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 static unsigned __percpu *pcpu_count_ptr(struct percpu_ref *ref) 35 { 36 return (unsigned __percpu *)(ref->pcpu_count_ptr & ~PCPU_REF_DEAD); 37 } 38 39 /** 40 * percpu_ref_init - initialize a percpu refcount 41 * @ref: percpu_ref to initialize 42 * @release: function which will be called when refcount hits 0 43 * 44 * Initializes the refcount in single atomic counter mode with a refcount of 1; 45 * analagous to atomic_set(ref, 1). 46 * 47 * Note that @release must not sleep - it may potentially be called from RCU 48 * callback context by percpu_ref_kill(). 49 */ 50 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release) 51 { 52 atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS); 53 54 ref->pcpu_count_ptr = (unsigned long)alloc_percpu(unsigned); 55 if (!ref->pcpu_count_ptr) 56 return -ENOMEM; 57 58 ref->release = release; 59 return 0; 60 } 61 EXPORT_SYMBOL_GPL(percpu_ref_init); 62 63 /** 64 * percpu_ref_reinit - re-initialize a percpu refcount 65 * @ref: perpcu_ref to re-initialize 66 * 67 * Re-initialize @ref so that it's in the same state as when it finished 68 * percpu_ref_init(). @ref must have been initialized successfully, killed 69 * and reached 0 but not exited. 70 * 71 * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while 72 * this function is in progress. 73 */ 74 void percpu_ref_reinit(struct percpu_ref *ref) 75 { 76 unsigned __percpu *pcpu_count = pcpu_count_ptr(ref); 77 int cpu; 78 79 BUG_ON(!pcpu_count); 80 WARN_ON(!percpu_ref_is_zero(ref)); 81 82 atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS); 83 84 /* 85 * Restore per-cpu operation. smp_store_release() is paired with 86 * smp_read_barrier_depends() in __pcpu_ref_alive() and guarantees 87 * that the zeroing is visible to all percpu accesses which can see 88 * the following PCPU_REF_DEAD clearing. 89 */ 90 for_each_possible_cpu(cpu) 91 *per_cpu_ptr(pcpu_count, cpu) = 0; 92 93 smp_store_release(&ref->pcpu_count_ptr, 94 ref->pcpu_count_ptr & ~PCPU_REF_DEAD); 95 } 96 EXPORT_SYMBOL_GPL(percpu_ref_reinit); 97 98 /** 99 * percpu_ref_exit - undo percpu_ref_init() 100 * @ref: percpu_ref to exit 101 * 102 * This function exits @ref. The caller is responsible for ensuring that 103 * @ref is no longer in active use. The usual places to invoke this 104 * function from are the @ref->release() callback or in init failure path 105 * where percpu_ref_init() succeeded but other parts of the initialization 106 * of the embedding object failed. 107 */ 108 void percpu_ref_exit(struct percpu_ref *ref) 109 { 110 unsigned __percpu *pcpu_count = pcpu_count_ptr(ref); 111 112 if (pcpu_count) { 113 free_percpu(pcpu_count); 114 ref->pcpu_count_ptr = PCPU_REF_DEAD; 115 } 116 } 117 EXPORT_SYMBOL_GPL(percpu_ref_exit); 118 119 static void percpu_ref_kill_rcu(struct rcu_head *rcu) 120 { 121 struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu); 122 unsigned __percpu *pcpu_count = pcpu_count_ptr(ref); 123 unsigned count = 0; 124 int cpu; 125 126 for_each_possible_cpu(cpu) 127 count += *per_cpu_ptr(pcpu_count, cpu); 128 129 pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count); 130 131 /* 132 * It's crucial that we sum the percpu counters _before_ adding the sum 133 * to &ref->count; since gets could be happening on one cpu while puts 134 * happen on another, adding a single cpu's count could cause 135 * @ref->count to hit 0 before we've got a consistent value - but the 136 * sum of all the counts will be consistent and correct. 137 * 138 * Subtracting the bias value then has to happen _after_ adding count to 139 * &ref->count; we need the bias value to prevent &ref->count from 140 * reaching 0 before we add the percpu counts. But doing it at the same 141 * time is equivalent and saves us atomic operations: 142 */ 143 144 atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count); 145 146 WARN_ONCE(atomic_read(&ref->count) <= 0, "percpu ref <= 0 (%i)", 147 atomic_read(&ref->count)); 148 149 /* @ref is viewed as dead on all CPUs, send out kill confirmation */ 150 if (ref->confirm_kill) 151 ref->confirm_kill(ref); 152 153 /* 154 * Now we're in single atomic_t mode with a consistent refcount, so it's 155 * safe to drop our initial ref: 156 */ 157 percpu_ref_put(ref); 158 } 159 160 /** 161 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation 162 * @ref: percpu_ref to kill 163 * @confirm_kill: optional confirmation callback 164 * 165 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if 166 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be 167 * called after @ref is seen as dead from all CPUs - all further 168 * invocations of percpu_ref_tryget() will fail. See percpu_ref_tryget() 169 * for more details. 170 * 171 * Due to the way percpu_ref is implemented, @confirm_kill will be called 172 * after at least one full RCU grace period has passed but this is an 173 * implementation detail and callers must not depend on it. 174 */ 175 void percpu_ref_kill_and_confirm(struct percpu_ref *ref, 176 percpu_ref_func_t *confirm_kill) 177 { 178 WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD, 179 "percpu_ref_kill() called more than once!\n"); 180 181 ref->pcpu_count_ptr |= PCPU_REF_DEAD; 182 ref->confirm_kill = confirm_kill; 183 184 call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu); 185 } 186 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm); 187