3 files changed, 27 insertions, 42 deletions

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index f2df1a00e0..26cec37f74 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -1521,18 +1521,15 @@ func gcBgMarkWorker(ready chan struct{}) {
 		}
 
 		systemstack(func() {
-			// Mark our goroutine preemptible so its stack
-			// can be scanned or observed by the execution
-			// tracer. This, for example, lets two mark workers
-			// scan each other (otherwise, they would
-			// deadlock). We must not modify anything on
-			// the G stack. However, stack shrinking is
-			// disabled for mark workers, so it is safe to
-			// read from the G stack.
+			// Mark our goroutine preemptible so its stack can be scanned or observed
+			// by the execution tracer. This, for example, lets two mark workers scan
+			// each other (otherwise, they would deadlock).
 			//
-			// N.B. The execution tracer is not aware of this status
-			// transition and handles it specially based on the
-			// wait reason.
+			// casGToWaitingForSuspendG marks the goroutine as ineligible for a
+			// stack shrink, effectively pinning the stack in memory for the duration.
+			//
+			// N.B. The execution tracer is not aware of this status transition and
+			// handles it specially based on the wait reason.
 			casGToWaitingForSuspendG(gp, _Grunning, waitReasonGCWorkerActive)
 			switch pp.gcMarkWorkerMode {
 			default:
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 25d39d9ba3..8d5f2fc793 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -1372,6 +1372,9 @@ func casGToWaiting(gp *g, old uint32, reason waitReason) {
 // casGToWaitingForSuspendG transitions gp from old to _Gwaiting, and sets the wait reason.
 // The wait reason must be a valid isWaitingForSuspendG wait reason.
 //
+// While a goroutine is in this state, it's stack is effectively pinned.
+// The garbage collector must not shrink or otherwise mutate the goroutine's stack.
+//
 // Use this over casgstatus when possible to ensure that a waitreason is set.
 func casGToWaitingForSuspendG(gp *g, old uint32, reason waitReason) {
 	if !reason.isWaitingForSuspendG() {
@@ -1608,18 +1611,11 @@ func stopTheWorldWithSema(reason stwReason) worldStop {
 	// stack while we try to stop the world since otherwise we could get
 	// in a mutual preemption deadlock.
 	//
-	// We must not modify anything on the G stack because a stack shrink
-	// may occur, now that we switched to _Gwaiting, specifically if we're
-	// doing this during the mark phase (mark termination excepted, since
-	// we know that stack scanning is done by that point). A stack shrink
-	// is otherwise OK though because in order to return from this function
-	// (and to leave the system stack) we must have preempted all
-	// goroutines, including any attempting to scan our stack, in which
-	// case, any stack shrinking will have already completed by the time we
-	// exit.
+	// casGToWaitingForSuspendG marks the goroutine as ineligible for a
+	// stack shrink, effectively pinning the stack in memory for the duration.
 	//
 	// N.B. The execution tracer is not aware of this status transition and
-	// andles it specially based on the wait reason.
+	// handles it specially based on the wait reason.
 	casGToWaitingForSuspendG(getg().m.curg, _Grunning, waitReasonStoppingTheWorld)
 
 	trace := traceAcquire()
@@ -2106,16 +2102,11 @@ func forEachP(reason waitReason, fn func(*p)) {
 		// deadlock as we attempt to preempt a goroutine that's trying
 		// to preempt us (e.g. for a stack scan).
 		//
-		// We must not modify anything on the G stack because a stack shrink
-		// may occur. A stack shrink is otherwise OK though because in order
-		// to return from this function (and to leave the system stack) we
-		// must have preempted all goroutines, including any attempting
-		// to scan our stack, in which case, any stack shrinking will
-		// have already completed by the time we exit.
+		// casGToWaitingForSuspendG marks the goroutine as ineligible for a
+		// stack shrink, effectively pinning the stack in memory for the duration.
 		//
-		// N.B. The execution tracer is not aware of this status
-		// transition and handles it specially based on the
-		// wait reason.
+		// N.B. The execution tracer is not aware of this status transition and
+		// handles it specially based on the wait reason.
 		casGToWaitingForSuspendG(gp, _Grunning, reason)
 		forEachPInternal(fn)
 		casgstatus(gp, _Gwaiting, _Grunning)
diff --git a/src/runtime/stack.go b/src/runtime/stack.go
index a338708d76..d809820b07 100644
--- a/src/runtime/stack.go
+++ b/src/runtime/stack.go
@@ -1214,15 +1214,18 @@ func isShrinkStackSafe(gp *g) bool {
 	if gp.parkingOnChan.Load() {
 		return false
 	}
-	// We also can't copy the stack while tracing is enabled, and
-	// gp is in _Gwaiting solely to make itself available to suspendG.
+	// We also can't copy the stack while a gp is in _Gwaiting solely
+	// to make itself available to suspendG.
+	//
 	// In these cases, the G is actually executing on the system
-	// stack, and the execution tracer may want to take a stack trace
-	// of the G's stack. Note: it's safe to access gp.waitreason here.
-	// We're only checking if this is true if we took ownership of the
+	// stack, and the execution tracer, mutex profiler, etc. may want
+	// to take a stack trace of the G's stack.
+	//
+	// Note: it's safe to access gp.waitreason here.
+	// We're only calling isShrinkStackSafe if we took ownership of the
 	// G with the _Gscan bit. This prevents the goroutine from transitioning,
 	// which prevents gp.waitreason from changing.
-	if traceEnabled() && readgstatus(gp)&^_Gscan == _Gwaiting && gp.waitreason.isWaitingForSuspendG() {
+	if readgstatus(gp)&^_Gscan == _Gwaiting && gp.waitreason.isWaitingForSuspendG() {
 		return false
 	}
 	return true
@@ -1258,12 +1261,6 @@ func shrinkstack(gp *g) {
 	if debug.gcshrinkstackoff > 0 {
 		return
 	}
-	f := findfunc(gp.startpc)
-	if f.valid() && f.funcID == abi.FuncID_gcBgMarkWorker {
-		// We're not allowed to shrink the gcBgMarkWorker
-		// stack (see gcBgMarkWorker for explanation).
-		return
-	}
 
 	oldsize := gp.stack.hi - gp.stack.lo
 	newsize := oldsize / 2