From 8279a736447839c374f0cd6d55e5621212ae8335 Mon Sep 17 00:00:00 2001
From: Brian Norris <banorris@uci.edu>
Date: Tue, 9 Oct 2012 17:13:47 -0700
Subject: [PATCH] mcs_lock: add mcs mutex
From:
http://cbloomrants.blogspot.com/2011/07/07-18-11-mcs-list-based-lock_18.html
---
mcs-lock/mcs-lock.h | 90 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 90 insertions(+)
create mode 100644 mcs-lock/mcs-lock.h
diff --git a/mcs-lock/mcs-lock.h b/mcs-lock/mcs-lock.h
new file mode 100644
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 );
+ }
+};
--
2.34.1