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