syscall: syscall.AllThreadsSyscall signal handling fixes

The runtime support for syscall.AllThreadsSyscall() functions had
some corner case deadlock issues when signal handling was in use.
This was observed in at least 3 build test failures on ppc64 and
amd64 architecture CGO_ENABLED=0 builds over the last few months.

The fixes involve more controlled handling of signals while the
AllThreads mechanism is being executed. Further details are
discussed in bug #44193.

The all-threads syscall support is new in go1.16, so earlier
releases are not affected by this bug.

Fixes #44193

Change-Id: I01ba8508a6e1bb2d872751f50da86dd07911a41d
Reviewed-on: https://go-review.googlesource.com/c/go/+/305149
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Pratt <mpratt@google.com>
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/os/signal/signal_test.go

@@ -16,6 +16,7 @@ import (
 	"os"
 	"os/exec"
 	"runtime"
+	"runtime/trace"
 	"strconv"
 	"sync"
 	"syscall"
@@ -854,3 +855,44 @@ func TestNotifyContextStringer(t *testing.T) {
 		t.Errorf("c.String() = %q, want %q", got, want)
 	}
 }
+
+// #44193 test signal handling while stopping and starting the world.
+func TestSignalTrace(t *testing.T) {
+	done := make(chan struct{})
+	quit := make(chan struct{})
+	c := make(chan os.Signal, 1)
+	Notify(c, syscall.SIGHUP)
+
+	// Source and sink for signals busy loop unsynchronized with
+	// trace starts and stops. We are ultimately validating that
+	// signals and runtime.(stop|start)TheWorldGC are compatible.
+	go func() {
+		defer close(done)
+		defer Stop(c)
+		pid := syscall.Getpid()
+		for {
+			select {
+			case <-quit:
+				return
+			default:
+				syscall.Kill(pid, syscall.SIGHUP)
+			}
+			waitSig(t, c, syscall.SIGHUP)
+		}
+	}()
+
+	for i := 0; i < 100; i++ {
+		buf := new(bytes.Buffer)
+		if err := trace.Start(buf); err != nil {
+			t.Fatalf("[%d] failed to start tracing: %v", i, err)
+		}
+		time.After(1 * time.Microsecond)
+		trace.Stop()
+		size := buf.Len()
+		if size == 0 {
+			t.Fatalf("[%d] trace is empty", i)
+		}
+	}
+	close(quit)
+	<-done
+}

src/runtime/proc.go

@@ -1402,6 +1402,9 @@ func mPark() {
 	g := getg()
 	for {
 		notesleep(&g.m.park)
+		// Note, because of signal handling by this parked m,
+		// a preemptive mDoFixup() may actually occur via
+		// mDoFixupAndOSYield(). (See golang.org/issue/44193)
 		noteclear(&g.m.park)
 		if !mDoFixup() {
 			return
@@ -1635,6 +1638,22 @@ func syscall_runtime_doAllThreadsSyscall(fn func(bool) bool) {
 	for atomic.Load(&sched.sysmonStarting) != 0 {
 		osyield()
 	}
+
+	// We don't want this thread to handle signals for the
+	// duration of this critical section. The underlying issue
+	// being that this locked coordinating m is the one monitoring
+	// for fn() execution by all the other m's of the runtime,
+	// while no regular go code execution is permitted (the world
+	// is stopped). If this present m were to get distracted to
+	// run signal handling code, and find itself waiting for a
+	// second thread to execute go code before being able to
+	// return from that signal handling, a deadlock will result.
+	// (See golang.org/issue/44193.)
+	lockOSThread()
+	var sigmask sigset
+	sigsave(&sigmask)
+	sigblock(false)
+
 	stopTheWorldGC("doAllThreadsSyscall")
 	if atomic.Load(&newmHandoff.haveTemplateThread) != 0 {
 		// Ensure that there are no in-flight thread
@@ -1686,6 +1705,7 @@ func syscall_runtime_doAllThreadsSyscall(fn func(bool) bool) {
 			// the possibility of racing with mp.
 			lock(&mp.mFixup.lock)
 			mp.mFixup.fn = fn
+			atomic.Store(&mp.mFixup.used, 1)
 			if mp.doesPark {
 				// For non-service threads this will
 				// cause the wakeup to be short lived
@@ -1702,9 +1722,7 @@ func syscall_runtime_doAllThreadsSyscall(fn func(bool) bool) {
 				if mp.procid == tid {
 					continue
 				}
-				lock(&mp.mFixup.lock)
-				done = done && (mp.mFixup.fn == nil)
-				unlock(&mp.mFixup.lock)
+				done = atomic.Load(&mp.mFixup.used) == 0
 			}
 			if done {
 				break
@@ -1731,6 +1749,8 @@ func syscall_runtime_doAllThreadsSyscall(fn func(bool) bool) {
 		unlock(&mFixupRace.lock)
 	}
 	startTheWorldGC()
+	msigrestore(sigmask)
+	unlockOSThread()
 }
 
