1 #include <linux/percpu.h> 2 #include <linux/sched.h> 3 #include <linux/osq_lock.h> 4 5 /* 6 * An MCS like lock especially tailored for optimistic spinning for sleeping 7 * lock implementations (mutex, rwsem, etc). 8 * 9 * Using a single mcs node per CPU is safe because sleeping locks should not be 10 * called from interrupt context and we have preemption disabled while 11 * spinning. 12 */ 13 static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node); 14 15 /* 16 * We use the value 0 to represent "no CPU", thus the encoded value 17 * will be the CPU number incremented by 1. 18 */ 19 static inline int encode_cpu(int cpu_nr) 20 { 21 return cpu_nr + 1; 22 } 23 24 static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) 25 { 26 int cpu_nr = encoded_cpu_val - 1; 27 28 return per_cpu_ptr(&osq_node, cpu_nr); 29 } 30 31 /* 32 * Get a stable @node->next pointer, either for unlock() or unqueue() purposes. 33 * Can return NULL in case we were the last queued and we updated @lock instead. 34 */ 35 static inline struct optimistic_spin_node * 36 osq_wait_next(struct optimistic_spin_queue *lock, 37 struct optimistic_spin_node *node, 38 struct optimistic_spin_node *prev) 39 { 40 struct optimistic_spin_node *next = NULL; 41 int curr = encode_cpu(smp_processor_id()); 42 int old; 43 44 /* 45 * If there is a prev node in queue, then the 'old' value will be 46 * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if 47 * we're currently last in queue, then the queue will then become empty. 48 */ 49 old = prev ? prev->cpu : OSQ_UNLOCKED_VAL; 50 51 for (;;) { 52 if (atomic_read(&lock->tail) == curr && 53 atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) { 54 /* 55 * We were the last queued, we moved @lock back. @prev 56 * will now observe @lock and will complete its 57 * unlock()/unqueue(). 58 */ 59 break; 60 } 61 62 /* 63 * We must xchg() the @node->next value, because if we were to 64 * leave it in, a concurrent unlock()/unqueue() from 65 * @node->next might complete Step-A and think its @prev is 66 * still valid. 67 * 68 * If the concurrent unlock()/unqueue() wins the race, we'll 69 * wait for either @lock to point to us, through its Step-B, or 70 * wait for a new @node->next from its Step-C. 71 */ 72 if (node->next) { 73 next = xchg(&node->next, NULL); 74 if (next) 75 break; 76 } 77 78 cpu_relax_lowlatency(); 79 } 80 81 return next; 82 } 83 84 bool osq_lock(struct optimistic_spin_queue *lock) 85 { 86 struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); 87 struct optimistic_spin_node *prev, *next; 88 int curr = encode_cpu(smp_processor_id()); 89 int old; 90 91 node->locked = 0; 92 node->next = NULL; 93 node->cpu = curr; 94 95 /* 96 * We need both ACQUIRE (pairs with corresponding RELEASE in 97 * unlock() uncontended, or fastpath) and RELEASE (to publish 98 * the node fields we just initialised) semantics when updating 99 * the lock tail. 100 */ 101 old = atomic_xchg(&lock->tail, curr); 102 if (old == OSQ_UNLOCKED_VAL) 103 return true; 104 105 prev = decode_cpu(old); 106 node->prev = prev; 107 WRITE_ONCE(prev->next, node); 108 109 /* 110 * Normally @prev is untouchable after the above store; because at that 111 * moment unlock can proceed and wipe the node element from stack. 112 * 113 * However, since our nodes are static per-cpu storage, we're 114 * guaranteed their existence -- this allows us to apply 115 * cmpxchg in an attempt to undo our queueing. 116 */ 117 118 while (!READ_ONCE(node->locked)) { 119 /* 120 * If we need to reschedule bail... so we can block. 121 */ 122 if (need_resched()) 123 goto unqueue; 124 125 cpu_relax_lowlatency(); 126 } 127 return true; 128 129 unqueue: 130 /* 131 * Step - A -- stabilize @prev 132 * 133 * Undo our @prev->next assignment; this will make @prev's 134 * unlock()/unqueue() wait for a next pointer since @lock points to us 135 * (or later). 136 */ 137 138 for (;;) { 139 if (prev->next == node && 140 cmpxchg(&prev->next, node, NULL) == node) 141 break; 142 143 /* 144 * We can only fail the cmpxchg() racing against an unlock(), 145 * in which case we should observe @node->locked becomming 146 * true. 147 */ 148 if (smp_load_acquire(&node->locked)) 149 return true; 150 151 cpu_relax_lowlatency(); 152 153 /* 154 * Or we race against a concurrent unqueue()'s step-B, in which 155 * case its step-C will write us a new @node->prev pointer. 156 */ 157 prev = READ_ONCE(node->prev); 158 } 159 160 /* 161 * Step - B -- stabilize @next 162 * 163 * Similar to unlock(), wait for @node->next or move @lock from @node 164 * back to @prev. 165 */ 166 167 next = osq_wait_next(lock, node, prev); 168 if (!next) 169 return false; 170 171 /* 172 * Step - C -- unlink 173 * 174 * @prev is stable because its still waiting for a new @prev->next 175 * pointer, @next is stable because our @node->next pointer is NULL and 176 * it will wait in Step-A. 177 */ 178 179 WRITE_ONCE(next->prev, prev); 180 WRITE_ONCE(prev->next, next); 181 182 return false; 183 } 184 185 void osq_unlock(struct optimistic_spin_queue *lock) 186 { 187 struct optimistic_spin_node *node, *next; 188 int curr = encode_cpu(smp_processor_id()); 189 190 /* 191 * Fast path for the uncontended case. 192 */ 193 if (likely(atomic_cmpxchg_release(&lock->tail, curr, 194 OSQ_UNLOCKED_VAL) == curr)) 195 return; 196 197 /* 198 * Second most likely case. 199 */ 200 node = this_cpu_ptr(&osq_node); 201 next = xchg(&node->next, NULL); 202 if (next) { 203 WRITE_ONCE(next->locked, 1); 204 return; 205 } 206 207 next = osq_wait_next(lock, node, NULL); 208 if (next) 209 WRITE_ONCE(next->locked, 1); 210 } 211