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(&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 old = atomic_xchg(&lock->tail, curr); 96 if (old == OSQ_UNLOCKED_VAL) 97 return true; 98 99 prev = decode_cpu(old); 100 node->prev = prev; 101 WRITE_ONCE(prev->next, node); 102 103 /* 104 * Normally @prev is untouchable after the above store; because at that 105 * moment unlock can proceed and wipe the node element from stack. 106 * 107 * However, since our nodes are static per-cpu storage, we're 108 * guaranteed their existence -- this allows us to apply 109 * cmpxchg in an attempt to undo our queueing. 110 */ 111 112 while (!READ_ONCE(node->locked)) { 113 /* 114 * If we need to reschedule bail... so we can block. 115 */ 116 if (need_resched()) 117 goto unqueue; 118 119 cpu_relax_lowlatency(); 120 } 121 return true; 122 123 unqueue: 124 /* 125 * Step - A -- stabilize @prev 126 * 127 * Undo our @prev->next assignment; this will make @prev's 128 * unlock()/unqueue() wait for a next pointer since @lock points to us 129 * (or later). 130 */ 131 132 for (;;) { 133 if (prev->next == node && 134 cmpxchg(&prev->next, node, NULL) == node) 135 break; 136 137 /* 138 * We can only fail the cmpxchg() racing against an unlock(), 139 * in which case we should observe @node->locked becomming 140 * true. 141 */ 142 if (smp_load_acquire(&node->locked)) 143 return true; 144 145 cpu_relax_lowlatency(); 146 147 /* 148 * Or we race against a concurrent unqueue()'s step-B, in which 149 * case its step-C will write us a new @node->prev pointer. 150 */ 151 prev = READ_ONCE(node->prev); 152 } 153 154 /* 155 * Step - B -- stabilize @next 156 * 157 * Similar to unlock(), wait for @node->next or move @lock from @node 158 * back to @prev. 159 */ 160 161 next = osq_wait_next(lock, node, prev); 162 if (!next) 163 return false; 164 165 /* 166 * Step - C -- unlink 167 * 168 * @prev is stable because its still waiting for a new @prev->next 169 * pointer, @next is stable because our @node->next pointer is NULL and 170 * it will wait in Step-A. 171 */ 172 173 WRITE_ONCE(next->prev, prev); 174 WRITE_ONCE(prev->next, next); 175 176 return false; 177 } 178 179 void osq_unlock(struct optimistic_spin_queue *lock) 180 { 181 struct optimistic_spin_node *node, *next; 182 int curr = encode_cpu(smp_processor_id()); 183 184 /* 185 * Fast path for the uncontended case. 186 */ 187 if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr)) 188 return; 189 190 /* 191 * Second most likely case. 192 */ 193 node = this_cpu_ptr(&osq_node); 194 next = xchg(&node->next, NULL); 195 if (next) { 196 WRITE_ONCE(next->locked, 1); 197 return; 198 } 199 200 next = osq_wait_next(lock, node, NULL); 201 if (next) 202 WRITE_ONCE(next->locked, 1); 203 } 204