 // runSafePointFn runs the safe point function, if any, for this P.
@@ -2225,9 +2245,21 @@ var mFixupRace struct {
 // mDoFixup runs any outstanding fixup function for the running m.
 // Returns true if a fixup was outstanding and actually executed.
 //
+// Note: to avoid deadlocks, and the need for the fixup function
+// itself to be async safe, signals are blocked for the working m
+// while it holds the mFixup lock. (See golang.org/issue/44193)
+//
 //go:nosplit
 func mDoFixup() bool {
 	_g_ := getg()
+	if used := atomic.Load(&_g_.m.mFixup.used); used == 0 {
+		return false
+	}
+
+	// slow path - if fixup fn is used, block signals and lock.
+	var sigmask sigset
+	sigsave(&sigmask)
+	sigblock(false)
 	lock(&_g_.m.mFixup.lock)
 	fn := _g_.m.mFixup.fn
 	if fn != nil {
@@ -2244,7 +2276,6 @@ func mDoFixup() bool {
 			// is more obviously safe.
 			throw("GC must be disabled to protect validity of fn value")
 		}
-		*(*uintptr)(unsafe.Pointer(&_g_.m.mFixup.fn)) = 0
 		if _g_.racectx != 0 || !raceenabled {
 			fn(false)
 		} else {
@@ -2259,11 +2290,24 @@ func mDoFixup() bool {
 			_g_.racectx = 0
 			unlock(&mFixupRace.lock)
 		}
+		*(*uintptr)(unsafe.Pointer(&_g_.m.mFixup.fn)) = 0
+		atomic.Store(&_g_.m.mFixup.used, 0)
 	}
 	unlock(&_g_.m.mFixup.lock)
+	msigrestore(sigmask)
 	return fn != nil
 }
 
+// mDoFixupAndOSYield is called when an m is unable to send a signal
+// because the allThreadsSyscall mechanism is in progress. That is, an
+// mPark() has been interrupted with this signal handler so we need to
+// ensure the fixup is executed from this context.
+//go:nosplit
+func mDoFixupAndOSYield() {
+	mDoFixup()
+	osyield()
+}
+
 // templateThread is a thread in a known-good state that exists solely
 // to start new threads in known-good states when the calling thread
 // may not be in a good state.

src/runtime/runtime2.go

@@ -554,10 +554,13 @@ type m struct {
 	syscalltick   uint32
 	freelink      *m // on sched.freem
 
-	// mFixup is used to synchronize OS related m state (credentials etc)
-	// use mutex to access.
+	// mFixup is used to synchronize OS related m state
+	// (credentials etc) use mutex to access. To avoid deadlocks
+	// an atomic.Load() of used being zero in mDoFixupFn()
+	// guarantees fn is nil.
 	mFixup struct {
 		lock mutex
+		used uint32
 		fn   func(bool) bool
 	}
 

src/runtime/sigqueue.go

@@ -120,7 +120,7 @@ Send:
 			}
 		case sigFixup:
 			// nothing to do - we need to wait for sigIdle.
-			osyield()
+			mDoFixupAndOSYield()
 		}
 	}