mcs_lock: add mcs mutex

From:
http://cbloomrants.blogspot.com/2011/07/07-18-11-mcs-list-based-lock_18.html

diff --git a/mcs-lock/mcs-lock.h b/mcs-lock/mcs-lock.h
new file mode 100644 (file)
index 0000000..df64d8b
--- /dev/null
+++ b/mcs-lock/mcs-lock.h
@@ -0,0 +1,90 @@
+// mcs on stack
+
+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($).load(std::mo_acquire) ) {
+                               bo.yield($);
+                       }
+               }
+       }
+
+       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($).load(std::mo_acquire);
+                               if ( next != NULL )
+                                       break;
+                               bo.yield($);
+                       }
+               }
+
+               // (*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 );
+       }
+};