1 /* 2 * Queued read/write locks 3 * 4 * This program is free software; you can redistribute it and/or modify 5 * it under the terms of the GNU General Public License as published by 6 * the Free Software Foundation; either version 2 of the License, or 7 * (at your option) any later version. 8 * 9 * This program is distributed in the hope that it will be useful, 10 * but WITHOUT ANY WARRANTY; without even the implied warranty of 11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 12 * GNU General Public License for more details. 13 * 14 * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P. 15 * 16 * Authors: Waiman Long <waiman.long@hp.com> 17 */ 18 #include <linux/smp.h> 19 #include <linux/bug.h> 20 #include <linux/cpumask.h> 21 #include <linux/percpu.h> 22 #include <linux/hardirq.h> 23 #include <asm/qrwlock.h> 24 25 /* 26 * This internal data structure is used for optimizing access to some of 27 * the subfields within the atomic_t cnts. 28 */ 29 struct __qrwlock { 30 union { 31 atomic_t cnts; 32 struct { 33 #ifdef __LITTLE_ENDIAN 34 u8 wmode; /* Writer mode */ 35 u8 rcnts[3]; /* Reader counts */ 36 #else 37 u8 rcnts[3]; /* Reader counts */ 38 u8 wmode; /* Writer mode */ 39 #endif 40 }; 41 }; 42 arch_spinlock_t lock; 43 }; 44 45 /** 46 * rspin_until_writer_unlock - inc reader count & spin until writer is gone 47 * @lock : Pointer to queue rwlock structure 48 * @writer: Current queue rwlock writer status byte 49 * 50 * In interrupt context or at the head of the queue, the reader will just 51 * increment the reader count & wait until the writer releases the lock. 52 */ 53 static __always_inline void 54 rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) 55 { 56 while ((cnts & _QW_WMASK) == _QW_LOCKED) { 57 cpu_relax_lowlatency(); 58 cnts = smp_load_acquire((u32 *)&lock->cnts); 59 } 60 } 61 62 /** 63 * queue_read_lock_slowpath - acquire read lock of a queue rwlock 64 * @lock: Pointer to queue rwlock structure 65 */ 66 void queue_read_lock_slowpath(struct qrwlock *lock) 67 { 68 u32 cnts; 69 70 /* 71 * Readers come here when they cannot get the lock without waiting 72 */ 73 if (unlikely(in_interrupt())) { 74 /* 75 * Readers in interrupt context will spin until the lock is 76 * available without waiting in the queue. 77 */ 78 cnts = smp_load_acquire((u32 *)&lock->cnts); 79 rspin_until_writer_unlock(lock, cnts); 80 return; 81 } 82 atomic_sub(_QR_BIAS, &lock->cnts); 83 84 /* 85 * Put the reader into the wait queue 86 */ 87 arch_spin_lock(&lock->lock); 88 89 /* 90 * At the head of the wait queue now, wait until the writer state 91 * goes to 0 and then try to increment the reader count and get 92 * the lock. It is possible that an incoming writer may steal the 93 * lock in the interim, so it is necessary to check the writer byte 94 * to make sure that the write lock isn't taken. 95 */ 96 while (atomic_read(&lock->cnts) & _QW_WMASK) 97 cpu_relax_lowlatency(); 98 99 cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS; 100 rspin_until_writer_unlock(lock, cnts); 101 102 /* 103 * Signal the next one in queue to become queue head 104 */ 105 arch_spin_unlock(&lock->lock); 106 } 107 EXPORT_SYMBOL(queue_read_lock_slowpath); 108 109 /** 110 * queue_write_lock_slowpath - acquire write lock of a queue rwlock 111 * @lock : Pointer to queue rwlock structure 112 */ 113 void queue_write_lock_slowpath(struct qrwlock *lock) 114 { 115 u32 cnts; 116 117 /* Put the writer into the wait queue */ 118 arch_spin_lock(&lock->lock); 119 120 /* Try to acquire the lock directly if no reader is present */ 121 if (!atomic_read(&lock->cnts) && 122 (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)) 123 goto unlock; 124 125 /* 126 * Set the waiting flag to notify readers that a writer is pending, 127 * or wait for a previous writer to go away. 128 */ 129 for (;;) { 130 struct __qrwlock *l = (struct __qrwlock *)lock; 131 132 if (!READ_ONCE(l->wmode) && 133 (cmpxchg(&l->wmode, 0, _QW_WAITING) == 0)) 134 break; 135 136 cpu_relax_lowlatency(); 137 } 138 139 /* When no more readers, set the locked flag */ 140 for (;;) { 141 cnts = atomic_read(&lock->cnts); 142 if ((cnts == _QW_WAITING) && 143 (atomic_cmpxchg(&lock->cnts, _QW_WAITING, 144 _QW_LOCKED) == _QW_WAITING)) 145 break; 146 147 cpu_relax_lowlatency(); 148 } 149 unlock: 150 arch_spin_unlock(&lock->lock); 151 } 152 EXPORT_SYMBOL(queue_write_lock_slowpath); 153