runtime: do not unmap work.spans until after checkmark phase

This is causing crashes.

Change-Id: I1832f33d114bc29894e491dd2baac45d7ab3a50d
Reviewed-on: https://go-review.googlesource.com/5330
Reviewed-by: Rick Hudson <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: rsc

src/runtime/mgc.go

@@ -616,24 +616,23 @@ func gc(start_time int64, eagersweep bool) {
 		sweep.npausesweep = 0
 	}
 
-	// See the comment in the beginning of this function as to why we need the following.
-	// Even if this is still stop-the-world, a concurrent exitsyscall can allocate a stack from heap.
-	lock(&mheap_.lock)
-	// Free the old cached mark array if necessary.
-	if work.spans != nil && &work.spans[0] != &h_allspans[0] {
-		sysFree(unsafe.Pointer(&work.spans[0]), uintptr(len(work.spans))*unsafe.Sizeof(work.spans[0]), &memstats.other_sys)
-	}
-
 	if debug.gccheckmark > 0 {
 		if !checkmarkphase {
 			// first half of two-pass; don't set up sweep
-			unlock(&mheap_.lock)
 			return
 		}
 		checkmarkphase = false // done checking marks
 		clearCheckmarks()
 	}
 
+	// See the comment in the beginning of this function as to why we need the following.
+	// Even if this is still stop-the-world, a concurrent exitsyscall can allocate a stack from heap.
+	lock(&mheap_.lock)
+	// Free the old cached mark array if necessary.
+	if work.spans != nil && &work.spans[0] != &h_allspans[0] {
+		sysFree(unsafe.Pointer(&work.spans[0]), uintptr(len(work.spans))*unsafe.Sizeof(work.spans[0]), &memstats.other_sys)
+	}
+
 	// Cache the current array for sweeping.
 	mheap_.gcspans = mheap_.allspans
 	mheap_.sweepgen += 2