1b2441318SGreg Kroah-Hartman /* SPDX-License-Identifier: GPL-2.0 */
2c9122da1SPeter Zijlstra /*
3c9122da1SPeter Zijlstra * MCS lock defines
4c9122da1SPeter Zijlstra *
5c9122da1SPeter Zijlstra * This file contains the main data structure and API definitions of MCS lock.
6c9122da1SPeter Zijlstra *
7c9122da1SPeter Zijlstra * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
8c9122da1SPeter Zijlstra * with the desirable properties of being fair, and with each cpu trying
9c9122da1SPeter Zijlstra * to acquire the lock spinning on a local variable.
10*e2db7592SIngo Molnar * It avoids expensive cache bounces that common test-and-set spin-lock
11c9122da1SPeter Zijlstra * implementations incur.
12c9122da1SPeter Zijlstra */
13c9122da1SPeter Zijlstra #ifndef __LINUX_MCS_SPINLOCK_H
14c9122da1SPeter Zijlstra #define __LINUX_MCS_SPINLOCK_H
15c9122da1SPeter Zijlstra
16c9122da1SPeter Zijlstra #include <asm/mcs_spinlock.h>
17c9122da1SPeter Zijlstra
18c9122da1SPeter Zijlstra struct mcs_spinlock {
19c9122da1SPeter Zijlstra struct mcs_spinlock *next;
20c9122da1SPeter Zijlstra int locked; /* 1 if lock acquired */
21a33fda35SWaiman Long int count; /* nesting count, see qspinlock.c */
22c9122da1SPeter Zijlstra };
23c9122da1SPeter Zijlstra
24c9122da1SPeter Zijlstra #ifndef arch_mcs_spin_lock_contended
25c9122da1SPeter Zijlstra /*
267f56b58aSJason Low * Using smp_cond_load_acquire() provides the acquire semantics
277f56b58aSJason Low * required so that subsequent operations happen after the
287f56b58aSJason Low * lock is acquired. Additionally, some architectures such as
297f56b58aSJason Low * ARM64 would like to do spin-waiting instead of purely
307f56b58aSJason Low * spinning, and smp_cond_load_acquire() provides that behavior.
31c9122da1SPeter Zijlstra */
32c9122da1SPeter Zijlstra #define arch_mcs_spin_lock_contended(l) \
33c9122da1SPeter Zijlstra do { \
347f56b58aSJason Low smp_cond_load_acquire(l, VAL); \
35c9122da1SPeter Zijlstra } while (0)
36c9122da1SPeter Zijlstra #endif
37c9122da1SPeter Zijlstra
38c9122da1SPeter Zijlstra #ifndef arch_mcs_spin_unlock_contended
39c9122da1SPeter Zijlstra /*
40c9122da1SPeter Zijlstra * smp_store_release() provides a memory barrier to ensure all
41c9122da1SPeter Zijlstra * operations in the critical section has been completed before
42c9122da1SPeter Zijlstra * unlocking.
43c9122da1SPeter Zijlstra */
44c9122da1SPeter Zijlstra #define arch_mcs_spin_unlock_contended(l) \
45c9122da1SPeter Zijlstra smp_store_release((l), 1)
46c9122da1SPeter Zijlstra #endif
47c9122da1SPeter Zijlstra
48c9122da1SPeter Zijlstra /*
49c9122da1SPeter Zijlstra * Note: the smp_load_acquire/smp_store_release pair is not
50c9122da1SPeter Zijlstra * sufficient to form a full memory barrier across
51c9122da1SPeter Zijlstra * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
52c9122da1SPeter Zijlstra * For applications that need a full barrier across multiple cpus
53c9122da1SPeter Zijlstra * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
54c9122da1SPeter Zijlstra * used after mcs_lock.
55c9122da1SPeter Zijlstra */
56c9122da1SPeter Zijlstra
57c9122da1SPeter Zijlstra /*
58c9122da1SPeter Zijlstra * In order to acquire the lock, the caller should declare a local node and
59c9122da1SPeter Zijlstra * pass a reference of the node to this function in addition to the lock.
60c9122da1SPeter Zijlstra * If the lock has already been acquired, then this will proceed to spin
61c9122da1SPeter Zijlstra * on this node->locked until the previous lock holder sets the node->locked
62c9122da1SPeter Zijlstra * in mcs_spin_unlock().
