kernel/locking/osq_lock.c

   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          * ACQUIRE semantics, pairs with corresponding RELEASE
  97          * in unlock() uncontended, or fastpath.
  98          */
  99         old = atomic_xchg_acquire(&lock->tail, curr);
 100         if (old == OSQ_UNLOCKED_VAL)
 101                 return true;
 102
 103         prev = decode_cpu(old);
 104         node->prev = prev;
 105         WRITE_ONCE(prev->next, node);
 106
 107         /*
 108          * Normally @prev is untouchable after the above store; because at that
 109          * moment unlock can proceed and wipe the node element from stack.
 110          *
 111          * However, since our nodes are static per-cpu storage, we're
 112          * guaranteed their existence -- this allows us to apply
 113          * cmpxchg in an attempt to undo our queueing.
 114          */
 115
 116         while (!READ_ONCE(node->locked)) {
 117                 /*
 118                  * If we need to reschedule bail... so we can block.
 119                  */
 120                 if (need_resched())
 121                         goto unqueue;
 122
 123                 cpu_relax_lowlatency();
 124         }
 125         return true;
 126
 127 unqueue:
 128         /*
 129          * Step - A  -- stabilize @prev
 130          *
 131          * Undo our @prev->next assignment; this will make @prev's
 132          * unlock()/unqueue() wait for a next pointer since @lock points to us
 133          * (or later).
 134          */
 135
 136         for (;;) {
 137                 if (prev->next == node &&
 138                     cmpxchg(&prev->next, node, NULL) == node)
 139                         break;
 140
 141                 /*
 142                  * We can only fail the cmpxchg() racing against an unlock(),
 143                  * in which case we should observe @node->locked becomming
 144                  * true.
 145                  */
 146                 if (smp_load_acquire(&node->locked))
 147                         return true;
 148
 149                 cpu_relax_lowlatency();
 150
 151                 /*
 152                  * Or we race against a concurrent unqueue()'s step-B, in which
 153                  * case its step-C will write us a new @node->prev pointer.
 154                  */
 155                 prev = READ_ONCE(node->prev);
 156         }
 157
 158         /*
 159          * Step - B -- stabilize @next
 160          *
 161          * Similar to unlock(), wait for @node->next or move @lock from @node
 162          * back to @prev.
 163          */
 164
 165         next = osq_wait_next(lock, node, prev);
 166         if (!next)
 167                 return false;
 168
 169         /*
 170          * Step - C -- unlink
 171          *
 172          * @prev is stable because its still waiting for a new @prev->next
 173          * pointer, @next is stable because our @node->next pointer is NULL and
 174          * it will wait in Step-A.
 175          */
 176
 177         WRITE_ONCE(next->prev, prev);
 178         WRITE_ONCE(prev->next, next);
 179
 180         return false;
 181 }
 182
 183 void osq_unlock(struct optimistic_spin_queue *lock)
 184 {
 185         struct optimistic_spin_node *node, *next;
 186         int curr = encode_cpu(smp_processor_id());
 187
 188         /*
 189          * Fast path for the uncontended case.
 190          */
 191         if (likely(atomic_cmpxchg_release(&lock->tail, curr,
 192                                           OSQ_UNLOCKED_VAL) == curr))
 193                 return;
 194
 195         /*
 196          * Second most likely case.
 197          */
 198         node = this_cpu_ptr(&osq_node);
 199         next = xchg(&node->next, NULL);
 200         if (next) {
 201                 WRITE_ONCE(next->locked, 1);
 202                 return;
 203         }
 204
 205         next = osq_wait_next(lock, node, NULL);
 206         if (next)
 207                 WRITE_ONCE(next->locked, 1);
 208 }