diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index f6e07f8ec0..b360f20ee7 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -1615,6 +1615,12 @@ func unlockextra(mp *m) {
 	atomic.Storeuintptr(&extram, uintptr(unsafe.Pointer(mp)))
 }
 
+// execLock serializes exec and clone to avoid bugs or unspecified behaviour
+// around exec'ing while creating/destroying threads.  See issue #19546.
+//
+// TODO: look into using a rwmutex, to avoid serializing thread creation.
+var execLock mutex
+
 // Create a new m. It will start off with a call to fn, or else the scheduler.
 // fn needs to be static and not a heap allocated closure.
 // May run with m.p==nil, so write barriers are not allowed.
@@ -1634,10 +1640,14 @@ func newm(fn func(), _p_ *p) {
 		if msanenabled {
 			msanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
 		}
+		lock(&execLock)
 		asmcgocall(_cgo_thread_start, unsafe.Pointer(&ts))
+		unlock(&execLock)
 		return
 	}
+	lock(&execLock)
 	newosproc(mp, unsafe.Pointer(mp.g0.stack.hi))
+	unlock(&execLock)
 }
 
 // Stops execution of the current m until new work is available.
@@ -2857,6 +2867,18 @@ func syscall_runtime_AfterForkInChild() {
 	msigrestore(getg().m.sigmask)
 }
 
+// Called from syscall package before Exec.
+//go:linkname syscall_runtime_BeforeExec syscall.runtime_BeforeExec
+func syscall_runtime_BeforeExec() {
+	lock(&execLock)
+}
+
+// Called from syscall package after Exec.
+//go:linkname syscall_runtime_AfterExec syscall.runtime_AfterExec
+func syscall_runtime_AfterExec() {
+	unlock(&execLock)
+}
+
 // Allocate a new g, with a stack big enough for stacksize bytes.
 func malg(stacksize int32) *g {
 	newg := new(g)
diff --git a/src/syscall/exec_unix.go b/src/syscall/exec_unix.go
index e4f047f3f4..e8ca29ab40 100644
--- a/src/syscall/exec_unix.go
+++ b/src/syscall/exec_unix.go
@@ -242,6 +242,10 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 	return pid, 0, err
 }
 
+// Implemented in runtime package.
+func runtime_BeforeExec()
+func runtime_AfterExec()
+
 // Exec invokes the execve(2) system call.
 func Exec(argv0 string, argv []string, envv []string) (err error) {
 	argv0p, err := BytePtrFromString(argv0)
@@ -256,9 +260,11 @@ func Exec(argv0 string, argv []string, envv []string) (err error) {
 	if err != nil {
 		return err
 	}
+	runtime_BeforeExec()
 	_, _, err1 := RawSyscall(SYS_EXECVE,
 		uintptr(unsafe.Pointer(argv0p)),
 		uintptr(unsafe.Pointer(&argvp[0])),
 		uintptr(unsafe.Pointer(&envvp[0])))
+	runtime_AfterExec()
 	return Errno(err1)
 }