1 #include <linux/atomic.h> 2 #include <linux/rwsem.h> 3 #include <linux/percpu.h> 4 #include <linux/wait.h> 5 #include <linux/lockdep.h> 6 #include <linux/percpu-rwsem.h> 7 #include <linux/rcupdate.h> 8 #include <linux/sched.h> 9 #include <linux/errno.h> 10 11 int __percpu_init_rwsem(struct percpu_rw_semaphore *brw, 12 const char *name, struct lock_class_key *rwsem_key) 13 { 14 brw->fast_read_ctr = alloc_percpu(int); 15 if (unlikely(!brw->fast_read_ctr)) 16 return -ENOMEM; 17 18 /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */ 19 __init_rwsem(&brw->rw_sem, name, rwsem_key); 20 rcu_sync_init(&brw->rss, RCU_SCHED_SYNC); 21 atomic_set(&brw->slow_read_ctr, 0); 22 init_waitqueue_head(&brw->write_waitq); 23 return 0; 24 } 25 EXPORT_SYMBOL_GPL(__percpu_init_rwsem); 26 27 void percpu_free_rwsem(struct percpu_rw_semaphore *brw) 28 { 29 /* 30 * XXX: temporary kludge. The error path in alloc_super() 31 * assumes that percpu_free_rwsem() is safe after kzalloc(). 32 */ 33 if (!brw->fast_read_ctr) 34 return; 35 36 rcu_sync_dtor(&brw->rss); 37 free_percpu(brw->fast_read_ctr); 38 brw->fast_read_ctr = NULL; /* catch use after free bugs */ 39 } 40 EXPORT_SYMBOL_GPL(percpu_free_rwsem); 41 42 /* 43 * This is the fast-path for down_read/up_read. If it succeeds we rely 44 * on the barriers provided by rcu_sync_enter/exit; see the comments in 45 * percpu_down_write() and percpu_up_write(). 46 * 47 * If this helper fails the callers rely on the normal rw_semaphore and 48 * atomic_dec_and_test(), so in this case we have the necessary barriers. 49 */ 50 static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val) 51 { 52 bool success; 53 54 preempt_disable(); 55 success = rcu_sync_is_idle(&brw->rss); 56 if (likely(success)) 57 __this_cpu_add(*brw->fast_read_ctr, val); 58 preempt_enable(); 59 60 return success; 61 } 62 63 /* 64 * Like the normal down_read() this is not recursive, the writer can 65 * come after the first percpu_down_read() and create the deadlock. 66 * 67 * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep, 68 * percpu_up_read() does rwsem_release(). This pairs with the usage 69 * of ->rw_sem in percpu_down/up_write(). 70 */ 71 void percpu_down_read(struct percpu_rw_semaphore *brw) 72 { 73 might_sleep(); 74 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_); 75 76 if (likely(update_fast_ctr(brw, +1))) 77 return; 78 79 /* Avoid rwsem_acquire_read() and rwsem_release() */ 80 __down_read(&brw->rw_sem); 81 atomic_inc(&brw->slow_read_ctr); 82 __up_read(&brw->rw_sem); 83 } 84 EXPORT_SYMBOL_GPL(percpu_down_read); 85 86 int percpu_down_read_trylock(struct percpu_rw_semaphore *brw) 87 { 88 if (unlikely(!update_fast_ctr(brw, +1))) { 89 if (!__down_read_trylock(&brw->rw_sem)) 90 return 0; 91 atomic_inc(&brw->slow_read_ctr); 92 __up_read(&brw->rw_sem); 93 } 94 95 rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_); 96 return 1; 97 } 98 99 void percpu_up_read(struct percpu_rw_semaphore *brw) 100 { 101 rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_); 102 103 if (likely(update_fast_ctr(brw, -1))) 104 return; 105 106 /* false-positive is possible but harmless */ 107 if (atomic_dec_and_test(&brw->slow_read_ctr)) 108 wake_up_all(&brw->write_waitq); 109 } 110 EXPORT_SYMBOL_GPL(percpu_up_read); 111 112 static int clear_fast_ctr(struct percpu_rw_semaphore *brw) 113 { 114 unsigned int sum = 0; 115 int cpu; 116 117 for_each_possible_cpu(cpu) { 118 sum += per_cpu(*brw->fast_read_ctr, cpu); 119 per_cpu(*brw->fast_read_ctr, cpu) = 0; 120 } 121 122 return sum; 123 } 124 125 void percpu_down_write(struct percpu_rw_semaphore *brw) 126 { 127 /* 128 * Make rcu_sync_is_idle() == F and thus disable the fast-path in 129 * percpu_down_read() and percpu_up_read(), and wait for gp pass. 130 * 131 * The latter synchronises us with the preceding readers which used 132 * the fast-past, so we can not miss the result of __this_cpu_add() 133 * or anything else inside their criticial sections. 134 */ 135 rcu_sync_enter(&brw->rss); 136 137 /* exclude other writers, and block the new readers completely */ 138 down_write(&brw->rw_sem); 139 140 /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */ 141 atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr); 142 143 /* wait for all readers to complete their percpu_up_read() */ 144 wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr)); 145 } 146 EXPORT_SYMBOL_GPL(percpu_down_write); 147 148 void percpu_up_write(struct percpu_rw_semaphore *brw) 149 { 150 /* release the lock, but the readers can't use the fast-path */ 151 up_write(&brw->rw_sem); 152 /* 153 * Enable the fast-path in percpu_down_read() and percpu_up_read() 154 * but only after another gp pass; this adds the necessary barrier 155 * to ensure the reader can't miss the changes done by us. 156 */ 157 rcu_sync_exit(&brw->rss); 158 } 159 EXPORT_SYMBOL_GPL(percpu_up_write); 160