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_load_acquire() provides a memory barrier that ensures 27 * subsequent operations happen after the lock is acquired. 28 */ 29 #define arch_mcs_spin_lock_contended(l) \ 30 do { \ 31 while (!(smp_load_acquire(l))) \ 32 cpu_relax(); \ 33 } while (0) 34 #endif 35 36 #ifndef arch_mcs_spin_unlock_contended 37 /* 38 * smp_store_release() provides a memory barrier to ensure all 39 * operations in the critical section has been completed before 40 * unlocking. 41 */ 42 #define arch_mcs_spin_unlock_contended(l) \ 43 smp_store_release((l), 1) 44 #endif 45 46 /* 47 * Note: the smp_load_acquire/smp_store_release pair is not 48 * sufficient to form a full memory barrier across 49 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock. 50 * For applications that need a full barrier across multiple cpus 51 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be 52 * used after mcs_lock. 53 */ 54 55 /* 56 * In order to acquire the lock, the caller should declare a local node and 57 * pass a reference of the node to this function in addition to the lock. 58 * If the lock has already been acquired, then this will proceed to spin 59 * on this node->locked until the previous lock holder sets the node->locked 60 * in mcs_spin_unlock(). 61 */ 62 static inline 63 void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) 64 { 65 struct mcs_spinlock *prev; 66 67 /* Init node */ 68 node->locked = 0; 69 node->next = NULL; 70 71 /* 72 * We rely on the full barrier with global transitivity implied by the 73 * below xchg() to order the initialization stores above against any 74 * observation of @node. And to provide the ACQUIRE ordering associated 75 * with a LOCK primitive. 76 */ 77 prev = xchg(lock, node); 78 if (likely(prev == NULL)) { 79 /* 80 * Lock acquired, don't need to set node->locked to 1. Threads 81 * only spin on its own node->locked value for lock acquisition. 82 * However, since this thread can immediately acquire the lock 83 * and does not proceed to spin on its own node->locked, this 84 * value won't be used. If a debug mode is needed to 85 * audit lock status, then set node->locked value here. 86 */ 87 return; 88 } 89 WRITE_ONCE(prev->next, node); 90 91 /* Wait until the lock holder passes the lock down. */ 92 arch_mcs_spin_lock_contended(&node->locked); 93 } 94 95 /* 96 * Releases the lock. The caller should pass in the corresponding node that 97 * was used to acquire the lock. 98 */ 99 static inline 100 void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) 101 { 102 struct mcs_spinlock *next = READ_ONCE(node->next); 103 104 if (likely(!next)) { 105 /* 106 * Release the lock by setting it to NULL 107 */ 108 if (likely(cmpxchg_release(lock, node, NULL) == node)) 109 return; 110 /* Wait until the next pointer is set */ 111 while (!(next = READ_ONCE(node->next))) 112 cpu_relax(); 113 } 114 115 /* Pass lock to next waiter. */ 116 arch_mcs_spin_unlock_contended(&next->locked); 117 } 118 119 #endif /* __LINUX_MCS_SPINLOCK_H */ 120