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