1 // SPDX-License-Identifier: GPL-2.0 2 #include <linux/percpu.h> 3 #include <linux/sched.h> 4 #include <linux/osq_lock.h> 5 6 /* 7 * An MCS like lock especially tailored for optimistic spinning for sleeping 8 * lock implementations (mutex, rwsem, etc). 9 * 10 * Using a single mcs node per CPU is safe because sleeping locks should not be 11 * called from interrupt context and we have preemption disabled while 12 * spinning. 13 */ 14 static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node); 15 16 /* 17 * We use the value 0 to represent "no CPU", thus the encoded value 18 * will be the CPU number incremented by 1. 19 */ 20 static inline int encode_cpu(int cpu_nr) 21 { 22 return cpu_nr + 1; 23 } 24 25 static inline int node_cpu(struct optimistic_spin_node *node) 26 { 27 return node->cpu - 1; 28 } 29 30 static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) 31 { 32 int cpu_nr = encoded_cpu_val - 1; 33 34 return per_cpu_ptr(&osq_node, cpu_nr); 35 } 36 37 /* 38 * Get a stable @node->next pointer, either for unlock() or unqueue() purposes. 39 * Can return NULL in case we were the last queued and we updated @lock instead. 40 */ 41 static inline struct optimistic_spin_node * 42 osq_wait_next(struct optimistic_spin_queue *lock, 43 struct optimistic_spin_node *node, 44 struct optimistic_spin_node *prev) 45 { 46 struct optimistic_spin_node *next = NULL; 47 int curr = encode_cpu(smp_processor_id()); 48 int old; 49 50 /* 51 * If there is a prev node in queue, then the 'old' value will be 52 * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if 53 * we're currently last in queue, then the queue will then become empty. 54 */ 55 old = prev ? prev->cpu : OSQ_UNLOCKED_VAL; 56 57 for (;;) { 58 if (atomic_read(&lock->tail) == curr && 59 atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) { 60 /* 61 * We were the last queued, we moved @lock back. @prev 62 * will now observe @lock and will complete its 63 * unlock()/unqueue(). 64 */ 65 break; 66 } 67 68 /* 69 * We must xchg() the @node->next value, because if we were to 70 * leave it in, a concurrent unlock()/unqueue() from 71 * @node->next might complete Step-A and think its @prev is 72 * still valid. 73 * 74 * If the concurrent unlock()/unqueue() wins the race, we'll 75 * wait for either @lock to point to us, through its Step-B, or 76 * wait for a new @node->next from its Step-C. 77 */ 78 if (node->next) { 79 next = xchg(&node->next, NULL); 80 if (next) 81 break; 82 } 83 84 cpu_relax(); 85 } 86 87 return next; 88 } 89 90 bool osq_lock(struct optimistic_spin_queue *lock) 91 { 92 struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); 93 struct optimistic_spin_node *prev, *next; 94 int curr = encode_cpu(smp_processor_id()); 95 int old; 96 97 node->locked = 0; 98 node->next = NULL; 99 node->cpu = curr; 100 101 /* 102 * We need both ACQUIRE (pairs with corresponding RELEASE in 103 * unlock() uncontended, or fastpath) and RELEASE (to publish 104 * the node fields we just initialised) semantics when updating 105 * the lock tail. 106 */ 107 old = atomic_xchg(&lock->tail, curr); 108 if (old == OSQ_UNLOCKED_VAL) 109 return true; 110 111 prev = decode_cpu(old); 112 node->prev = prev; 113 114 /* 115 * osq_lock() unqueue 116 * 117 * node->prev = prev osq_wait_next() 118 * WMB MB 119 * prev->next = node next->prev = prev // unqueue-C 120 * 121 * Here 'node->prev' and 'next->prev' are the same variable and we need 122 * to ensure these stores happen in-order to avoid corrupting the list. 123 */ 124 smp_wmb(); 125 126 WRITE_ONCE(prev->next, node); 127 128 /* 129 * Normally @prev is untouchable after the above store; because at that 130 * moment unlock can proceed and wipe the node element from stack. 131 * 132 * However, since our nodes are static per-cpu storage, we're 133 * guaranteed their existence -- this allows us to apply 134 * cmpxchg in an attempt to undo our queueing. 135 */ 136 137 while (!READ_ONCE(node->locked)) { 138 /* 139 * If we need to reschedule bail... so we can block. 140 * Use vcpu_is_preempted() to avoid waiting for a preempted 141 * lock holder: 142 */ 143 if (need_resched() || vcpu_is_preempted(node_cpu(node->prev))) 144 goto unqueue; 145 146 cpu_relax(); 147 } 148 return true; 149 150 unqueue: 151 /* 152 * Step - A -- stabilize @prev 153 * 154 * Undo our @prev->next assignment; this will make @prev's 155 * unlock()/unqueue() wait for a next pointer since @lock points to us 156 * (or later). 157 */ 158 159 for (;;) { 160 if (prev->next == node && 161 cmpxchg(&prev->next, node, NULL) == node) 162 break; 163 164 /* 165 * We can only fail the cmpxchg() racing against an unlock(), 166 * in which case we should observe @node->locked becomming 167 * true. 168 */ 169 if (smp_load_acquire(&node->locked)) 170 return true; 171 172 cpu_relax(); 173 174 /* 175 * Or we race against a concurrent unqueue()'s step-B, in which 176 * case its step-C will write us a new @node->prev pointer. 177 */ 178 prev = READ_ONCE(node->prev); 179 } 180 181 /* 182 * Step - B -- stabilize @next 183 * 184 * Similar to unlock(), wait for @node->next or move @lock from @node 185 * back to @prev. 186 */ 187 188 next = osq_wait_next(lock, node, prev); 189 if (!next) 190 return false; 191 192 /* 193 * Step - C -- unlink 194 * 195 * @prev is stable because its still waiting for a new @prev->next 196 * pointer, @next is stable because our @node->next pointer is NULL and 197 * it will wait in Step-A. 198 */ 199 200 WRITE_ONCE(next->prev, prev); 201 WRITE_ONCE(prev->next, next); 202 203 return false; 204 } 205 206 void osq_unlock(struct optimistic_spin_queue *lock) 207 { 208 struct optimistic_spin_node *node, *next; 209 int curr = encode_cpu(smp_processor_id()); 210 211 /* 212 * Fast path for the uncontended case. 213 */ 214 if (likely(atomic_cmpxchg_release(&lock->tail, curr, 215 OSQ_UNLOCKED_VAL) == curr)) 216 return; 217 218 /* 219 * Second most likely case. 220 */ 221 node = this_cpu_ptr(&osq_node); 222 next = xchg(&node->next, NULL); 223 if (next) { 224 WRITE_ONCE(next->locked, 1); 225 return; 226 } 227 228 next = osq_wait_next(lock, node, NULL); 229 if (next) 230 WRITE_ONCE(next->locked, 1); 231 } 232