1#define pr_fmt(fmt) "%s: " fmt "\n", __func__ 2 3#include <linux/kernel.h> 4#include <linux/percpu-refcount.h> 5 6/* 7 * Initially, a percpu refcount is just a set of percpu counters. Initially, we 8 * don't try to detect the ref hitting 0 - which means that get/put can just 9 * increment or decrement the local counter. Note that the counter on a 10 * particular cpu can (and will) wrap - this is fine, when we go to shutdown the 11 * percpu counters will all sum to the correct value 12 * 13 * (More precisely: because moduler arithmatic is commutative the sum of all the 14 * pcpu_count vars will be equal to what it would have been if all the gets and 15 * puts were done to a single integer, even if some of the percpu integers 16 * overflow or underflow). 17 * 18 * The real trick to implementing percpu refcounts is shutdown. We can't detect 19 * the ref hitting 0 on every put - this would require global synchronization 20 * and defeat the whole purpose of using percpu refs. 21 * 22 * What we do is require the user to keep track of the initial refcount; we know 23 * the ref can't hit 0 before the user drops the initial ref, so as long as we 24 * convert to non percpu mode before the initial ref is dropped everything 25 * works. 26 * 27 * Converting to non percpu mode is done with some RCUish stuff in 28 * percpu_ref_kill. Additionally, we need a bias value so that the atomic_t 29 * can't hit 0 before we've added up all the percpu refs. 30 */ 31 32#define PCPU_COUNT_BIAS (1U << 31) 33 34/** 35 * percpu_ref_init - initialize a percpu refcount 36 * @ref: percpu_ref to initialize 37 * @release: function which will be called when refcount hits 0 38 * 39 * Initializes the refcount in single atomic counter mode with a refcount of 1; 40 * analagous to atomic_set(ref, 1). 41 * 42 * Note that @release must not sleep - it may potentially be called from RCU 43 * callback context by percpu_ref_kill(). 44 */ 45int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release) 46{ 47 atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS); 48 49 ref->pcpu_count = alloc_percpu(unsigned); 50 if (!ref->pcpu_count) 51 return -ENOMEM; 52 53 ref->release = release; 54 return 0; 55} 56 57/** 58 * percpu_ref_cancel_init - cancel percpu_ref_init() 59 * @ref: percpu_ref to cancel init for 60 * 61 * Once a percpu_ref is initialized, its destruction is initiated by 62 * percpu_ref_kill() and completes asynchronously, which can be painful to 63 * do when destroying a half-constructed object in init failure path. 64 * 65 * This function destroys @ref without invoking @ref->release and the 66 * memory area containing it can be freed immediately on return. To 67 * prevent accidental misuse, it's required that @ref has finished 68 * percpu_ref_init(), whether successful or not, but never used. 69 * 70 * The weird name and usage restriction are to prevent people from using 71 * this function by mistake for normal shutdown instead of 72 * percpu_ref_kill(). 73 */ 74void percpu_ref_cancel_init(struct percpu_ref *ref) 75{ 76 unsigned __percpu *pcpu_count = ref->pcpu_count; 77 int cpu; 78 79 WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS); 80 81 if (pcpu_count) { 82 for_each_possible_cpu(cpu) 83 WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu)); 84 free_percpu(ref->pcpu_count); 85 } 86} 87 88static void percpu_ref_kill_rcu(struct rcu_head *rcu) 89{ 90 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu); 91 unsigned __percpu *pcpu_count = ref->pcpu_count; 92 unsigned count = 0; 93 int cpu; 94 95 /* Mask out PCPU_REF_DEAD */ 96 pcpu_count = (unsigned __percpu *) 97 (((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK); 98 99 for_each_possible_cpu(cpu) 100 count += *per_cpu_ptr(pcpu_count, cpu); 101 102 free_percpu(pcpu_count); 103 104 pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count); 105 106 /* 107 * It's crucial that we sum the percpu counters _before_ adding the sum 108 * to &ref->count; since gets could be happening on one cpu while puts 109 * happen on another, adding a single cpu's count could cause 110 * @ref->count to hit 0 before we've got a consistent value - but the 111 * sum of all the counts will be consistent and correct. 112 * 113 * Subtracting the bias value then has to happen _after_ adding count to 114 * &ref->count; we need the bias value to prevent &ref->count from 115 * reaching 0 before we add the percpu counts. But doing it at the same 116 * time is equivalent and saves us atomic operations: 117 */ 118 119 atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count); 120 121 /* @ref is viewed as dead on all CPUs, send out kill confirmation */ 122 if (ref->confirm_kill) 123 ref->confirm_kill(ref); 124 125 /* 126 * Now we're in single atomic_t mode with a consistent refcount, so it's 127 * safe to drop our initial ref: 128 */ 129 percpu_ref_put(ref); 130} 131 132/** 133 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation 134 * @ref: percpu_ref to kill 135 * @confirm_kill: optional confirmation callback 136 * 137 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if 138 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be 139 * called after @ref is seen as dead from all CPUs - all further 140 * invocations of percpu_ref_tryget() will fail. See percpu_ref_tryget() 141 * for more details. 142 * 143 * Due to the way percpu_ref is implemented, @confirm_kill will be called 144 * after at least one full RCU grace period has passed but this is an 145 * implementation detail and callers must not depend on it. 146 */ 147void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 148 percpu_ref_func_t *confirm_kill) 149{ 150 WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD, 151 "percpu_ref_kill() called more than once!\n"); 152 153 ref->pcpu_count = (unsigned __percpu *) 154 (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD); 155 ref->confirm_kill = confirm_kill; 156 157 call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu); 158} 159