[release-branch.go1.15] runtime: block signals in needm before allocating M

Otherwise, if a signal occurs just after we allocated the M,
we can deadlock if the signal handler needs to allocate an M
itself.

For #42207
Fixes #42636

Change-Id: I76f44547f419e8b1c14cbf49bf602c6e645d8c14
Reviewed-on: https://go-review.googlesource.com/c/go/+/265759
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 368c401164)
Reviewed-on: https://go-review.googlesource.com/c/go/+/271847

src/runtime/crash_cgo_test.go

@@ -600,3 +600,16 @@ func TestEINTR(t *testing.T) {
 		t.Fatalf("want %s, got %s\n", want, output)
 	}
 }
+
+// Issue #42207.
+func TestNeedmDeadlock(t *testing.T) {
+	switch runtime.GOOS {
+	case "plan9", "windows":
+		t.Skipf("no signals on %s", runtime.GOOS)
+	}
+	output := runTestProg(t, "testprogcgo", "NeedmDeadlock")
+	want := "OK\n"
+	if output != want {
+		t.Fatalf("want %s, got %s\n", want, output)
+	}
+}

src/runtime/os_js.go

@@ -59,7 +59,7 @@ func mpreinit(mp *m) {
 }
 
 //go:nosplit
-func msigsave(mp *m) {
+func sigsave(p *sigset) {
 }
 
 //go:nosplit

src/runtime/os_plan9.go

@@ -167,7 +167,7 @@ func mpreinit(mp *m) {
 	mp.errstr = (*byte)(mallocgc(_ERRMAX, nil, true))
 }
 
-func msigsave(mp *m) {
+func sigsave(p *sigset) {
 }
 
 func msigrestore(sigmask sigset) {

src/runtime/os_windows.go

@@ -830,7 +830,7 @@ func mpreinit(mp *m) {
 }
 
 //go:nosplit
-func msigsave(mp *m) {
+func sigsave(p *sigset) {
 }
 
 //go:nosplit

src/runtime/proc.go

@@ -569,7 +569,7 @@ func schedinit() {
 	typelinksinit() // uses maps, activeModules
 	itabsinit()     // uses activeModules
 
-	msigsave(_g_.m)
+	sigsave(&_g_.m.sigmask)
 	initSigmask = _g_.m.sigmask
 
 	goargs()
@@ -1536,6 +1536,18 @@ func needm(x byte) {
 		exit(1)
 	}
 
+	// Save and block signals before getting an M.
+	// The signal handler may call needm itself,
+	// and we must avoid a deadlock. Also, once g is installed,
+	// any incoming signals will try to execute,
+	// but we won't have the sigaltstack settings and other data
+	// set up appropriately until the end of minit, which will
+	// unblock the signals. This is the same dance as when
+	// starting a new m to run Go code via newosproc.
+	var sigmask sigset
+	sigsave(&sigmask)
+	sigblock()
+
 	// Lock extra list, take head, unlock popped list.
 	// nilokay=false is safe here because of the invariant above,
 	// that the extra list always contains or will soon contain
@@ -1553,14 +1565,8 @@ func needm(x byte) {
 	extraMCount--
 	unlockextra(mp.schedlink.ptr())
 
-	// Save and block signals before installing g.
+	// Store the original signal mask for use by minit.
-	// Once g is installed, any incoming signals will try to execute,
+	mp.sigmask = sigmask
-	// but we won't have the sigaltstack settings and other data
-	// set up appropriately until the end of minit, which will
-	// unblock the signals. This is the same dance as when
-	// starting a new m to run Go code via newosproc.
-	msigsave(mp)
-	sigblock()
 
 	// Install g (= m->g0) and set the stack bounds
 	// to match the current stack. We don't actually know
@@ -3417,7 +3423,7 @@ func beforefork() {
 	// a signal handler before exec if a signal is sent to the process
 	// group. See issue #18600.
 	gp.m.locks++
-	msigsave(gp.m)
+	sigsave(&gp.m.sigmask)
 	sigblock()
 
