xref: /openbmc/linux/kernel/locking/qrwlock.c (revision b830f94f)
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 <asm/qrwlock.h>
16 
17 /**
18  * queued_read_lock_slowpath - acquire read lock of a queue rwlock
19  * @lock: Pointer to queue 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 	/*
39 	 * Put the reader into the wait queue
40 	 */
41 	arch_spin_lock(&lock->wait_lock);
42 	atomic_add(_QR_BIAS, &lock->cnts);
43 
44 	/*
45 	 * The ACQUIRE semantics of the following spinning code ensure
46 	 * that accesses can't leak upwards out of our subsequent critical
47 	 * section in the case that the lock is currently held for write.
48 	 */
49 	atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
50 
51 	/*
52 	 * Signal the next one in queue to become queue head
53 	 */
54 	arch_spin_unlock(&lock->wait_lock);
55 }
56 EXPORT_SYMBOL(queued_read_lock_slowpath);
57 
58 /**
59  * queued_write_lock_slowpath - acquire write lock of a queue rwlock
60  * @lock : Pointer to queue rwlock structure
61  */
62 void queued_write_lock_slowpath(struct qrwlock *lock)
63 {
64 	/* Put the writer into the wait queue */
65 	arch_spin_lock(&lock->wait_lock);
66 
67 	/* Try to acquire the lock directly if no reader is present */
68 	if (!atomic_read(&lock->cnts) &&
69 	    (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
70 		goto unlock;
71 
72 	/* Set the waiting flag to notify readers that a writer is pending */
73 	atomic_add(_QW_WAITING, &lock->cnts);
74 
75 	/* When no more readers or writers, set the locked flag */
76 	do {
77 		atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
78 	} while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
79 					_QW_LOCKED) != _QW_WAITING);
80 unlock:
81 	arch_spin_unlock(&lock->wait_lock);
82 }
83 EXPORT_SYMBOL(queued_write_lock_slowpath);
84