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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 it has already been killed. Returns 125: * %true on success; %false on failure. 126: * 127: * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget 128: * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be 129: * used. After the confirm_kill callback is invoked, it's guaranteed that 130: * no new reference will be given out by percpu_ref_tryget(). 131: */ 132: static inline bool percpu_ref_tryget(struct percpu_ref *ref) 133: { 134: unsigned __percpu *pcpu_count; 135: int ret = false; 136: 137: rcu_read_lock_sched(); 138: 139: pcpu_count = ACCESS_ONCE(ref->pcpu_count); 140: 141: if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) { 142: __this_cpu_inc(*pcpu_count); 143: ret = true; 144: } 145: 146: rcu_read_unlock_sched(); 147: 148: return ret; 149: } 150: 151: /** 152: * percpu_ref_put - decrement a percpu refcount 153: * @ref: percpu_ref to put 154: * 155: * Decrement the refcount, and if 0, call the release function (which was passed 156: * to percpu_ref_init()) 157: */ 158: static inline void percpu_ref_put(struct percpu_ref *ref) 159: { 160: unsigned __percpu *pcpu_count; 161: 162: rcu_read_lock_sched(); 163: 164: pcpu_count = ACCESS_ONCE(ref->pcpu_count); 165: 166: if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) 167: __this_cpu_dec(*pcpu_count); 168: else if (unlikely(atomic_dec_and_test(&ref->count))) 169: ref->release(ref); 170: 171: rcu_read_unlock_sched(); 172: } 173: 174: #endif 175: