runtime: in semasleep, subtract time spent so far from timeout

When pthread_cond_timedwait_relative_np gets a spurious wakeup
(due to a signal, typically), we used to retry with the same
relative timeout. That's incorrect, we should lower the timeout
by the time we've spent in this function so far.

In the worst case, signals come in and cause spurious wakeups
faster than the timeout, causing semasleep to never time out.

Also fix nacl and netbsd while we're here. They have similar issues.

Fixes #27520

Change-Id: I6601e120e44a4b8ef436eef75a1e7c8cf1d39e39
Reviewed-on: https://go-review.googlesource.com/133655
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os_darwin.go

@@ -34,6 +34,10 @@ func semacreate(mp *m) {
 
 //go:nosplit
 func semasleep(ns int64) int32 {
+	var start int64
+	if ns >= 0 {
+		start = nanotime()
+	}
 	mp := getg().m
 	pthread_mutex_lock(&mp.mutex)
 	for {
@@ -43,8 +47,13 @@ func semasleep(ns int64) int32 {
 			return 0
 		}
 		if ns >= 0 {
+			spent := nanotime() - start
+			if spent >= ns {
+				pthread_mutex_unlock(&mp.mutex)
+				return -1
+			}
 			var t timespec
-			t.set_nsec(ns)
+			t.set_nsec(ns - spent)
 			err := pthread_cond_timedwait_relative_np(&mp.cond, &mp.mutex, &t)
 			if err == _ETIMEDOUT {
 				pthread_mutex_unlock(&mp.mutex)

src/runtime/os_nacl.go

@@ -197,23 +197,23 @@ func semacreate(mp *m) {
 //go:nosplit
 func semasleep(ns int64) int32 {
 	var ret int32
-
 	systemstack(func() {
 		_g_ := getg()
 		if nacl_mutex_lock(_g_.m.waitsemalock) < 0 {
 			throw("semasleep")
 		}
-
+		var ts timespec
+		if ns >= 0 {
+			end := ns + nanotime()
+			ts.tv_sec = end / 1e9
+			ts.tv_nsec = int32(end % 1e9)
+		}
 		for _g_.m.waitsemacount == 0 {
 			if ns < 0 {
 				if nacl_cond_wait(_g_.m.waitsema, _g_.m.waitsemalock) < 0 {
 					throw("semasleep")
 				}
 			} else {
-				var ts timespec
-				end := ns + nanotime()
-				ts.tv_sec = end / 1e9
-				ts.tv_nsec = int32(end % 1e9)
 				r := nacl_cond_timed_wait_abs(_g_.m.waitsema, _g_.m.waitsemalock, &ts)
 				if r == -_ETIMEDOUT {
 					nacl_mutex_unlock(_g_.m.waitsemalock)

src/runtime/os_netbsd.go

@@ -126,15 +126,9 @@ func semacreate(mp *m) {
 //go:nosplit
 func semasleep(ns int64) int32 {
 	_g_ := getg()
-
-	// Compute sleep deadline.
-	var tsp *timespec
-	var ts timespec
+	var deadline int64
 	if ns >= 0 {
-		var nsec int32
-		ts.set_sec(timediv(ns, 1000000000, &nsec))
-		ts.set_nsec(nsec)
-		tsp = &ts
+		deadline = nanotime() + ns
 	}
 
 	for {
@@ -147,18 +141,21 @@ func semasleep(ns int64) int32 {
 		}
 
 		// Sleep until unparked by semawakeup or timeout.
+		var tsp *timespec
+		var ts timespec
+		if ns >= 0 {
+			wait := deadline - nanotime()
+			if wait <= 0 {
+				return -1
+			}
+			var nsec int32
+			ts.set_sec(timediv(wait, 1000000000, &nsec))
+			ts.set_nsec(nsec)
+			tsp = &ts
+		}
 		ret := lwp_park(_CLOCK_MONOTONIC, _TIMER_RELTIME, tsp, 0, unsafe.Pointer(&_g_.m.waitsemacount), nil)
 		if ret == _ETIMEDOUT {
 			return -1
-		} else if ret == _EINTR && ns >= 0 {
-			// Avoid sleeping forever if we keep getting
-			// interrupted (for example by the profiling
-			// timer). It would be if tsp upon return had the
-			// remaining time to sleep, but this is good enough.
-			var nsec int32
-			ns /= 2
-			ts.set_sec(timediv(ns, 1000000000, &nsec))
-			ts.set_nsec(nsec)
 		}
 	}
 }