Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 1 | #include <linux/percpu.h> |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 2 | #include <linux/sched.h> |
| 3 | #include "mcs_spinlock.h" |
| 4 | |
| 5 | #ifdef CONFIG_SMP |
| 6 | |
| 7 | /* |
| 8 | * An MCS like lock especially tailored for optimistic spinning for sleeping |
| 9 | * lock implementations (mutex, rwsem, etc). |
| 10 | * |
| 11 | * Using a single mcs node per CPU is safe because sleeping locks should not be |
| 12 | * called from interrupt context and we have preemption disabled while |
| 13 | * spinning. |
| 14 | */ |
Jason Low | 046a619 | 2014-07-14 10:27:48 -0700 | [diff] [blame] | 15 | static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node); |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 16 | |
| 17 | /* |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 18 | * We use the value 0 to represent "no CPU", thus the encoded value |
| 19 | * will be the CPU number incremented by 1. |
| 20 | */ |
| 21 | static inline int encode_cpu(int cpu_nr) |
| 22 | { |
| 23 | return cpu_nr + 1; |
| 24 | } |
| 25 | |
| 26 | static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val) |
| 27 | { |
| 28 | int cpu_nr = encoded_cpu_val - 1; |
| 29 | |
| 30 | return per_cpu_ptr(&osq_node, cpu_nr); |
| 31 | } |
| 32 | |
| 33 | /* |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 34 | * Get a stable @node->next pointer, either for unlock() or unqueue() purposes. |
| 35 | * Can return NULL in case we were the last queued and we updated @lock instead. |
| 36 | */ |
Jason Low | 046a619 | 2014-07-14 10:27:48 -0700 | [diff] [blame] | 37 | static inline struct optimistic_spin_node * |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 38 | osq_wait_next(struct optimistic_spin_queue *lock, |
Jason Low | 046a619 | 2014-07-14 10:27:48 -0700 | [diff] [blame] | 39 | struct optimistic_spin_node *node, |
| 40 | struct optimistic_spin_node *prev) |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 41 | { |
Jason Low | 046a619 | 2014-07-14 10:27:48 -0700 | [diff] [blame] | 42 | struct optimistic_spin_node *next = NULL; |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 43 | int curr = encode_cpu(smp_processor_id()); |
| 44 | int old; |
| 45 | |
| 46 | /* |
| 47 | * If there is a prev node in queue, then the 'old' value will be |
| 48 | * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if |
| 49 | * we're currently last in queue, then the queue will then become empty. |
| 50 | */ |
| 51 | old = prev ? prev->cpu : OSQ_UNLOCKED_VAL; |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 52 | |
| 53 | for (;;) { |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 54 | if (atomic_read(&lock->tail) == curr && |
| 55 | atomic_cmpxchg(&lock->tail, curr, old) == curr) { |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 56 | /* |
| 57 | * We were the last queued, we moved @lock back. @prev |
| 58 | * will now observe @lock and will complete its |
| 59 | * unlock()/unqueue(). |
| 60 | */ |
| 61 | break; |
| 62 | } |
| 63 | |
| 64 | /* |
| 65 | * We must xchg() the @node->next value, because if we were to |
| 66 | * leave it in, a concurrent unlock()/unqueue() from |
| 67 | * @node->next might complete Step-A and think its @prev is |
| 68 | * still valid. |
| 69 | * |
| 70 | * If the concurrent unlock()/unqueue() wins the race, we'll |
| 71 | * wait for either @lock to point to us, through its Step-B, or |
| 72 | * wait for a new @node->next from its Step-C. |
| 73 | */ |
| 74 | if (node->next) { |
| 75 | next = xchg(&node->next, NULL); |
| 76 | if (next) |
| 77 | break; |
| 78 | } |
| 79 | |
Davidlohr Bueso | 3a6bfbc | 2014-06-29 15:09:33 -0700 | [diff] [blame] | 80 | cpu_relax_lowlatency(); |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 81 | } |
| 82 | |
| 83 | return next; |
| 84 | } |
| 85 | |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 86 | bool osq_lock(struct optimistic_spin_queue *lock) |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 87 | { |
Jason Low | 046a619 | 2014-07-14 10:27:48 -0700 | [diff] [blame] | 88 | struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); |
| 89 | struct optimistic_spin_node *prev, *next; |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 90 | int curr = encode_cpu(smp_processor_id()); |
| 91 | int old; |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 92 | |
| 93 | node->locked = 0; |
