diff --git a/builder/testdata/binary-size.txt b/builder/testdata/binary-size.txt
index 024a7d8326..96e0415d9d 100644
--- a/builder/testdata/binary-size.txt
+++ b/builder/testdata/binary-size.txt
@@ -4,5 +4,5 @@ microbit      examples/serial        z    2985    439    8  2272
 wioterminal   examples/pininterrupt  z    8314   1790  148  7512
 cortex-m-qemu ./testdata/size-corpus z    9923   1301   80  4188
 cortex-m-qemu ./testdata/size-corpus 2   17482    862   72  4188
-riscv-qemu    ./testdata/size-corpus z   17364   1028   88 16660
-riscv-qemu    ./testdata/size-corpus 2   24960    744   80 16652
+riscv-qemu    ./testdata/size-corpus z   17396   1028   88 16660
+riscv-qemu    ./testdata/size-corpus 2   24976    744   80 16652
diff --git a/src/runtime/gc_stack_cores.go b/src/runtime/gc_stack_cores.go
index 66aff00879..c32653fc34 100644
--- a/src/runtime/gc_stack_cores.go
+++ b/src/runtime/gc_stack_cores.go
@@ -17,7 +17,7 @@ func gcMarkReachable() {
 	// If the other cores haven't started yet (for example, when a GC cycle
 	// happens during init()), we only need to scan the stack of the current
 	// core.
-	if !secondaryCoresStarted {
+	if secondaryCoresReady.Load() == 0 {
 		// Scan the stack(s) of the current core.
 		scanCurrentStack()
 		if !task.OnSystemStack() {
@@ -98,7 +98,7 @@ func scanstack(sp uintptr) {
 
 // Resume the world after a call to gcMarkReachable.
 func gcResumeWorld() {
-	if !secondaryCoresStarted {
+	if secondaryCoresReady.Load() == 0 {
 		// Nothing to do: the world wasn't stopped in gcMarkReachable.
 		return
 	}
diff --git a/src/runtime/runtime_rp2.go b/src/runtime/runtime_rp2.go
index 9cebcd24f9..b41b034ead 100644
--- a/src/runtime/runtime_rp2.go
+++ b/src/runtime/runtime_rp2.go
@@ -176,6 +176,10 @@ func runCore1() {
 	// interrupts can still happen while the GC is running.
 	enableSIOFifoInterruptCore1()
 
+	// The stop-the-world interrupt must be enabled before the
+	// secondary core is allowed to enter the scheduler.
+	waitForSecondaryCoresReady()
+
 	// Now start running the scheduler on this core.
 	schedulerLock.Lock()
 	scheduler(false)
diff --git a/src/runtime/runtime_rp2040_flashsafe_cores.go b/src/runtime/runtime_rp2040_flashsafe_cores.go
index 67e9a4a232..d4adfd6df8 100644
--- a/src/runtime/runtime_rp2040_flashsafe_cores.go
+++ b/src/runtime/runtime_rp2040_flashsafe_cores.go
@@ -23,9 +23,7 @@ var rp2040FlashSafeState volatile.Register8
 // With scheduler=cores it must not be called from an interrupt handler or with
 // interrupts disabled; the GC stop-the-world path has the same constraint (see #5610).
 func rp2040EnterFlashSafeSection() (interrupt.State, bool) {
-	// secondaryCoresStarted is set after startSecondaryCores() returns, so core 1
-	// may already run Go code in this window. The GC shares it (see #5610).
-	multicore := secondaryCoresStarted
+	multicore := secondaryCoresReady.Load() != 0
 	if !multicore {
 		return interrupt.Disable(), false
 	}
diff --git a/src/runtime/runtime_tinygoriscv_qemu.go b/src/runtime/runtime_tinygoriscv_qemu.go
index 049891b81b..05c2a7efbf 100644
--- a/src/runtime/runtime_tinygoriscv_qemu.go
+++ b/src/runtime/runtime_tinygoriscv_qemu.go
@@ -48,6 +48,10 @@ func main() {
 		// Also enable timer interrupts, for sleepTicksMulticore.
 		riscv.MIE.SetBits(riscv.MIE_MTIE)
 
+		// The stop-the-world interrupt must be enabled before the
+		// secondary core is allowed to enter the scheduler.
+		waitForSecondaryCoresReady()
+
 		// Now start running the scheduler on this core.
 		schedulerLock.Lock()
 		scheduler(false)
diff --git a/src/runtime/scheduler_cores.go b/src/runtime/scheduler_cores.go
index 294662681b..2f94dba56b 100644
--- a/src/runtime/scheduler_cores.go
+++ b/src/runtime/scheduler_cores.go
@@ -14,8 +14,13 @@ const hasParallelism = true
 
 var mainExited atomic.Uint32
 
-// True after the secondary cores have started.
-var secondaryCoresStarted bool
+// Non-zero when secondary cores may enter the scheduler.
+var secondaryCoresReady atomic.Uint32
+
+func waitForSecondaryCoresReady() {
+	for secondaryCoresReady.Load() == 0 {
+	}
+}
 
 // Which task is running on a given core (or nil if there is no task running on
 // the core).
@@ -190,7 +195,7 @@ func run() {
 
 		// After package initializers have finished, start all the other cores.
 		startSecondaryCores()
-		secondaryCoresStarted = true
+		secondaryCoresReady.Store(1)
 
 		// Run main.main.
 		callMain()
diff --git a/src/runtime/scheduler_notcores.go b/src/runtime/scheduler_notcores.go
new file mode 100644
index 0000000000..dcfd12645c
--- /dev/null
+++ b/src/runtime/scheduler_notcores.go
@@ -0,0 +1,5 @@
+//go:build !scheduler.cores
+
+package runtime
+
+func waitForSecondaryCoresReady() {}