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 *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); 23 rcuwait_init(&sem->writer); 24 sem->readers_block = 0; 25 #ifdef CONFIG_DEBUG_LOCK_ALLOC 26 debug_check_no_locks_freed((void *)sem, sizeof(*sem)); 27 lockdep_init_map(&sem->dep_map, name, key, 0); 28 #endif 29 return 0; 30 } 31 EXPORT_SYMBOL_GPL(__percpu_init_rwsem); 32 33 void percpu_free_rwsem(struct percpu_rw_semaphore *sem) 34 { 35 /* 36 * XXX: temporary kludge. The error path in alloc_super() 37 * assumes that percpu_free_rwsem() is safe after kzalloc(). 38 */ 39 if (!sem->read_count) 40 return; 41 42 rcu_sync_dtor(&sem->rss); 43 free_percpu(sem->read_count); 44 sem->read_count = NULL; /* catch use after free bugs */ 45 } 46 EXPORT_SYMBOL_GPL(percpu_free_rwsem); 47 48 bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try) 49 { 50 /* 51 * Due to having preemption disabled the decrement happens on 52 * the same CPU as the increment, avoiding the 53 * increment-on-one-CPU-and-decrement-on-another problem. 54 * 55 * If the reader misses the writer's assignment of readers_block, then 56 * the writer is guaranteed to see the reader's increment. 57 * 58 * Conversely, any readers that increment their sem->read_count after 59 * the writer looks are guaranteed to see the readers_block value, 60 * which in turn means that they are guaranteed to immediately 61 * decrement their sem->read_count, so that it doesn't matter that the 62 * writer missed them. 63 */ 64 65 smp_mb(); /* A matches D */ 66 67 /* 68 * If !readers_block the critical section starts here, matched by the 69 * release in percpu_up_write(). 70 */ 71 if (likely(!smp_load_acquire(&sem->readers_block))) 72 return true; 73 74 /* 75 * Per the above comment; we still have preemption disabled and 76 * will thus decrement on the same CPU as we incremented. 77 */ 78 __percpu_up_read(sem); 79 80 if (try) 81 return false; 82 83 /* 84 * We either call schedule() in the wait, or we'll fall through 85 * and reschedule on the preempt_enable() in percpu_down_read(). 86 */ 87 preempt_enable_no_resched(); 88 89 /* 90 * Avoid lockdep for the down/up_read() we already have them. 91 */ 92 __down_read(&sem->rw_sem); 93 this_cpu_inc(*sem->read_count); 94 __up_read(&sem->rw_sem); 95 96 preempt_disable(); 97 return true; 98 } 99 EXPORT_SYMBOL_GPL(__percpu_down_read); 100 101 void __percpu_up_read(struct percpu_rw_semaphore *sem) 102 { 103 smp_mb(); /* B matches C */ 104 /* 105 * In other words, if they see our decrement (presumably to aggregate 106 * zero, as that is the only time it matters) they will also see our 107 * critical section. 108 */ 109 __this_cpu_dec(*sem->read_count); 110 111 /* Prod writer to recheck readers_active */ 112 rcuwait_wake_up(&sem->writer); 113 } 114 EXPORT_SYMBOL_GPL(__percpu_up_read); 115 116 #define per_cpu_sum(var) \ 117 ({ \ 118 typeof(var) __sum = 0; \ 119 int cpu; \ 120 compiletime_assert_atomic_type(__sum); \ 121 for_each_possible_cpu(cpu) \ 122 __sum += per_cpu(var, cpu); \ 123 __sum; \ 124 }) 125 126 /* 127 * Return true if the modular sum of the sem->read_count per-CPU variable is 128 * zero. If this sum is zero, then it is stable due to the fact that if any 129 * newly arriving readers increment a given counter, they will immediately 130 * decrement that same counter. 131 */ 132 static bool readers_active_check(struct percpu_rw_semaphore *sem) 133 { 134 if (per_cpu_sum(*sem->read_count) != 0) 135 return false; 136 137 /* 138 * If we observed the decrement; ensure we see the entire critical 139 * section. 140 */ 141 142 smp_mb(); /* C matches B */ 143 144 return true; 145 } 146 147 void percpu_down_write(struct percpu_rw_semaphore *sem) 148 { 149 rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_); 150 151 /* Notify readers to take the slow path. */ 152 rcu_sync_enter(&sem->rss); 153 154 __down_write(&sem->rw_sem); 155 156 /* 157 * Notify new readers to block; up until now, and thus throughout the 158 * longish rcu_sync_enter() above, new readers could still come in. 159 */ 160 WRITE_ONCE(sem->readers_block, 1); 161 162 smp_mb(); /* D matches A */ 163 164 /* 165 * If they don't see our writer of readers_block, then we are 166 * guaranteed to see their sem->read_count increment, and therefore 167 * will wait for them. 168 */ 169 170 /* Wait for all now active readers to complete. */ 171 rcuwait_wait_event(&sem->writer, readers_active_check(sem)); 172 } 173 EXPORT_SYMBOL_GPL(percpu_down_write); 174 175 void percpu_up_write(struct percpu_rw_semaphore *sem) 176 { 177 rwsem_release(&sem->dep_map, _RET_IP_); 178 179 /* 180 * Signal the writer is done, no fast path yet. 181 * 182 * One reason that we cannot just immediately flip to readers_fast is 183 * that new readers might fail to see the results of this writer's 184 * critical section. 185 * 186 * Therefore we force it through the slow path which guarantees an 187 * acquire and thereby guarantees the critical section's consistency. 188 */ 189 smp_store_release(&sem->readers_block, 0); 190 191 /* 192 * Release the write lock, this will allow readers back in the game. 193 */ 194 __up_write(&sem->rw_sem); 195 196 /* 197 * Once this completes (at least one RCU-sched grace period hence) the 198 * reader fast path will be available again. Safe to use outside the 199 * exclusive write lock because its counting. 200 */ 201 rcu_sync_exit(&sem->rss); 202 } 203 EXPORT_SYMBOL_GPL(percpu_up_write); 204