runtime,time: use atomic.Int32 for isSending

This change switches isSending to be an atomic.Int32 instead of an
atomic.Uint8. The Int32 version is managed as a counter, which is
something that we couldn't do with Uint8 without adding a new intrinsic
which may not be available on all architectures.

That is, instead of only being able to support 8 concurrent timer
firings on the same timer because we only have 8 independent bits to set
for each concurrent timer firing, we can now have 2^31-1 concurrent
timer firings before running into any issues. Like the fact that each
bit-set was matched with a clear, here we match increments with
decrements to indicate that we're in the "sending on a channel" critical
section in the timer code, so we can report the correct result back on
Stop or Reset.

We choose an Int32 instead of a Uint32 because it's easier to check for
obviously bad values (negative values are always bad) and 2^31-1
concurrent timer firings should be enough for anyone.

Previously, we avoided anything bigger than a Uint8 because we could
pack it into some padding in the runtime.timer struct. But it turns out
that the type that actually matters, runtime.timeTimer, is exactly 96
bytes in size. This means its in the next size class up in the 112 byte
size class because of an allocation header. We thus have some free space
to work with. This change increases the size of this struct from 96
bytes to 104 bytes.

(I'm not sure if runtime.timer is often allocated directly, but if it
is, we get lucky in the same way too. It's exactly 80 bytes in size,
which means its in the 96-byte size class, leaving us with some space to
work with.)

Fixes #69969.
Related to #69880 and #69312.

Change-Id: I9fd59cb6a69365c62971d1f225490a65c58f3e77
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/621616
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/time.go

@@ -30,36 +30,6 @@ type timer struct {
 	state  uint8        // state bits
 	isChan bool         // timer has a channel; immutable; can be read without lock
 
-	// isSending is used to handle races between running a
-	// channel timer and stopping or resetting the timer.
-	// It is used only for channel timers (t.isChan == true).
-	// It is not used for tickers.
-	// The lowest zero bit is set when about to send a value on the channel,
-	// and cleared after sending the value.
-	// The stop/reset code uses this to detect whether it
-	// stopped the channel send.
-	//
-	// An isSending bit is set only when t.mu is held.
-	// An isSending bit is cleared only when t.sendLock is held.
-	// isSending is read only when both t.mu and t.sendLock are held.
-	//
-	// Setting and clearing Uint8 bits handles the case of
-	// a timer that is reset concurrently with unlockAndRun.
-	// If the reset timer runs immediately, we can wind up with
-	// concurrent calls to unlockAndRun for the same timer.
-	// Using matched bit set and clear in unlockAndRun
-	// ensures that the value doesn't get temporarily out of sync.
-	//
-	// We use a uint8 to keep the timer struct small.
-	// This means that we can only support up to 8 concurrent
-	// runs of a timer, where a concurrent run can only occur if
-	// we start a run, unlock the timer, the timer is reset to a new
-	// value (or the ticker fires again), it is ready to run,
-	// and it is actually run, all before the first run completes.
-	// Since completing a run is fast, even 2 concurrent timer runs are
-	// nearly impossible, so this should be safe in practice.
-	isSending atomic.Uint8
-
 	blocked uint32 // number of goroutines blocked on timer's channel
 
 	// Timer wakes up at when, and then at when+period, ... (period > 0 only)
@@ -99,6 +69,20 @@ type timer struct {
 	// sendLock protects sends on the timer's channel.
 	// Not used for async (pre-Go 1.23) behavior when debug.asynctimerchan.Load() != 0.
 	sendLock mutex
+
+	// isSending is used to handle races between running a
+	// channel timer and stopping or resetting the timer.
+	// It is used only for channel timers (t.isChan == true).
+	// It is not used for tickers.
+	// The value is incremented when about to send a value on the channel,
+	// and decremented after sending the value.
+	// The stop/reset code uses this to detect whether it
+	// stopped the channel send.
+	//
+	// isSending is incremented only when t.mu is held.
+	// isSending is decremented only when t.sendLock is held.
+	// isSending is read only when both t.mu and t.sendLock are held.
+	isSending atomic.Int32
 }
 
 // init initializes a newly allocated timer t.
@@ -1065,20 +1049,11 @@ func (t *timer) unlockAndRun(now int64) {
 	}
 
 	async := debug.asynctimerchan.Load() != 0
-	var isSendingClear uint8
 	if !async && t.isChan && t.period == 0 {
 		// Tell Stop/Reset that we are sending a value.
-		// Set the lowest zero bit.
+		if t.isSending.Add(1) < 0 {
-		// We do this awkward step because atomic.Uint8
-		// doesn't support Add or CompareAndSwap.
-		// We only set bits with t locked.
-		v := t.isSending.Load()
-		i := sys.TrailingZeros8(^v)
-		if i == 8 {
 			throw("too many concurrent timer firings")
 		}
-		isSendingClear = 1 << i
-		t.isSending.Or(isSendingClear)
 	}
 
 	t.unlock()
@@ -1121,7 +1096,9 @@ func (t *timer) unlockAndRun(now int64) {
 			// We are committed to possibly sending a value
 			// based on seq, so no need to keep telling
 			// stop/modify that we are sending.
-			t.isSending.And(^isSendingClear)
+			if t.isSending.Add(-1) < 0 {
+				throw("mismatched isSending updates")
+			}
 		}
 
 		if t.seq != seq {

src/time/sleep_test.go

@@ -847,9 +847,9 @@ func testStopResetResultGODEBUG(t *testing.T, testStop bool, godebug string) {
 	wg.Wait()
 }
 
-// Test having a large number of goroutines wake up a timer simultaneously.
+// Test having a large number of goroutines wake up a ticker simultaneously.
 // This used to trigger a crash when run under x/tools/cmd/stress.
-func TestMultiWakeup(t *testing.T) {
+func TestMultiWakeupTicker(t *testing.T) {
 	if testing.Short() {
 		t.Skip("-short")
 	}
@@ -872,6 +872,32 @@ func TestMultiWakeup(t *testing.T) {
 	wg.Wait()
 }
 
+// Test having a large number of goroutines wake up a timer simultaneously.
+// This used to trigger a crash when run under x/tools/cmd/stress.
+func TestMultiWakeupTimer(t *testing.T) {
+	if testing.Short() {
+		t.Skip("-short")
+	}
+
+	goroutines := runtime.GOMAXPROCS(0)
+	timer := NewTimer(Nanosecond)
+	var wg sync.WaitGroup
+	wg.Add(goroutines)
+	for range goroutines {
+		go func() {
+			defer wg.Done()
+			for range 10000 {
+				select {
+				case <-timer.C:
+				default:
+				}
+				timer.Reset(Nanosecond)
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 // Benchmark timer latency when the thread that creates the timer is busy with
 // other work and the timers must be serviced by other threads.
 // https://golang.org/issue/38860