| 94 | node->next = NULL; |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 95 | node->cpu = curr; |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 96 | |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 97 | old = atomic_xchg(&lock->tail, curr); |
| 98 | if (old == OSQ_UNLOCKED_VAL) |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 99 | return true; |
| 100 | |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 101 | prev = decode_cpu(old); |
| 102 | node->prev = prev; |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 103 | ACCESS_ONCE(prev->next) = node; |
| 104 | |
| 105 | /* |
| 106 | * Normally @prev is untouchable after the above store; because at that |
| 107 | * moment unlock can proceed and wipe the node element from stack. |
| 108 | * |
| 109 | * However, since our nodes are static per-cpu storage, we're |
| 110 | * guaranteed their existence -- this allows us to apply |
| 111 | * cmpxchg in an attempt to undo our queueing. |
| 112 | */ |
| 113 | |
| 114 | while (!smp_load_acquire(&node->locked)) { |
| 115 | /* |
| 116 | * If we need to reschedule bail... so we can block. |
| 117 | */ |
| 118 | if (need_resched()) |
| 119 | goto unqueue; |
| 120 | |
Davidlohr Bueso | 3a6bfbc | 2014-06-29 15:09:33 -0700 | [diff] [blame] | 121 | cpu_relax_lowlatency(); |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 122 | } |
| 123 | return true; |
| 124 | |
| 125 | unqueue: |
| 126 | /* |
| 127 | * Step - A -- stabilize @prev |
| 128 | * |
| 129 | * Undo our @prev->next assignment; this will make @prev's |
| 130 | * unlock()/unqueue() wait for a next pointer since @lock points to us |
| 131 | * (or later). |
| 132 | */ |
| 133 | |
| 134 | for (;;) { |
| 135 | if (prev->next == node && |
| 136 | cmpxchg(&prev->next, node, NULL) == node) |
| 137 | break; |
| 138 | |
| 139 | /* |
| 140 | * We can only fail the cmpxchg() racing against an unlock(), |
| 141 | * in which case we should observe @node->locked becomming |
| 142 | * true. |
| 143 | */ |
| 144 | if (smp_load_acquire(&node->locked)) |
| 145 | return true; |
| 146 | |
Davidlohr Bueso | 3a6bfbc | 2014-06-29 15:09:33 -0700 | [diff] [blame] | 147 | cpu_relax_lowlatency(); |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 148 | |
| 149 | /* |
| 150 | * Or we race against a concurrent unqueue()'s step-B, in which |
| 151 | * case its step-C will write us a new @node->prev pointer. |
| 152 | */ |
| 153 | prev = ACCESS_ONCE(node->prev); |
| 154 | } |
| 155 | |
| 156 | /* |
| 157 | * Step - B -- stabilize @next |
| 158 | * |
| 159 | * Similar to unlock(), wait for @node->next or move @lock from @node |
| 160 | * back to @prev. |
| 161 | */ |
| 162 | |
| 163 | next = osq_wait_next(lock, node, prev); |
| 164 | if (!next) |
| 165 | return false; |
| 166 | |
| 167 | /* |
| 168 | * Step - C -- unlink |
| 169 | * |
| 170 | * @prev is stable because its still waiting for a new @prev->next |
| 171 | * pointer, @next is stable because our @node->next pointer is NULL and |
| 172 | * it will wait in Step-A. |
| 173 | */ |
| 174 | |
| 175 | ACCESS_ONCE(next->prev) = prev; |
| 176 | ACCESS_ONCE(prev->next) = next; |
| 177 | |
| 178 | return false; |
| 179 | } |
| 180 | |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 181 | void osq_unlock(struct optimistic_spin_queue *lock) |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 182 | { |
Jason Low | 33ecd20 | 2014-07-14 10:27:51 -0700 | [diff] [blame] | 183 | struct optimistic_spin_node *node, *next; |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 184 | int curr = encode_cpu(smp_processor_id()); |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 185 | |
| 186 | /* |
| 187 | * Fast path for the uncontended case. |
| 188 | */ |
Jason Low | 9063182 | 2014-07-14 10:27:49 -0700 | [diff] [blame] | 189 | if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr)) |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 190 | return; |
| 191 | |
| 192 | /* |
| 193 | * Second most likely case. |
| 194 | */ |
Jason Low | 33ecd20 | 2014-07-14 10:27:51 -0700 | [diff] [blame] | 195 | node = this_cpu_ptr(&osq_node); |
Peter Zijlstra | fb0527b | 2014-01-29 12:51:42 +0100 | [diff] [blame] | 196 | next = xchg(&node->next, NULL); |
| 197 | if (next) { |
| 198 | ACCESS_ONCE(next->locked) = 1; |
| 199 | return; |
| 200 | } |
| 201 | |
| 202 | next = osq_wait_next(lock, node, NULL); |
| 203 | if (next) |
| 204 | ACCESS_ONCE(next->locked) = 1; |
| 205 | } |
| 206 | |
| 207 | #endif |
| 208 | |