libgo/runtime/lock_futex.c

   1 // Copyright 2011 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 // +build freebsd linux
   6
   7 #include "runtime.h"
   8
   9 // This implementation depends on OS-specific implementations of
  10 //
  11 //      runtime_futexsleep(uint32 *addr, uint32 val, int64 ns)
  12 //              Atomically,
  13 //                      if(*addr == val) sleep
  14 //              Might be woken up spuriously; that's allowed.
  15 //              Don't sleep longer than ns; ns < 0 means forever.
  16 //
  17 //      runtime_futexwakeup(uint32 *addr, uint32 cnt)
  18 //              If any procs are sleeping on addr, wake up at most cnt.
  19
  20 enum
  21 {
  22         MUTEX_UNLOCKED = 0,
  23         MUTEX_LOCKED = 1,
  24         MUTEX_SLEEPING = 2,
  25
  26         ACTIVE_SPIN = 4,
  27         ACTIVE_SPIN_CNT = 30,
  28         PASSIVE_SPIN = 1,
  29 };
  30
  31 // Possible lock states are MUTEX_UNLOCKED, MUTEX_LOCKED and MUTEX_SLEEPING.
  32 // MUTEX_SLEEPING means that there is presumably at least one sleeping thread.
  33 // Note that there can be spinning threads during all states - they do not
  34 // affect mutex's state.
  35 void
  36 runtime_lock(Lock *l)
  37 {
  38         uint32 i, v, wait, spin;
  39
  40         if(runtime_m()->locks++ < 0)
  41                 runtime_throw("runtime_lock: lock count");
  42
  43         // Speculative grab for lock.
  44         v = runtime_xchg(&l->key, MUTEX_LOCKED);
  45         if(v == MUTEX_UNLOCKED)
  46                 return;
  47
  48         // wait is either MUTEX_LOCKED or MUTEX_SLEEPING
  49         // depending on whether there is a thread sleeping
  50         // on this mutex.  If we ever change l->key from
  51         // MUTEX_SLEEPING to some other value, we must be
  52         // careful to change it back to MUTEX_SLEEPING before
  53         // returning, to ensure that the sleeping thread gets
  54         // its wakeup call.
  55         wait = v;
  56
  57         // On uniprocessor's, no point spinning.
  58         // On multiprocessors, spin for ACTIVE_SPIN attempts.
  59         spin = 0;
  60         if(runtime_ncpu > 1)
  61                 spin = ACTIVE_SPIN;
  62
  63         for(;;) {
  64                 // Try for lock, spinning.
  65                 for(i = 0; i < spin; i++) {
  66                         while(l->key == MUTEX_UNLOCKED)
  67                                 if(runtime_cas(&l->key, MUTEX_UNLOCKED, wait))
  68                                         return;
  69                         runtime_procyield(ACTIVE_SPIN_CNT);
  70                 }
  71
  72                 // Try for lock, rescheduling.
  73                 for(i=0; i < PASSIVE_SPIN; i++) {
  74                         while(l->key == MUTEX_UNLOCKED)
  75                                 if(runtime_cas(&l->key, MUTEX_UNLOCKED, wait))
  76                                         return;
  77                         runtime_osyield();
  78                 }
  79
  80                 // Sleep.
  81                 v = runtime_xchg(&l->key, MUTEX_SLEEPING);
  82                 if(v == MUTEX_UNLOCKED)
  83                         return;
  84                 wait = MUTEX_SLEEPING;
  85                 runtime_futexsleep(&l->key, MUTEX_SLEEPING, -1);
  86         }
  87 }
  88
  89 void
  90 runtime_unlock(Lock *l)
  91 {
  92         uint32 v;
  93
  94         if(--runtime_m()->locks < 0)
  95                 runtime_throw("runtime_unlock: lock count");
  96
  97         v = runtime_xchg(&l->key, MUTEX_UNLOCKED);
  98         if(v == MUTEX_UNLOCKED)
  99                 runtime_throw("unlock of unlocked lock");
 100         if(v == MUTEX_SLEEPING)
 101                 runtime_futexwakeup(&l->key, 1);
 102 }
 103
 104 // One-time notifications.
 105 void
 106 runtime_noteclear(Note *n)
 107 {
 108         n->key = 0;
 109 }
 110
 111 void
 112 runtime_notewakeup(Note *n)
 113 {
 114         runtime_xchg(&n->key, 1);
 115         runtime_futexwakeup(&n->key, 1);
 116 }
 117
 118 void
 119 runtime_notesleep(Note *n)
 120 {
 121         while(runtime_atomicload(&n->key) == 0)
 122                 runtime_futexsleep(&n->key, 0, -1);
 123 }
 124
 125 void
 126 runtime_notetsleep(Note *n, int64 ns)
 127 {
 128         int64 deadline, now;
 129
 130         if(ns < 0) {
 131                 runtime_notesleep(n);
 132                 return;
 133         }
 134
 135         if(runtime_atomicload(&n->key) != 0)
 136                 return;
 137
 138         deadline = runtime_nanotime() + ns;
 139         for(;;) {
 140                 runtime_futexsleep(&n->key, 0, ns);
 141                 if(runtime_atomicload(&n->key) != 0)
 142                         return;
 143                 now = runtime_nanotime();
 144                 if(now >= deadline)
 145                         return;
 146                 ns = deadline - now;
 147         }
 148 }