1 /* SPDX-License-Identifier: GPL-2.0 */ 2 /* 3 * MCS lock defines 4 * 5 * This file contains the main data structure and API definitions of MCS lock. 6 * 7 * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock 8 * with the desirable properties of being fair, and with each cpu trying 9 * to acquire the lock spinning on a local variable. 10 * It avoids expensive cache bouncings that common test-and-set spin-lock 11 * implementations incur. 12 */ 13 #ifndef __LINUX_MCS_SPINLOCK_H 14 #define __LINUX_MCS_SPINLOCK_H 15 16 #include <asm/mcs_spinlock.h> 17 18 struct mcs_spinlock { 19 struct mcs_spinlock *next; 20 int locked; /* 1 if lock acquired */ 21 int count; /* nesting count, see qspinlock.c */ 22 }; 23 24 #ifndef arch_mcs_spin_lock_contended 25 /* 26 * Using smp_cond_load_acquire() provides the acquire semantics 27 * required so that subsequent operations happen after the 28 * lock is acquired. Additionally, some architectures such as 29 * ARM64 would like to do spin-waiting instead of purely 30 * spinning, and smp_cond_load_acquire() provides that behavior. 31 */ 32 #define arch_mcs_spin_lock_contended(l) \ 33 do { \ 34 smp_cond_load_acquire(l, VAL); \ 35 } while (0) 36 #endif 37 38 #ifndef arch_mcs_spin_unlock_contended 39 /* 40 * smp_store_release() provides a memory barrier to ensure all 41 * operations in the critical section has been completed before 42 * unlocking. 43 */ 44 #define arch_mcs_spin_unlock_contended(l) \ 45 smp_store_release((l), 1) 46 #endif 47 48 /* 49 * Note: the smp_load_acquire/smp_store_release pair is not 50 * sufficient to form a full memory barrier across 51 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock. 52 * For applications that need a full barrier across multiple cpus 53 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be 54 * used after mcs_lock. 55 */ 56 57 /* 58 * In order to acquire the lock, the caller should declare a local node and 59 * pass a reference of the node to this function in addition to the lock. 60 * If the lock has already been acquired, then this will proceed to spin 61 * on this node->locked until the previous lock holder sets the node->locked 62 * in mcs_spin_unlock(). 63 */ 64 static inline 65 void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) 66 { 67 struct mcs_spinlock *prev; 68 69 /* Init node */ 70 node->locked = 0; 71 node->next = NULL; 72 73 /* 74 * We rely on the full barrier with global transitivity implied by the 75 * below xchg() to order the initialization stores above against any 76 * observation of @node. And to provide the ACQUIRE ordering associated 77 * with a LOCK primitive. 78 */ 79 prev = xchg(lock, node); 80 if (likely(prev == NULL)) { 81 /* 82 * Lock acquired, don't need to set node->locked to 1. Threads 83 * only spin on its own node->locked value for lock acquisition. 84 * However, since this thread can immediately acquire the lock 85 * and does not proceed to spin on its own node->locked, this 86 * value won't be used. If a debug mode is needed to 87 * audit lock status, then set node->locked value here. 88 */ 89 return; 90 } 91 WRITE_ONCE(prev->next, node); 92 93 /* Wait until the lock holder passes the lock down. */ 94 arch_mcs_spin_lock_contended(&node->locked); 95 } 96 97 /* 98 * Releases the lock. The caller should pass in the corresponding node that 99 * was used to acquire the lock. 100 */ 101 static inline 102 void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) 103 { 104 struct mcs_spinlock *next = READ_ONCE(node->next); 105 106 if (likely(!next)) { 107 /* 108 * Release the lock by setting it to NULL 109 */ 110 if (likely(cmpxchg_release(lock, node, NULL) == node)) 111 return; 112 /* Wait until the next pointer is set */ 113 while (!(next = READ_ONCE(node->next))) 114 cpu_relax(); 115 } 116 117 /* Pass lock to next waiter. */ 118 arch_mcs_spin_unlock_contended(&next->locked); 119 } 120 121 #endif /* __LINUX_MCS_SPINLOCK_H */ 122