--- /dev/null
+// mcs on stack
+
+#include <atomic>
+#include <unrelacy.h>
+
+struct mcs_node {
+ std::atomic<mcs_node *> next;
+ std::atomic<int> gate;
+
+ mcs_node() {
+ next.store(0);
+ gate.store(0);
+ }
+};
+
+struct mcs_mutex {
+public:
+ // tail is null when lock is not held
+ std::atomic<mcs_node *> m_tail;
+
+ mcs_mutex() {
+ m_tail.store( NULL );
+ }
+ ~mcs_mutex() {
+ ASSERT( m_tail.load() == NULL );
+ }
+
+ class guard {
+ public:
+ mcs_mutex * m_t;
+ mcs_node m_node; // node held on the stack
+
+ guard(mcs_mutex * t) : m_t(t) { t->lock(this); }
+ ~guard() { m_t->unlock(this); }
+ };
+
+ void lock(guard * I) {
+ mcs_node * me = &(I->m_node);
+
+ // set up my node :
+ // not published yet so relaxed :
+ me->next.store(NULL, std::mo_relaxed );
+ me->gate.store(1, std::mo_relaxed );
+
+ // publish my node as the new tail :
+ mcs_node * pred = m_tail.exchange(me, std::mo_acq_rel);
+ if ( pred != NULL ) {
+ // (*1) race here
+ // unlock of pred can see me in the tail before I fill next
+
+ // publish me to previous lock-holder :
+ pred->next.store(me, std::mo_release );
+
+ // (*2) pred not touched any more
+
+ // now this is the spin -
+ // wait on predecessor setting my flag -
+ rl::linear_backoff bo;
+ while ( me->gate.load(std::mo_relaxed) ) {
+ thrd_yield();
+ }
+ }
+ }
+
+ void unlock(guard * I) {
+ mcs_node * me = &(I->m_node);
+
+ mcs_node * next = me->next.load(std::mo_acquire);
+ if ( next == NULL )
+ {
+ mcs_node * tail_was_me = me;
+ if ( m_tail.compare_exchange_strong( tail_was_me,NULL,std::mo_acq_rel) ) {
+ // got null in tail, mutex is unlocked
+ return;
+ }
+
+ // (*1) catch the race :
+ rl::linear_backoff bo;
+ for(;;) {
+ next = me->next.load(std::mo_relaxed);
+ if ( next != NULL )
+ break;
+ thrd_yield();
+ }
+ }
+
+ // (*2) - store to next must be done,
+ // so no locker can be viewing my node any more
+
+ // let next guy in :
+ next->gate.store( 0, std::mo_release );
+ }
+};