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()/PCPU_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 53 struct percpu_ref; 54 typedef void (percpu_ref_func_t)(struct percpu_ref *); 55 56 struct percpu_ref { 57 atomic_t count; 58 /* 59 * The low bit of the pointer indicates whether the ref is in percpu 60 * mode; if set, then get/put will manipulate the atomic_t (this is a 61 * hack because we need to keep the pointer around for 62 * percpu_ref_kill_rcu()) 63 */ 64 unsigned __percpu *pcpu_count; 65 percpu_ref_func_t *release; 66 percpu_ref_func_t *confirm_kill; 67 struct rcu_head rcu; 68 }; 69 70 int __must_check percpu_ref_init(struct percpu_ref *ref, 71 percpu_ref_func_t *release); 72 void percpu_ref_cancel_init(struct percpu_ref *ref); 73 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 74 percpu_ref_func_t *confirm_kill); 75 76 /** 77 * percpu_ref_kill - drop the initial ref 78 * @ref: percpu_ref to kill 79 * 80 * Must be used to drop the initial ref on a percpu refcount; must be called 81 * precisely once before shutdown. 82 * 83 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the 84 * percpu counters and dropping the initial ref. 85 */ 86 static inline void percpu_ref_kill(struct percpu_ref *ref) 87 { 88 return percpu_ref_kill_and_confirm(ref, NULL); 89 } 90 91 #define PCPU_STATUS_BITS 2 92 #define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1) 93 #define PCPU_REF_PTR 0 94 #define PCPU_REF_DEAD 1 95 96 #define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK) 97 98 /** 99 * percpu_ref_get - increment a percpu refcount 100 * @ref: percpu_ref to get 101 * 102 * Analagous to atomic_inc(). 103 */ 104 static inline void percpu_ref_get(struct percpu_ref *ref) 105 { 106 unsigned __percpu *pcpu_count; 107 108 rcu_read_lock_sched(); 109 110 pcpu_count = ACCESS_ONCE(ref->pcpu_count); 111 112 if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) 113 this_cpu_inc(*pcpu_count); 114 else 115 atomic_inc(&ref->count); 116 117 rcu_read_unlock_sched(); 118 } 119 120 /** 121 * percpu_ref_tryget - try to increment a percpu refcount 122 * @ref: percpu_ref to try-get 123 * 124 * Increment a percpu refcount unless its count already reached zero. 125 * Returns %true on success; %false on failure. 126 * 127 * The caller is responsible for ensuring that @ref stays accessible. 128 */ 129 static inline bool percpu_ref_tryget(struct percpu_ref *ref) 130 { 131 unsigned __percpu *pcpu_count; 132 int ret = false; 133 134 rcu_read_lock_sched(); 135 136 pcpu_count = ACCESS_ONCE(ref->pcpu_count); 137 138 if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) { 139 this_cpu_inc(*pcpu_count); 140 ret = true; 141 } else { 142 ret = atomic_inc_not_zero(&ref->count); 143 } 144 145 rcu_read_unlock_sched(); 146 147 return ret; 148 } 149 150 /** 151 * percpu_ref_tryget_live - try to increment a live percpu refcount 152 * @ref: percpu_ref to try-get 153 * 154 * Increment a percpu refcount unless it has already been killed. Returns 155 * %true on success; %false on failure. 156 * 157 * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget 158 * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be 159 * used. After the confirm_kill callback is invoked, it's guaranteed that 160 * no new reference will be given out by percpu_ref_tryget(). 161 * 162 * The caller is responsible for ensuring that @ref stays accessible. 163 */ 164 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref) 165 { 166 unsigned __percpu *pcpu_count; 167 int ret = false; 168 169 rcu_read_lock_sched(); 170 171 pcpu_count = ACCESS_ONCE(ref->pcpu_count); 172 173 if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) { 174 this_cpu_inc(*pcpu_count); 175 ret = true; 176 } 177 178 rcu_read_unlock_sched(); 179 180 return ret; 181 } 182 183 /** 184 * percpu_ref_put - decrement a percpu refcount 185 * @ref: percpu_ref to put 186 * 187 * Decrement the refcount, and if 0, call the release function (which was passed 188 * to percpu_ref_init()) 189 */ 190 static inline void percpu_ref_put(struct percpu_ref *ref) 191 { 192 unsigned __percpu *pcpu_count; 193 194 rcu_read_lock_sched(); 195 196 pcpu_count = ACCESS_ONCE(ref->pcpu_count); 197 198 if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) 199 this_cpu_dec(*pcpu_count); 200 else if (unlikely(atomic_dec_and_test(&ref->count))) 201 ref->release(ref); 202 203 rcu_read_unlock_sched(); 204 } 205 206 #endif 207