diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index d75893dc43..8c8f7d936b 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -320,11 +320,20 @@ var work struct {
 	nwait  uint32
 
 	// Number of roots of various root types. Set by gcMarkRootPrepare.
+	//
+	// nStackRoots == len(stackRoots), but we have nStackRoots for
+	// consistency.
 	nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int
 
 	// Base indexes of each root type. Set by gcMarkRootPrepare.
 	baseData, baseBSS, baseSpans, baseStacks, baseEnd uint32
 
+	// stackRoots is a snapshot of all of the Gs that existed
+	// before the beginning of concurrent marking. The backing
+	// store of this must not be modified because it might be
+	// shared with allgs.
+	stackRoots []*g
+
 	// Each type of GC state transition is protected by a lock.
 	// Since multiple threads can simultaneously detect the state
 	// transition condition, any thread that detects a transition
@@ -1368,6 +1377,11 @@ func gcMark(startTime int64) {
 		throw("work.full != 0")
 	}
 
+	// Drop allg snapshot. allgs may have grown, in which case
+	// this is the only reference to the old backing store and
+	// there's no need to keep it around.
+	work.stackRoots = nil
+
 	// Clear out buffers and double-check that all gcWork caches
 	// are empty. This should be ensured by gcMarkDone before we
 	// enter mark termination.
diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index a5129bd1ee..a15c62cc49 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -102,7 +102,8 @@ func gcMarkRootPrepare() {
 	// ignore them because they begin life without any roots, so
 	// there's nothing to scan, and any roots they create during
 	// the concurrent phase will be caught by the write barrier.
-	work.nStackRoots = int(atomic.Loaduintptr(&allglen))
+	work.stackRoots = allGsSnapshot()
+	work.nStackRoots = len(work.stackRoots)
 
 	work.markrootNext = 0
 	work.markrootJobs = uint32(fixedRootCount + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots)
@@ -194,15 +195,12 @@ func markroot(gcw *gcWork, i uint32, flushBgCredit bool) int64 {
 	default:
 		// the rest is scanning goroutine stacks
 		workCounter = &gcController.stackScanWork
-		var gp *g
-		if work.baseStacks <= i && i < work.baseEnd {
-			// N.B. Atomic read of allglen in gcMarkRootPrepare
-			// acts as a barrier to ensure that allgs must be large
-			// enough to contain all relevant Gs.
-			gp = allgs[i-work.baseStacks]
-		} else {
+		if i < work.baseStacks || work.baseEnd <= i {
+			printlock()
+			print("runtime: markroot index ", i, " not in stack roots range [", work.baseStacks, ", ", work.baseEnd, ")\n")
 			throw("markroot: bad index")
 		}
+		gp := work.stackRoots[i-work.baseStacks]
 
 		// remember when we've first observed the G blocked
 		// needed only to output in traceback
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index a238ea77f3..f375b67981 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -547,6 +547,20 @@ func allgadd(gp *g) {
 	unlock(&allglock)
 }
 
+// allGsSnapshot returns a snapshot of the slice of all Gs.
+//
+// The world must be stopped or allglock must be held.
+func allGsSnapshot() []*g {
+	assertWorldStoppedOrLockHeld(&allglock)
+
+	// Because the world is stopped or allglock is held, allgadd
+	// cannot happen concurrently with this. allgs grows
+	// monotonically and existing entries never change, so we can
+	// simply return a copy of the slice header. For added safety,
+	// we trim everything past len because that can still change.
+	return allgs[:len(allgs):len(allgs)]
+}
+
 // atomicAllG returns &allgs[0] and len(allgs) for use with atomicAllGIndex.
 func atomicAllG() (**g, uintptr) {
 	length := atomic.Loaduintptr(&allglen)