benchmark/mcs-lock/mcs-lock.h

   1 // mcs on stack
   2
   3 #include <stdatomic.h>
   4 #include <unrelacy.h>
   5
   6 #include <spec_lib.h>
   7 #include <stdlib.h>
   8 #include <cdsannotate.h>
   9 #include <specannotation.h>
  10 #include <model_memory.h>
  11 #include "common.h"
  12
  13 struct mcs_node {
  14         std::atomic<mcs_node *> next;
  15         std::atomic<int> gate;
  16
  17         mcs_node() {
  18                 next.store(0);
  19                 gate.store(0);
  20         }
  21 };
  22
  23
  24 struct mcs_mutex {
  25 public:
  26         // tail is null when lock is not held
  27         std::atomic<mcs_node *> m_tail;
  28
  29         mcs_mutex() {
  30                 /**
  31                         @Begin
  32                 @Entry_point
  33                         @End
  34                 */
  35                 m_tail.store( NULL );
  36         }
  37         ~mcs_mutex() {
  38                 //ASSERT( m_tail.load() == NULL );
  39         }
  40
  41         // Each thread will have their own guard.
  42         class guard {
  43         public:
  44                 mcs_mutex * m_t;
  45                 mcs_node    m_node; // node held on the stack
  46
  47                 guard(mcs_mutex * t) : m_t(t) { t->lock(this); }
  48                 ~guard() { m_t->unlock(this); }
  49         };
  50
  51         /**
  52                 @Begin
  53                 @Options:
  54                         LANG = CPP;
  55                         CLASS = mcs_mutex;
  56                 @Global_define:
  57                         @DeclareVar:
  58                                 bool _lock_acquired;
  59                         @InitVar:
  60                                 _lock_acquired = false;
  61                 @Happens_before:
  62                         Unlock -> Lock
  63                 @End
  64         */
  65
  66         /**
  67                 @Begin
  68                 @Interface: Lock
  69                 @Commit_point_set: Lock_Enqueue_Point1 | Lock_Enqueue_Point2
  70                 @Check:
  71                         _lock_acquired == false;
  72                 @Action:
  73                         _lock_acquired = true;
  74                 @End
  75         */
  76         void lock(guard * I) {
  77                 mcs_node * me = &(I->m_node);
  78
  79                 // set up my node :
  80                 // not published yet so relaxed :
  81                 me->next.store(NULL, std::mo_relaxed );
  82                 me->gate.store(1, std::mo_relaxed );
  83
  84                 // publish my node as the new tail :
  85                 mcs_node * pred = m_tail.exchange(me, std::mo_acq_rel);
  86                 /**
  87                         @Begin
  88                         @Commit_point_define_check: pred == NULL
  89                         @Label: Lock_Enqueue_Point1
  90                         @End
  91                 */
  92                 if ( pred != NULL ) {
  93                         // (*1) race here
  94                         // unlock of pred can see me in the tail before I fill next
  95
  96                         // publish me to previous lock-holder :
  97                         pred->next.store(me, std::mo_release );
  98
  99                         // (*2) pred not touched any more
 100
 101                         // now this is the spin -
 102                         // wait on predecessor setting my flag -
 103                         rl::linear_backoff bo;
 104                         int my_gate = 1;
 105                         while (my_gate ) {
 106                                 my_gate = me->gate.load(std::mo_acquire);
 107                                 /**
 108                                         @Begin
 109                                         @Commit_point_define_check: my_gate == 0
 110                                         @Label: Lock_Enqueue_Point2
 111                                         @End
 112                                 */
 113                                 thrd_yield();
 114                         }
 115                 }
 116         }
 117
 118         /**
 119                 @Begin
 120                 @Interface: Unlock
 121                 @Commit_point_set:
 122                         Unlock_Point_Success_1 | Unlock_Point_Success_2
 123                 @Check:
 124                         _lock_acquired == true
 125                 @Action:
 126                         _lock_acquired = false;
 127                 @End
 128         */
 129         void unlock(guard * I) {
 130                 mcs_node * me = &(I->m_node);
 131
 132                 mcs_node * next = me->next.load(std::mo_acquire);
 133                 if ( next == NULL )
 134                 {
 135                         mcs_node * tail_was_me = me;
 136                         bool success;
 137                         success = m_tail.compare_exchange_strong(
 138                                 tail_was_me,NULL,std::mo_acq_rel);
 139                         /**
 140                                 @Begin
 141                                 @Commit_point_define_check: success == true
 142                                 @Label: Unlock_Point_Success_1
 143                                 @End
 144                         */
 145                         if (success) {
 146
 147                                 // got null in tail, mutex is unlocked
 148                                 return;
 149                         }
 150
 151                         // (*1) catch the race :
 152                         rl::linear_backoff bo;
 153                         for(;;) {
 154                                 next = me->next.load(std::mo_acquire);
 155                                 if ( next != NULL )
 156                                         break;
 157                                 thrd_yield();
 158                         }
 159                 }
 160
 161                 // (*2) - store to next must be done,
 162                 //  so no locker can be viewing my node any more
 163
 164                 // let next guy in :
 165                 next->gate.store( 0, std::mo_release );
 166                 /**
 167                         @Begin
 168                         @Commit_point_define_check: true
 169                         @Label: Unlock_Point_Success_2
 170                         @End
 171                 */
 172         }
 173 };
 174 /**
 175         @Begin
 176         @Class_end
 177         @End
 178 */