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 */ 45 int 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 EXPORT_SYMBOL_GPL(percpu_ref_init); 57 58 /** 59 * percpu_ref_cancel_init - cancel percpu_ref_init() 60 * @ref: percpu_ref to cancel init for 61 * 62 * Once a percpu_ref is initialized, its destruction is initiated by 63 * percpu_ref_kill() and completes asynchronously, which can be painful to 64 * do when destroying a half-constructed object in init failure path. 65 * 66 * This function destroys @ref without invoking @ref->release and the 67 * memory area containing it can be freed immediately on return. To 68 * prevent accidental misuse, it's required that @ref has finished 69 * percpu_ref_init(), whether successful or not, but never used. 70 * 71 * The weird name and usage restriction are to prevent people from using 72 * this function by mistake for normal shutdown instead of 73 * percpu_ref_kill(). 74 */ 75 void percpu_ref_cancel_init(struct percpu_ref *ref) 76 { 77 unsigned __percpu *pcpu_count = ref->pcpu_count; 78 int cpu; 79 80 WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS); 81 82 if (pcpu_count) { 83 for_each_possible_cpu(cpu) 84 WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu)); 85 free_percpu(ref->pcpu_count); 86 } 87 } 88 EXPORT_SYMBOL_GPL(percpu_ref_cancel_init); 89 90 static void percpu_ref_kill_rcu(struct rcu_head *rcu) 91 { 92 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu); 93 unsigned __percpu *pcpu_count = ref->pcpu_count; 94 unsigned count = 0; 95 int cpu; 96 97 /* Mask out PCPU_REF_DEAD */ 98 pcpu_count = (unsigned __percpu *) 99 (((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK); 100 101 for_each_possible_cpu(cpu) 102 count += *per_cpu_ptr(pcpu_count, cpu); 103 104 free_percpu(pcpu_count); 105 106 pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count); 107 108 /* 109 * It's crucial that we sum the percpu counters _before_ adding the sum 110 * to &ref->count; since gets could be happening on one cpu while puts 111 * happen on another, adding a single cpu's count could cause 112 * @ref->count to hit 0 before we've got a consistent value - but the 113 * sum of all the counts will be consistent and correct. 114 * 115 * Subtracting the bias value then has to happen _after_ adding count to 116 * &ref->count; we need the bias value to prevent &ref->count from 117 * reaching 0 before we add the percpu counts. But doing it at the same 118 * time is equivalent and saves us atomic operations: 119 */ 120 121 atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count); 122 123 WARN_ONCE(atomic_read(&ref->count) <= 0, "percpu ref <= 0 (%i)", 124 atomic_read(&ref->count)); 125 126 /* @ref is viewed as dead on all CPUs, send out kill confirmation */ 127 if (ref->confirm_kill) 128 ref->confirm_kill(ref); 129 130 /* 131 * Now we're in single atomic_t mode with a consistent refcount, so it's 132 * safe to drop our initial ref: 133 */ 134 percpu_ref_put(ref); 135 } 136 137 /** 138 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation 139 * @ref: percpu_ref to kill 140 * @confirm_kill: optional confirmation callback 141 * 142 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if 143 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be 144 * called after @ref is seen as dead from all CPUs - all further 145 * invocations of percpu_ref_tryget() will fail. See percpu_ref_tryget() 146 * for more details. 147 * 148 * Due to the way percpu_ref is implemented, @confirm_kill will be called 149 * after at least one full RCU grace period has passed but this is an 150 * implementation detail and callers must not depend on it. 151 */ 152 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 153 percpu_ref_func_t *confirm_kill) 154 { 155 WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD, 156 "percpu_ref_kill() called more than once!\n"); 157 158 ref->pcpu_count = (unsigned __percpu *) 159 (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD); 160 ref->confirm_kill = confirm_kill; 161 162 call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu); 163 } 164 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm); 165