 	// This function is called before fork in syscall package.

src/runtime/signal_unix.go

@@ -1032,15 +1032,15 @@ func sigfwdgo(sig uint32, info *siginfo, ctx unsafe.Pointer) bool {
 	return true
 }
 
-// msigsave saves the current thread's signal mask into mp.sigmask.
+// sigsave saves the current thread's signal mask into *p.
 // This is used to preserve the non-Go signal mask when a non-Go
 // thread calls a Go function.
 // This is nosplit and nowritebarrierrec because it is called by needm
 // which may be called on a non-Go thread with no g available.
 //go:nosplit
 //go:nowritebarrierrec
-func msigsave(mp *m) {
+func sigsave(p *sigset) {
-	sigprocmask(_SIG_SETMASK, nil, &mp.sigmask)
+	sigprocmask(_SIG_SETMASK, nil, p)
 }
 
 // msigrestore sets the current thread's signal mask to sigmask.
@@ -1112,7 +1112,7 @@ func minitSignalStack() {
 // thread's signal mask. When this is called all signals have been
 // blocked for the thread.  This starts with m.sigmask, which was set
 // either from initSigmask for a newly created thread or by calling
-// msigsave if this is a non-Go thread calling a Go function. It
+// sigsave if this is a non-Go thread calling a Go function. It
 // removes all essential signals from the mask, thus causing those
 // signals to not be blocked. Then it sets the thread's signal mask.
 // After this is called the thread can receive signals.

src/runtime/testdata/testprogcgo/needmdeadlock.go

@@ -0,0 +1,95 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !plan9,!windows
+
+package main
+
+// This is for issue #42207.
+// During a call to needm we could get a SIGCHLD signal
+// which would itself call needm, causing a deadlock.
+
+/*
+#include <signal.h>
+#include <pthread.h>
+#include <sched.h>
+#include <unistd.h>
+
+extern void GoNeedM();
+
+#define SIGNALERS 10
+
+static void* needmSignalThread(void* p) {
+	pthread_t* pt = (pthread_t*)(p);
+	int i;
+
+	for (i = 0; i < 100; i++) {
+		if (pthread_kill(*pt, SIGCHLD) < 0) {
+			return NULL;
+		}
+		usleep(1);
+	}
+	return NULL;
+}
+
+// We don't need many calls, as the deadlock is only likely
+// to occur the first couple of times that needm is called.
+// After that there will likely be an extra M available.
+#define CALLS 10
+
+static void* needmCallbackThread(void* p) {
+	int i;
+
+	for (i = 0; i < SIGNALERS; i++) {
+		sched_yield(); // Help the signal threads get started.
+	}
+	for (i = 0; i < CALLS; i++) {
+		GoNeedM();
+	}
+	return NULL;
+}
+
+static void runNeedmSignalThread() {
+	int i;
+	pthread_t caller;
+	pthread_t s[SIGNALERS];
+
+	pthread_create(&caller, NULL, needmCallbackThread, NULL);
+	for (i = 0; i < SIGNALERS; i++) {
+		pthread_create(&s[i], NULL, needmSignalThread, &caller);
+	}
+	for (i = 0; i < SIGNALERS; i++) {
+		pthread_join(s[i], NULL);
+	}
+	pthread_join(caller, NULL);
+}
+*/
+import "C"
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+func init() {
+	register("NeedmDeadlock", NeedmDeadlock)
+}
+
+//export GoNeedM
+func GoNeedM() {
+}
+
+func NeedmDeadlock() {
+	// The failure symptom is that the program hangs because of a
+	// deadlock in needm, so set an alarm.
+	go func() {
+		time.Sleep(5 * time.Second)
+		fmt.Println("Hung for 5 seconds")
+		os.Exit(1)
+	}()
+
+	C.runNeedmSignalThread()
+	fmt.Println("OK")
+}