[release-branch.go1.3] undo 941ef9ddbada / 125150044

It broke the build across all platforms. The original change wasn't even reviewed. Probably should never have been ported to this branch. LGTM=r R=r CC=golang-codereviews https://golang.org/cl/128130043

Author: adg

src/pkg/runtime/mgc0.c

@@ -2760,7 +2760,7 @@ runtime·markscan(void *v)
 void
 runtime·markfreed(void *v)
 {
-uintptr *b, off, shift, xbits;
+uintptr *b, off, shift;
 
 if(0)
 runtime·printf("markfreed %p\n", v);
@@ -2771,18 +2771,7 @@ runtime·markfreed(void *v)
 off = (uintptr*)v - (uintptr*)runtime·mheap.arena_start; // word offset
 b = (uintptr*)runtime·mheap.arena_start - off/wordsPerBitmapWord - 1;
 shift = off % wordsPerBitmapWord;
-if(!g->m->gcing || work.nproc == 1) {
-// During normal operation (not GC), the span bitmap is not updated concurrently,
-// because either the span is cached or accesses are protected with MCentral lock.
-*b = (*b & ~(bitMask<<shift)) | (bitAllocated<<shift);
-} else {
-// During GC other threads concurrently mark heap.
-for(;;) {
-xbits = *b;
-if(runtime·casp((void**)b, (void*)xbits, (void*)((xbits & ~(bitMask<<shift)) | (bitAllocated<<shift))))
-break;
-}
-}
+*b = (*b & ~(bitMask<<shift)) | (bitAllocated<<shift);
 }
 
 // check that the block at v of size n is marked freed.