; 261 return !atomic_long_read(&ref->count); 262 } 263 264 #endif 265