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: bool _lock_acquired;
  58                         @InitVar: _lock_acquired = false;
  59                 @Happens_before: Unlock -> Lock
  60                 @End
  61         */
  62
  63         /**
  64                 @Begin
  65                 @Interface: Lock
  66                 @Commit_point_set: Lock_Enqueue_Point1 | Lock_Enqueue_Point2
  67                 @Check: _lock_acquired == false;
  68                 @Action: _lock_acquired = true;
  69                 @End
  70         */
  71         void lock(guard * I) {
  72                 mcs_node * me = &(I->m_node);
  73
  74                 // set up my node :
  75                 // not published yet so relaxed :
  76                 me->next.store(NULL, std::mo_relaxed );
  77                 me->gate.store(1, std::mo_relaxed );
  78
  79                 /** Run this in the -Y mode to expose the HB bug */
  80                 // publish my node as the new tail :
  81                 mcs_node * pred = m_tail.exchange(me, std::mo_acq_rel);
  82                 /**
  83                         @Begin
  84                         @Commit_point_define_check: pred == NULL
  85                         @Label: Lock_Enqueue_Point1
  86                         @End
  87                 */
  88                 if ( pred != NULL ) {
  89                         // (*1) race here
  90                         // unlock of pred can see me in the tail before I fill next
  91
  92                         // publish me to previous lock-holder :
  93                         // FIXME: detection miss, execution never ends
  94                         // If this is relaxed, the store 0 to gate will be read before and
  95                         // that lock will never ends.
  96                         pred->next.store(me, std::mo_release );
  97                         //printf("lock_miss1\n");
  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                                 //if (my_gate == 0)
 108                                         //printf("lock at gate!\n");
 109                                 /**
 110                                         @Begin
 111                                         @Commit_point_define_check: my_gate == 0
 112                                         @Label: Lock_Enqueue_Point2
 113                                         @End
 114                                 */
 115                                 thrd_yield();
 116                         }
 117                 }
 118         }
 119
 120         /**
 121                 @Begin
 122                 @Interface: Unlock
 123                 @Commit_point_set: Unlock_Point_Success_1 | Unlock_Point_Success_2
 124                 @Check: _lock_acquired == true
 125                 @Action: _lock_acquired = false;
 126                 @End
 127         */
 128         void unlock(guard * I) {
 129                 mcs_node * me = &(I->m_node);
 130
 131                 // FIXME: detection miss, execution never ends
 132                 mcs_node * next = me->next.load(std::mo_acquire);
 133                 //printf("unlock_miss2\n");
 134                 if ( next == NULL )
 135                 {
 136                         mcs_node * tail_was_me = me;
 137                         bool success;
 138                         success = m_tail.compare_exchange_strong(
 139                                 tail_was_me,NULL,std::mo_acq_rel);
 140                         /**
 141                                 @Begin
 142                                 @Commit_point_define_check: success == true
 143                                 @Label: Unlock_Point_Success_1
 144                                 @End
 145                         */
 146                         if (success) {
 147
 148                                 // got null in tail, mutex is unlocked
 149                                 return;
 150                         }
 151
 152                         // (*1) catch the race :
 153                         rl::linear_backoff bo;
 154                         for(;;) {
 155                                 // FIXME: detection miss, execution never ends
 156                                 next = me->next.load(std::mo_acquire);
 157                                 //printf("unlock_miss3\n");
 158                                 if ( next != NULL )
 159                                         break;
 160                                 thrd_yield();
 161                         }
 162                 }
 163
 164                 // (*2) - store to next must be done,
 165                 //  so no locker can be viewing my node any more
 166
 167                 // let next guy in :
 168                 next->gate.store( 0, std::mo_release );
 169                 /**
 170                         @Begin
 171                         @Commit_point_define_check: true
 172                         @Label: Unlock_Point_Success_2
 173                         @End
 174                 */
 175         }
 176 };
 177 /**
 178         @Begin
 179         @Class_end
 180         @End
 181 */