fix another gc bug, one that i have only imagined,

not observed: do not use malloc to allocate stacks
during garbage collection, because it would make the
malloc data structures change underfoot.

R=r
DELTA=6  (3 added, 0 deleted, 3 changed)
OCL=30323
CL=30326

src/pkg/runtime/malloc.c

@@ -274,7 +274,7 @@ stackalloc(uint32 n)
 	uint32 *ref;
 
 //return oldmal(n);
-	if(m->mallocing) {
+	if(m->mallocing || m->gcing) {
 		lock(&stacks);
 		if(stacks.size == 0)
 			FixAlloc_Init(&stacks, n, SysAlloc, nil, nil);
@@ -298,7 +298,7 @@ stackfree(void *v)
 {
 //return;
 
-	if(m->mallocing) {
+	if(m->mallocing || m->gcing) {
 		lock(&stacks);
 		FixAlloc_Free(&stacks, v);
 		unlock(&stacks);

src/pkg/runtime/mgc0.c

@@ -215,6 +215,7 @@ gc(int32 force)
 	if(gcpercent < 0)
 		return;
 
+	m->gcing = 1;
 	semacquire(&gcsema);
 	gosave(&g->sched);	// update g's stack pointer for scanstack
 	stoptheworld();
@@ -228,4 +229,5 @@ gc(int32 force)
 	starttheworld();
 	gosave(&g->sched);	// update g's stack pointer for debugging
 	semrelease(&gcsema);
+	m->gcing = 0;
 }

src/pkg/runtime/runtime.h

@@ -177,6 +177,7 @@ struct	M
 	int32	siz2;
 	int32	id;
 	int32	mallocing;
+	int32	gcing;
 	int32	locks;
 	Note	havenextg;
 	G*	nextg;