1 // SPDX-License-Identifier: GPL-2.0-or-later 2 /* 3 * Queued read/write locks 4 * 5 * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P. 6 * 7 * Authors: Waiman Long <waiman.long@hp.com> 8 */ 9 #include <linux/smp.h> 10 #include <linux/bug.h> 11 #include <linux/cpumask.h> 12 #include <linux/percpu.h> 13 #include <linux/hardirq.h> 14 #include <linux/spinlock.h> 15 16 /** 17 * queued_read_lock_slowpath - acquire read lock of a queue rwlock 18 * @lock: Pointer to queue rwlock structure 19 */ 20 void queued_read_lock_slowpath(struct qrwlock *lock) 21 { 22 /* 23 * Readers come here when they cannot get the lock without waiting 24 */ 25 if (unlikely(in_interrupt())) { 26 /* 27 * Readers in interrupt context will get the lock immediately 28 * if the writer is just waiting (not holding the lock yet), 29 * so spin with ACQUIRE semantics until the lock is available 30 * without waiting in the queue. 31 */ 32 atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED)); 33 return; 34 } 35 atomic_sub(_QR_BIAS, &lock->cnts); 36 37 /* 38 * Put the reader into the wait queue 39 */ 40 arch_spin_lock(&lock->wait_lock); 41 atomic_add(_QR_BIAS, &lock->cnts); 42 43 /* 44 * The ACQUIRE semantics of the following spinning code ensure 45 * that accesses can't leak upwards out of our subsequent critical 46 * section in the case that the lock is currently held for write. 47 */ 48 atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED)); 49 50 /* 51 * Signal the next one in queue to become queue head 52 */ 53 arch_spin_unlock(&lock->wait_lock); 54 } 55 EXPORT_SYMBOL(queued_read_lock_slowpath); 56 57 /** 58 * queued_write_lock_slowpath - acquire write lock of a queue rwlock 59 * @lock : Pointer to queue rwlock structure 60 */ 61 void queued_write_lock_slowpath(struct qrwlock *lock) 62 { 63 int cnts; 64 65 /* Put the writer into the wait queue */ 66 arch_spin_lock(&lock->wait_lock); 67 68 /* Try to acquire the lock directly if no reader is present */ 69 if (!(cnts = atomic_read(&lock->cnts)) && 70 atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)) 71 goto unlock; 72 73 /* Set the waiting flag to notify readers that a writer is pending */ 74 atomic_or(_QW_WAITING, &lock->cnts); 75 76 /* When no more readers or writers, set the locked flag */ 77 do { 78 cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING); 79 } while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)); 80 unlock: 81 arch_spin_unlock(&lock->wait_lock); 82 } 83 EXPORT_SYMBOL(queued_write_lock_slowpath); 84