1 /* 2 * MCS lock defines 3 * 4 * This file contains the main data structure and API definitions of MCS lock. 5 * 6 * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock 7 * with the desirable properties of being fair, and with each cpu trying 8 * to acquire the lock spinning on a local variable. 9 * It avoids expensive cache bouncings that common test-and-set spin-lock 10 * implementations incur. 11 */ 12 #ifndef __LINUX_MCS_SPINLOCK_H 13 #define __LINUX_MCS_SPINLOCK_H 14 15 #include <asm/mcs_spinlock.h> 16 17 struct mcs_spinlock { 18 struct mcs_spinlock *next; 19 int locked; /* 1 if lock acquired */ 20 }; 21 22 #ifndef arch_mcs_spin_lock_contended 23 /* 24 * Using smp_load_acquire() provides a memory barrier that ensures 25 * subsequent operations happen after the lock is acquired. 26 */ 27 #define arch_mcs_spin_lock_contended(l) \ 28 do { \ 29 while (!(smp_load_acquire(l))) \ 30 cpu_relax_lowlatency(); \ 31 } while (0) 32 #endif 33 34 #ifndef arch_mcs_spin_unlock_contended 35 /* 36 * smp_store_release() provides a memory barrier to ensure all 37 * operations in the critical section has been completed before 38 * unlocking. 39 */ 40 #define arch_mcs_spin_unlock_contended(l) \ 41 smp_store_release((l), 1) 42 #endif 43 44 /* 45 * Note: the smp_load_acquire/smp_store_release pair is not 46 * sufficient to form a full memory barrier across 47 * cpus for many architectures (except x86) for mcs_unlock and mcs_lock. 48 * For applications that need a full barrier across multiple cpus 49 * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be 50 * used after mcs_lock. 51 */ 52 53 /* 54 * In order to acquire the lock, the caller should declare a local node and 55 * pass a reference of the node to this function in addition to the lock. 56 * If the lock has already been acquired, then this will proceed to spin 57 * on this node->locked until the previous lock holder sets the node->locked 58 * in mcs_spin_unlock(). 59 * 60 * We don't inline mcs_spin_lock() so that perf can correctly account for the 61 * time spent in this lock function. 62 */ 63 static inline 64 void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) 65 { 66 struct mcs_spinlock *prev; 67 68 /* Init node */ 69 node->locked = 0; 70 node->next = NULL; 71 72 prev = xchg(lock, node); 73 if (likely(prev == NULL)) { 74 /* 75 * Lock acquired, don't need to set node->locked to 1. Threads 76 * only spin on its own node->locked value for lock acquisition. 77 * However, since this thread can immediately acquire the lock 78 * and does not proceed to spin on its own node->locked, this 79 * value won't be used. If a debug mode is needed to 80 * audit lock status, then set node->locked value here. 81 */ 82 return; 83 } 84 ACCESS_ONCE(prev->next) = node; 85 86 /* Wait until the lock holder passes the lock down. */ 87 arch_mcs_spin_lock_contended(&node->locked); 88 } 89 90 /* 91 * Releases the lock. The caller should pass in the corresponding node that 92 * was used to acquire the lock. 93 */ 94 static inline 95 void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node) 96 { 97 struct mcs_spinlock *next = ACCESS_ONCE(node->next); 98 99 if (likely(!next)) { 100 /* 101 * Release the lock by setting it to NULL 102 */ 103 if (likely(cmpxchg(lock, node, NULL) == node)) 104 return; 105 /* Wait until the next pointer is set */ 106 while (!(next = ACCESS_ONCE(node->next))) 107 cpu_relax_lowlatency(); 108 } 109 110 /* Pass lock to next waiter. */ 111 arch_mcs_spin_unlock_contended(&next->locked); 112 } 113 114 /* 115 * Cancellable version of the MCS lock above. 116 * 117 * Intended for adaptive spinning of sleeping locks: 118 * mutex_lock()/rwsem_down_{read,write}() etc. 119 */ 120 121 struct optimistic_spin_node { 122 struct optimistic_spin_node *next, *prev; 123 int locked; /* 1 if lock acquired */ 124 int cpu; /* encoded CPU # value */ 125 }; 126 127 extern bool osq_lock(struct optimistic_spin_queue *lock); 128 extern void osq_unlock(struct optimistic_spin_queue *lock); 129 130 #endif /* __LINUX_MCS_SPINLOCK_H */ 131