[release-branch.go1.15] runtime: wait for preemption signals before syscall.Exec

For #41702
For #41704
For #42023

Change-Id: If07f40b1d73b8f276ee28ffb8b7214175e56c24d
Reviewed-on: https://go-review.googlesource.com/c/go/+/262817
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit 05739d6f17)
Reviewed-on: https://go-review.googlesource.com/c/go/+/264022

src/runtime/proc.go

@@ -1281,6 +1281,14 @@ found:
 	checkdead()
 	unlock(&sched.lock)
 
+	if GOOS == "darwin" {
+		// Make sure pendingPreemptSignals is correct when an M exits.
+		// For #41702.
+		if atomic.Load(&m.signalPending) != 0 {
+			atomic.Xadd(&pendingPreemptSignals, -1)
+		}
+	}
+
 	if osStack {
 		// Return from mstart and let the system thread
 		// library free the g0 stack and terminate the thread.
@@ -3475,11 +3483,24 @@ func syscall_runtime_AfterForkInChild() {
 	inForkedChild = false
 }
 
+// pendingPreemptSignals is the number of preemption signals
+// that have been sent but not received. This is only used on Darwin.
+// For #41702.
+var pendingPreemptSignals uint32
+
 // Called from syscall package before Exec.
 //go:linkname syscall_runtime_BeforeExec syscall.runtime_BeforeExec
 func syscall_runtime_BeforeExec() {
 	// Prevent thread creation during exec.
 	execLock.lock()
+
+	// On Darwin, wait for all pending preemption signals to
+	// be received. See issue #41702.
+	if GOOS == "darwin" {
+		for int32(atomic.Load(&pendingPreemptSignals)) > 0 {
+			osyield()
+		}
+	}
 }
 
 // Called from syscall package after Exec.

src/runtime/signal_unix.go

@@ -336,6 +336,10 @@ func doSigPreempt(gp *g, ctxt *sigctxt) {
 	// Acknowledge the preemption.
 	atomic.Xadd(&gp.m.preemptGen, 1)
 	atomic.Store(&gp.m.signalPending, 0)
+
+	if GOOS == "darwin" {
+		atomic.Xadd(&pendingPreemptSignals, -1)
+	}
 }
 
 const preemptMSupported = true
@@ -365,6 +369,10 @@ func preemptM(mp *m) {
 	}
 
 	if atomic.Cas(&mp.signalPending, 0, 1) {
+		if GOOS == "darwin" {
+			atomic.Xadd(&pendingPreemptSignals, 1)
+		}
+
 		// If multiple threads are preempting the same M, it may send many
 		// signals to the same M such that it hardly make progress, causing
 		// live-lock problem. Apparently this could happen on darwin. See
@@ -436,6 +444,9 @@ func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
 			// no non-Go signal handler for sigPreempt.
 			// The default behavior for sigPreempt is to ignore
 			// the signal, so badsignal will be a no-op anyway.
+			if GOOS == "darwin" {
+				atomic.Xadd(&pendingPreemptSignals, -1)
+			}
 			return
 		}
 		c.fixsigcode(sig)