63c9122da1SPeter Zijlstra */
64c9122da1SPeter Zijlstra static inline
mcs_spin_lock(struct mcs_spinlock ** lock,struct mcs_spinlock * node)65c9122da1SPeter Zijlstra void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
66c9122da1SPeter Zijlstra {
67c9122da1SPeter Zijlstra struct mcs_spinlock *prev;
68c9122da1SPeter Zijlstra
69c9122da1SPeter Zijlstra /* Init node */
70c9122da1SPeter Zijlstra node->locked = 0;
71c9122da1SPeter Zijlstra node->next = NULL;
72c9122da1SPeter Zijlstra
73920c720aSPeter Zijlstra /*
74920c720aSPeter Zijlstra * We rely on the full barrier with global transitivity implied by the
75920c720aSPeter Zijlstra * below xchg() to order the initialization stores above against any
76920c720aSPeter Zijlstra * observation of @node. And to provide the ACQUIRE ordering associated
77920c720aSPeter Zijlstra * with a LOCK primitive.
78920c720aSPeter Zijlstra */
79920c720aSPeter Zijlstra prev = xchg(lock, node);
80c9122da1SPeter Zijlstra if (likely(prev == NULL)) {
81c9122da1SPeter Zijlstra /*
82c9122da1SPeter Zijlstra * Lock acquired, don't need to set node->locked to 1. Threads
83c9122da1SPeter Zijlstra * only spin on its own node->locked value for lock acquisition.
84c9122da1SPeter Zijlstra * However, since this thread can immediately acquire the lock
85c9122da1SPeter Zijlstra * and does not proceed to spin on its own node->locked, this
86c9122da1SPeter Zijlstra * value won't be used. If a debug mode is needed to
87c9122da1SPeter Zijlstra * audit lock status, then set node->locked value here.
88c9122da1SPeter Zijlstra */
89c9122da1SPeter Zijlstra return;
90c9122da1SPeter Zijlstra }
914d3199e4SDavidlohr Bueso WRITE_ONCE(prev->next, node);
92c9122da1SPeter Zijlstra
93c9122da1SPeter Zijlstra /* Wait until the lock holder passes the lock down. */
94c9122da1SPeter Zijlstra arch_mcs_spin_lock_contended(&node->locked);
95c9122da1SPeter Zijlstra }
96c9122da1SPeter Zijlstra
97c9122da1SPeter Zijlstra /*
98c9122da1SPeter Zijlstra * Releases the lock. The caller should pass in the corresponding node that
99c9122da1SPeter Zijlstra * was used to acquire the lock.
100c9122da1SPeter Zijlstra */
101c9122da1SPeter Zijlstra static inline
mcs_spin_unlock(struct mcs_spinlock ** lock,struct mcs_spinlock * node)102c9122da1SPeter Zijlstra void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
103c9122da1SPeter Zijlstra {
1044d3199e4SDavidlohr Bueso struct mcs_spinlock *next = READ_ONCE(node->next);
105c9122da1SPeter Zijlstra
106c9122da1SPeter Zijlstra if (likely(!next)) {
107c9122da1SPeter Zijlstra /*
108c9122da1SPeter Zijlstra * Release the lock by setting it to NULL
109c9122da1SPeter Zijlstra */
1103552a07aSDavidlohr Bueso if (likely(cmpxchg_release(lock, node, NULL) == node))
111c9122da1SPeter Zijlstra return;
112c9122da1SPeter Zijlstra /* Wait until the next pointer is set */
1134d3199e4SDavidlohr Bueso while (!(next = READ_ONCE(node->next)))
114f2f09a4cSChristian Borntraeger cpu_relax();
115c9122da1SPeter Zijlstra }
116c9122da1SPeter Zijlstra
117c9122da1SPeter Zijlstra /* Pass lock to next waiter. */
118c9122da1SPeter Zijlstra arch_mcs_spin_unlock_contended(&next->locked);
119c9122da1SPeter Zijlstra }
120c9122da1SPeter Zijlstra
121c9122da1SPeter Zijlstra #endif /* __LINUX_MCS_SPINLOCK_H */
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