1 #include <linux/atomic.h> 2 #include <linux/rwsem.h> 3 #include <linux/percpu.h> 4 #include <linux/lockdep.h> 5 #include <linux/percpu-rwsem.h> 6 #include <linux/rcupdate.h> 7 #include <linux/sched.h> 8 #include <linux/errno.h> 9 10 #include "rwsem.h" 11 12 int __percpu_init_rwsem(struct percpu_rw_semaphore *sem, 13 const char *name, struct lock_class_key *rwsem_key) 14 { 15 sem->read_count = alloc_percpu(int); 16 if (unlikely(!sem->read_count)) 17 return -ENOMEM; 18 19 /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */ 20 rcu_sync_init(&sem->rss, RCU_SCHED_SYNC); 21 __init_rwsem(&sem->rw_sem, name, rwsem_key); 22 rcuwait_init(&sem->writer); 23 sem->readers_block = 0; 24 return 0; 25 } 26 EXPORT_SYMBOL_GPL(__percpu_init_rwsem); 27 28 void percpu_free_rwsem(struct percpu_rw_semaphore *sem) 29 { 30 /* 31 * XXX: temporary kludge. The error path in alloc_super() 32 * assumes that percpu_free_rwsem() is safe after kzalloc(). 33 */ 34 if (!sem->read_count) 35 return; 36 37 rcu_sync_dtor(&sem->rss); 38 free_percpu(sem->read_count); 39 sem->read_count = NULL; /* catch use after free bugs */ 40 } 41 EXPORT_SYMBOL_GPL(percpu_free_rwsem); 42 43 int __percpu_down_read(struct percpu_rw_semaphore *sem, int try) 44 { 45 /* 46 * Due to having preemption disabled the decrement happens on 47 * the same CPU as the increment, avoiding the 48 * increment-on-one-CPU-and-decrement-on-another problem. 49 * 50 * If the reader misses the writer's assignment of readers_block, then 51 * the writer is guaranteed to see the reader's increment. 52 * 53 * Conversely, any readers that increment their sem->read_count after 54 * the writer looks are guaranteed to see the readers_block value, 55 * which in turn means that they are guaranteed to immediately 56 * decrement their sem->read_count, so that it doesn't matter that the 57 * writer missed them. 58 */ 59 60 smp_mb(); /* A matches D */ 61 62 /* 63 * If !readers_block the critical section starts here, matched by the 64 * release in percpu_up_write(). 65 */ 66 if (likely(!smp_load_acquire(&sem->readers_block))) 67 return 1; 68 69 /* 70 * Per the above comment; we still have preemption disabled and 71 * will thus decrement on the same CPU as we incremented. 72 */ 73 __percpu_up_read(sem); 74 75 if (try) 76 return 0; 77 78 /* 79 * We either call schedule() in the wait, or we'll fall through 80 * and reschedule on the preempt_enable() in percpu_down_read(). 81 */ 82 preempt_enable_no_resched(); 83 84 /* 85 * Avoid lockdep for the down/up_read() we already have them. 86 */ 87 __down_read(&sem->rw_sem); 88 this_cpu_inc(*sem->read_count); 89 __up_read(&sem->rw_sem); 90 91 preempt_disable(); 92 return 1; 93 } 94 EXPORT_SYMBOL_GPL(__percpu_down_read); 95 96 void __percpu_up_read(struct percpu_rw_semaphore *sem) 97 { 98 smp_mb(); /* B matches C */ 99 /* 100 * In other words, if they see our decrement (presumably to aggregate 101 * zero, as that is the only time it matters) they will also see our 102 * critical section. 103 */ 104 __this_cpu_dec(*sem->read_count); 105 106 /* Prod writer to recheck readers_active */ 107 rcuwait_wake_up(&sem->writer); 108 } 109 EXPORT_SYMBOL_GPL(__percpu_up_read); 110 111 #define per_cpu_sum(var) \ 112 ({ \ 113 typeof(var) __sum = 0; \ 114 int cpu; \ 115 compiletime_assert_atomic_type(__sum); \ 116 for_each_possible_cpu(cpu) \ 117 __sum += per_cpu(var, cpu); \ 118 __sum; \ 119 }) 120 121 /* 122 * Return true if the modular sum of the sem->read_count per-CPU variable is 123 * zero. If this sum is zero, then it is stable due to the fact that if any 124 * newly arriving readers increment a given counter, they will immediately 125 * decrement that same counter. 126 */ 127 static bool readers_active_check(struct percpu_rw_semaphore *sem) 128 { 129 if (per_cpu_sum(*sem->read_count) != 0) 130 return false; 131 132 /* 133 * If we observed the decrement; ensure we see the entire critical 134 * section. 135 */ 136 137 smp_mb(); /* C matches B */ 138 139 return true; 140 } 141 142 void percpu_down_write(struct percpu_rw_semaphore *sem) 143 { 144 /* Notify readers to take the slow path. */ 145 rcu_sync_enter(&sem->rss); 146 147 down_write(&sem->rw_sem); 148 149 /* 150 * Notify new readers to block; up until now, and thus throughout the 151 * longish rcu_sync_enter() above, new readers could still come in. 152 */ 153 WRITE_ONCE(sem->readers_block, 1); 154 155 smp_mb(); /* D matches A */ 156 157 /* 158 * If they don't see our writer of readers_block, then we are 159 * guaranteed to see their sem->read_count increment, and therefore 160 * will wait for them. 161 */ 162 163 /* Wait for all now active readers to complete. */ 164 rcuwait_wait_event(&sem->writer, readers_active_check(sem)); 165 } 166 EXPORT_SYMBOL_GPL(percpu_down_write); 167 168 void percpu_up_write(struct percpu_rw_semaphore *sem) 169 { 170 /* 171 * Signal the writer is done, no fast path yet. 172 * 173 * One reason that we cannot just immediately flip to readers_fast is 174 * that new readers might fail to see the results of this writer's 175 * critical section. 176 * 177 * Therefore we force it through the slow path which guarantees an 178 * acquire and thereby guarantees the critical section's consistency. 179 */ 180 smp_store_release(&sem->readers_block, 0); 181 182 /* 183 * Release the write lock, this will allow readers back in the game. 184 */ 185 up_write(&sem->rw_sem); 186 187 /* 188 * Once this completes (at least one RCU-sched grace period hence) the 189 * reader fast path will be available again. Safe to use outside the 190 * exclusive write lock because its counting. 191 */ 192 rcu_sync_exit(&sem->rss); 193 } 194 EXPORT_SYMBOL_GPL(percpu_up_write); 195