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 = atomic_read_acquire(&lock->cnts); 59 } 60 } 61 62 /** 63 * queued_read_lock_slowpath - acquire read lock of a queue rwlock 64 * @lock: Pointer to queue rwlock structure 65 * @cnts: Current qrwlock lock value 66 */ 67 void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts) 68 { 69 /* 70 * Readers come here when they cannot get the lock without waiting 71 */ 72 if (unlikely(in_interrupt())) { 73 /* 74 * Readers in interrupt context will get the lock immediately 75 * if the writer is just waiting (not holding the lock yet). 76 * The rspin_until_writer_unlock() function returns immediately 77 * in this case. Otherwise, they will spin (with ACQUIRE 78 * semantics) until the lock is available without waiting in 79 * the queue. 80 */ 81 rspin_until_writer_unlock(lock, cnts); 82 return; 83 } 84 atomic_sub(_QR_BIAS, &lock->cnts); 85 86 /* 87 * Put the reader into the wait queue 88 */ 89 arch_spin_lock(&lock->lock); 90 91 /* 92 * The ACQUIRE semantics of the following spinning code ensure 93 * that accesses can't leak upwards out of our subsequent critical 94 * section in the case that the lock is currently held for write. 95 */ 96 cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts) - _QR_BIAS; 97 rspin_until_writer_unlock(lock, cnts); 98 99 /* 100 * Signal the next one in queue to become queue head 101 */ 102 arch_spin_unlock(&lock->lock); 103 } 104 EXPORT_SYMBOL(queued_read_lock_slowpath); 105 106 /** 107 * queued_write_lock_slowpath - acquire write lock of a queue rwlock 108 * @lock : Pointer to queue rwlock structure 109 */ 110 void queued_write_lock_slowpath(struct qrwlock *lock) 111 { 112 u32 cnts; 113 114 /* Put the writer into the wait queue */ 115 arch_spin_lock(&lock->lock); 116 117 /* Try to acquire the lock directly if no reader is present */ 118 if (!atomic_read(&lock->cnts) && 119 (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)) 120 goto unlock; 121 122 /* 123 * Set the waiting flag to notify readers that a writer is pending, 124 * or wait for a previous writer to go away. 125 */ 126 for (;;) { 127 struct __qrwlock *l = (struct __qrwlock *)lock; 128 129 if (!READ_ONCE(l->wmode) && 130 (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0)) 131 break; 132 133 cpu_relax_lowlatency(); 134 } 135 136 /* When no more readers, set the locked flag */ 137 for (;;) { 138 cnts = atomic_read(&lock->cnts); 139 if ((cnts == _QW_WAITING) && 140 (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING, 141 _QW_LOCKED) == _QW_WAITING)) 142 break; 143 144 cpu_relax_lowlatency(); 145 } 146 unlock: 147 arch_spin_unlock(&lock->lock); 148 } 149 EXPORT_SYMBOL(queued_write_lock_slowpath); 150