1 // SPDX-License-Identifier: GPL-2.0-only 2 #include <linux/atomic.h> 3 #include <linux/rwsem.h> 4 #include <linux/percpu.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 #include "rwsem.h" 12 13 int __percpu_init_rwsem(struct percpu_rw_semaphore *sem, 14 const char *name, struct lock_class_key *rwsem_key) 15 { 16 sem->read_count = alloc_percpu(int); 17 if (unlikely(!sem->read_count)) 18 return -ENOMEM; 19 20 /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */ 21 rcu_sync_init(&sem->rss); 22 __init_rwsem(&sem->rw_sem, name, rwsem_key); 23 rcuwait_init(&sem->writer); 24 sem->readers_block = 0; 25 return 0; 26 } 27 EXPORT_SYMBOL_GPL(__percpu_init_rwsem); 28 29 void percpu_free_rwsem(struct percpu_rw_semaphore *sem) 30 { 31 /* 32 * XXX: temporary kludge. The error path in alloc_super() 33 * assumes that percpu_free_rwsem() is safe after kzalloc(). 34 */ 35 if (!sem->read_count) 36 return; 37 38 rcu_sync_dtor(&sem->rss); 39 free_percpu(sem->read_count); 40 sem->read_count = NULL; /* catch use after free bugs */ 41 } 42 EXPORT_SYMBOL_GPL(percpu_free_rwsem); 43 44 int __percpu_down_read(struct percpu_rw_semaphore *sem, int try) 45 { 46 /* 47 * Due to having preemption disabled the decrement happens on 48 * the same CPU as the increment, avoiding the 49 * increment-on-one-CPU-and-decrement-on-another problem. 50 * 51 * If the reader misses the writer's assignment of readers_block, then 52 * the writer is guaranteed to see the reader's increment. 53 * 54 * Conversely, any readers that increment their sem->read_count after 55 * the writer looks are guaranteed to see the readers_block value, 56 * which in turn means that they are guaranteed to immediately 57 * decrement their sem->read_count, so that it doesn't matter that the 58 * writer missed them. 59 */ 60 61 smp_mb(); /* A matches D */ 62 63 /* 64 * If !readers_block the critical section starts here, matched by the 65 * release in percpu_up_write(). 66 */ 67 if (likely(!smp_load_acquire(&sem->readers_block))) 68 return 1; 69 70 /* 71 * Per the above comment; we still have preemption disabled and 72 * will thus decrement on the same CPU as we incremented. 73 */ 74 __percpu_up_read(sem); 75 76 if (try) 77 return 0; 78 79 /* 80 * We either call schedule() in the wait, or we'll fall through 81 * and reschedule on the preempt_enable() in percpu_down_read(). 82 */ 83 preempt_enable_no_resched(); 84 85 /* 86 * Avoid lockdep for the down/up_read() we already have them. 87 */ 88 __down_read(&sem->rw_sem); 89 this_cpu_inc(*sem->read_count); 90 __up_read(&sem->rw_sem); 91 92 preempt_disable(); 93 return 1; 94 } 95 EXPORT_SYMBOL_GPL(__percpu_down_read); 96 97 void __percpu_up_read(struct percpu_rw_semaphore *sem) 98 { 99 smp_mb(); /* B matches C */ 100 /* 101 * In other words, if they see our decrement (presumably to aggregate 102 * zero, as that is the only time it matters) they will also see our 103 * critical section. 104 */ 105 __this_cpu_dec(*sem->read_count); 106 107 /* Prod writer to recheck readers_active */ 108 rcuwait_wake_up(&sem->writer); 109 } 110 EXPORT_SYMBOL_GPL(__percpu_up_read); 111 112 #define per_cpu_sum(var) \ 113 ({ \ 114 typeof(var) __sum = 0; \ 115 int cpu; \ 116 compiletime_assert_atomic_type(__sum); \ 117 for_each_possible_cpu(cpu) \ 118 __sum += per_cpu(var, cpu); \ 119 __sum; \ 120 }) 121 122 /* 123 * Return true if the modular sum of the sem->read_count per-CPU variable is 124 * zero. If this sum is zero, then it is stable due to the fact that if any 125 * newly arriving readers increment a given counter, they will immediately 126 * decrement that same counter. 127 */ 128 static bool readers_active_check(struct percpu_rw_semaphore *sem) 129 { 130 if (per_cpu_sum(*sem->read_count) != 0) 131 return false; 132 133 /* 134 * If we observed the decrement; ensure we see the entire critical 135 * section. 136 */ 137 138 smp_mb(); /* C matches B */ 139 140 return true; 141 } 142 143 void percpu_down_write(struct percpu_rw_semaphore *sem) 144 { 145 /* Notify readers to take the slow path. */ 146 rcu_sync_enter(&sem->rss); 147 148 down_write(&sem->rw_sem); 149 150 /* 151 * Notify new readers to block; up until now, and thus throughout the 152 * longish rcu_sync_enter() above, new readers could still come in. 153 */ 154 WRITE_ONCE(sem->readers_block, 1); 155 156 smp_mb(); /* D matches A */ 157 158 /* 159 * If they don't see our writer of readers_block, then we are 160 * guaranteed to see their sem->read_count increment, and therefore 161 * will wait for them. 162 */ 163 164 /* Wait for all now active readers to complete. */ 165 rcuwait_wait_event(&sem->writer, readers_active_check(sem)); 166 } 167 EXPORT_SYMBOL_GPL(percpu_down_write); 168 169 void percpu_up_write(struct percpu_rw_semaphore *sem) 170 { 171 /* 172 * Signal the writer is done, no fast path yet. 173 * 174 * One reason that we cannot just immediately flip to readers_fast is 175 * that new readers might fail to see the results of this writer's 176 * critical section. 177 * 178 * Therefore we force it through the slow path which guarantees an 179 * acquire and thereby guarantees the critical section's consistency. 180 */ 181 smp_store_release(&sem->readers_block, 0); 182 183 /* 184 * Release the write lock, this will allow readers back in the game. 185 */ 186 up_write(&sem->rw_sem); 187 188 /* 189 * Once this completes (at least one RCU-sched grace period hence) the 190 * reader fast path will be available again. Safe to use outside the 191 * exclusive write lock because its counting. 192 */ 193 rcu_sync_exit(&sem->rss); 194 } 195 EXPORT_SYMBOL_GPL(percpu_up_write); 196