runtime: change Lock from union to struct

Unions can break precise GC.
Update #5193.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/8457043

src/pkg/runtime/lock_futex.c

@@ -41,7 +41,7 @@ runtime·lock(Lock *l)
 		runtime·throw("runtime·lock: lock count");
 
 	// Speculative grab for lock.
-	v = runtime·xchg(&l->key, MUTEX_LOCKED);
+	v = runtime·xchg((uint32*)&l->key, MUTEX_LOCKED);
 	if(v == MUTEX_UNLOCKED)
 		return;
 
@@ -64,7 +64,7 @@ runtime·lock(Lock *l)
 		// Try for lock, spinning.
 		for(i = 0; i < spin; i++) {
 			while(l->key == MUTEX_UNLOCKED)
-				if(runtime·cas(&l->key, MUTEX_UNLOCKED, wait))
+				if(runtime·cas((uint32*)&l->key, MUTEX_UNLOCKED, wait))
 					return;
 			runtime·procyield(ACTIVE_SPIN_CNT);
 		}
@@ -72,17 +72,17 @@ runtime·lock(Lock *l)
 		// Try for lock, rescheduling.
 		for(i=0; i < PASSIVE_SPIN; i++) {
 			while(l->key == MUTEX_UNLOCKED)
-				if(runtime·cas(&l->key, MUTEX_UNLOCKED, wait))
+				if(runtime·cas((uint32*)&l->key, MUTEX_UNLOCKED, wait))
 					return;
 			runtime·osyield();
 		}
 
 		// Sleep.
-		v = runtime·xchg(&l->key, MUTEX_SLEEPING);
+		v = runtime·xchg((uint32*)&l->key, MUTEX_SLEEPING);
 		if(v == MUTEX_UNLOCKED)
 			return;
 		wait = MUTEX_SLEEPING;
-		runtime·futexsleep(&l->key, MUTEX_SLEEPING, -1);
+		runtime·futexsleep((uint32*)&l->key, MUTEX_SLEEPING, -1);
 	}
 }
 
@@ -94,11 +94,11 @@ runtime·unlock(Lock *l)
 	if(--m->locks < 0)
 		runtime·throw("runtime·unlock: lock count");
 
-	v = runtime·xchg(&l->key, MUTEX_UNLOCKED);
+	v = runtime·xchg((uint32*)&l->key, MUTEX_UNLOCKED);
 	if(v == MUTEX_UNLOCKED)
 		runtime·throw("unlock of unlocked lock");
 	if(v == MUTEX_SLEEPING)
-		runtime·futexwakeup(&l->key, 1);
+		runtime·futexwakeup((uint32*)&l->key, 1);
 }
 
 // One-time notifications.

src/pkg/runtime/lock_sema.c

@@ -41,7 +41,7 @@ runtime·lock(Lock *l)
 		runtime·throw("runtime·lock: lock count");
 
 	// Speculative grab for lock.
-	if(runtime·casp(&l->waitm, nil, (void*)LOCKED))
+	if(runtime·casp((void**)&l->key, nil, (void*)LOCKED))
 		return;
 
 	if(m->waitsema == 0)
@@ -54,10 +54,10 @@ runtime·lock(Lock *l)
 		spin = ACTIVE_SPIN;
 
 	for(i=0;; i++) {
-		v = (uintptr)runtime·atomicloadp(&l->waitm);
+		v = (uintptr)runtime·atomicloadp((void**)&l->key);
 		if((v&LOCKED) == 0) {
 unlocked:
-			if(runtime·casp(&l->waitm, (void*)v, (void*)(v|LOCKED)))
+			if(runtime·casp((void**)&l->key, (void*)v, (void*)(v|LOCKED)))
 				return;
 			i = 0;
 		}
@@ -72,9 +72,9 @@ unlocked:
 			// Queue this M.
 			for(;;) {
 				m->nextwaitm = (void*)(v&~LOCKED);
-				if(runtime·casp(&l->waitm, (void*)v, (void*)((uintptr)m|LOCKED)))
+				if(runtime·casp((void**)&l->key, (void*)v, (void*)((uintptr)m|LOCKED)))
 					break;
-				v = (uintptr)runtime·atomicloadp(&l->waitm);
+				v = (uintptr)runtime·atomicloadp((void**)&l->key);
 				if((v&LOCKED) == 0)
 					goto unlocked;
 			}
@@ -97,15 +97,15 @@ runtime·unlock(Lock *l)
 		runtime·throw("runtime·unlock: lock count");
 
 	for(;;) {
-		v = (uintptr)runtime·atomicloadp(&l->waitm);
+		v = (uintptr)runtime·atomicloadp((void**)&l->key);
 		if(v == LOCKED) {
-			if(runtime·casp(&l->waitm, (void*)LOCKED, nil))
+			if(runtime·casp((void**)&l->key, (void*)LOCKED, nil))
 				break;
 		} else {
 			// Other M's are waiting for the lock.
 			// Dequeue an M.
 			mp = (void*)(v&~LOCKED);
-			if(runtime·casp(&l->waitm, (void*)v, mp->nextwaitm)) {
+			if(runtime·casp((void**)&l->key, (void*)v, mp->nextwaitm)) {
 				// Dequeued an M.  Wake it.
 				runtime·semawakeup(mp);
 				break;

src/pkg/runtime/runtime.h

@@ -50,7 +50,7 @@ typedef	uint8			byte;
 typedef	struct	Func		Func;
 typedef	struct	G		G;
 typedef	struct	Gobuf		Gobuf;
-typedef	union	Lock		Lock;
+typedef	struct	Lock		Lock;
 typedef	struct	M		M;
 typedef	struct	P		P;
 typedef	struct	Mem		Mem;
@@ -156,10 +156,12 @@ enum
 /*
  * structures
  */
-union	Lock
+struct	Lock
 {
-	uint32	key;	// futex-based impl
-	M*	waitm;	// linked list of waiting M's (sema-based impl)
+	// Futex-based impl treats it as uint32 key,
+	// while sema-based impl as M* waitm.
+	// Used to be a union, but unions break precise GC.
+	uintptr	key;
 };
 union	Note
 {