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 #include <trace/events/lock.h> 16 17 /** 18 * queued_read_lock_slowpath - acquire read lock of a queued rwlock 19 * @lock: Pointer to queued rwlock structure 20 */ 21 void queued_read_lock_slowpath(struct qrwlock *lock) 22 { 23 /* 24 * Readers come here when they cannot get the lock without waiting 25 */ 26 if (unlikely(in_interrupt())) { 27 /* 28 * Readers in interrupt context will get the lock immediately 29 * if the writer is just waiting (not holding the lock yet), 30 * so spin with ACQUIRE semantics until the lock is available 31 * without waiting in the queue. 32 */ 33 atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED)); 34 return; 35 } 36 atomic_sub(_QR_BIAS, &lock->cnts); 37 38 trace_contention_begin(lock, LCB_F_SPIN | LCB_F_READ); 39 40 /* 41 * Put the reader into the wait queue 42 */ 43 arch_spin_lock(&lock->wait_lock); 44 atomic_add(_QR_BIAS, &lock->cnts); 45 46 /* 47 * The ACQUIRE semantics of the following spinning code ensure 48 * that accesses can't leak upwards out of our subsequent critical 49 * section in the case that the lock is currently held for write. 50 */ 51 atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED)); 52 53 /* 54 * Signal the next one in queue to become queue head 55 */ 56 arch_spin_unlock(&lock->wait_lock); 57 58 trace_contention_end(lock, 0); 59 } 60 EXPORT_SYMBOL(queued_read_lock_slowpath); 61 62 /** 63 * queued_write_lock_slowpath - acquire write lock of a queued rwlock 64 * @lock : Pointer to queued rwlock structure 65 */ 66 void queued_write_lock_slowpath(struct qrwlock *lock) 67 { 68 int cnts; 69 70 trace_contention_begin(lock, LCB_F_SPIN | LCB_F_WRITE); 71 72 /* Put the writer into the wait queue */ 73 arch_spin_lock(&lock->wait_lock); 74 75 /* Try to acquire the lock directly if no reader is present */ 76 if (!(cnts = atomic_read(&lock->cnts)) && 77 atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)) 78 goto unlock; 79 80 /* Set the waiting flag to notify readers that a writer is pending */ 81 atomic_or(_QW_WAITING, &lock->cnts); 82 83 /* When no more readers or writers, set the locked flag */ 84 do { 85 cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING); 86 } while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)); 87 unlock: 88 arch_spin_unlock(&lock->wait_lock); 89 90 trace_contention_end(lock, 0); 91 } 92 EXPORT_SYMBOL(queued_write_lock_slowpath); 93