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Comment the code.

--- a/lib/thread/mutex+linux.myr

+++ b/lib/thread/mutex+linux.myr

@@ -16,7 +16,7 @@

 const Unlocked = 0

 const Locked = 1

-const Lockedcontended = 2

+const Contended = 2

 generic Zptr = 0 castto(@a#)

 var nspin = 1	/* FIXME: pick a sane number, based on CPU count */

@@ -27,7 +27,9 @@

 const mtxlock = {mtx

 	var c

-	/* uncontended case: we get an unlocked mutex, and we lock it */

+	/* 

+	Uncontended case: we get an unlocked mutex, and we lock it.

+	*/

 	for var i = 0; i < nspin; i++

 		c = xcas(&mtx._state, Unlocked, Locked) 

 		if c == Unlocked

@@ -36,14 +38,18 @@

 		relax()

 	;;

-	/* contended: we set the lock _state to sleep */

+	/*

+	Contended case: we set the lock state to Contended. This indicates that there

+	the lock is locked, and we potentially have threads waiting on it, which means

+	that we will need to wake them up.

+	*/

 	if c == Locked

-		c = xchg(&mtx._state, Lockedcontended)

+		c = xchg(&mtx._state, Contended)

 	;;

 	while c != Unlocked

-		sys.futex(&mtx._state, sys.Futexwait | sys.Futexpriv, Lockedcontended, Zptr, Zptr, 0)

-		c = xchg(&mtx._state, Lockedcontended)

+		sys.futex(&mtx._state, sys.Futexwait | sys.Futexpriv, Contended, Zptr, Zptr, 0)

+		c = xchg(&mtx._state, Contended)

 	;;

 }

@@ -52,17 +58,30 @@

 }

 const mtxunlock = {mtx

-	/* uncontended sleep means we can just unlock and move on */

-	if mtx._state == Lockedcontended

-		mtx._state = Unlocked

-	elif xchg(&mtx._state, Unlocked) == Locked

+	/*

+	Uncontended case: If the mutex state is not contended, and we still

+	are uncontended by the xchg() call, then it's safe to simply return;

+	nobody was waiting for us.

+	*/

+	if mtx._state != Contended && xchg(&mtx._state, Unlocked) == Locked

 		->

 	;;

+	/*

+	Contended case: Now we unlock, and wait a little bit to see if anyone

+	takes the lock. If someone takes the lock, we can avoid waking entering

+	the kernel to wake processes. However, we need to set the lock to contended

+	so that if there are still waiters, the thread that took the lock takes

+	responsibility for unlocking.

+	*/

+	mtx._state = Unlocked

 	for var i = 0; i < nspin; i++

 		if mtx._state != Unlocked

-			/* there might have been waiters, but we set the _state to unlocked */

-			if xcas(&mtx._state, Locked, Lockedcontended) == Contendedlock

+			/*

+			If we're locked, set the state to contended to avoid

+			missed wakes.

+			*/

+			if xcas(&mtx._state, Locked, Contended) == Contended

 				->

 			;;

 		;;

@@ -73,4 +92,5 @@

 }

 const relax = {

+	/* FIXME: what is an appropriate way to kill CPU time without entering kernel? */

 }