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