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 */
queued_read_lock_slowpath(struct qrwlock * lock)21 void __lockfunc 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 */
queued_write_lock_slowpath(struct qrwlock * lock)66 void __lockfunc 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