runtime: make checking if tracing is enabled non-atomic

Tracing is currently broken when using iter.Pull from the rangefunc
experiment partly because the "tracing is off" fast path in traceAcquire
was deemed too expensive to check (an atomic load) during the coroutine
switch.

This change adds trace.enabled, a non-atomic indicator of whether
tracing is enabled. It doubles trace.gen, which is the source of truth
on whether tracing is enabled. The semantics around trace.enabled are
subtle.

When tracing is enabled, we need to be careful to make sure that if gen
!= 0, goroutines enter the tracer on traceAcquire. This is enforced by
making sure trace.enabled is published atomically with trace.gen. The
STW takes care of synchronization with most Ms, but there's still sysmon
and goroutines exiting syscalls. We need to synchronize with those
explicitly anyway, which luckily takes care of trace.enabled as well.

When tracing is disabled, it's always OK for trace.enabled to be stale,
since traceAcquire will always double-check gen before proceeding.

For #61897.

Change-Id: I47c2a530fb5339c15e419312fbb1e22d782cd453
Reviewed-on: https://go-review.googlesource.com/c/go/+/565935
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/trace2.go

@@ -98,6 +98,20 @@ var trace struct {
 	goStopReasons    [2][len(traceGoStopReasonStrings)]traceArg
 	goBlockReasons   [2][len(traceBlockReasonStrings)]traceArg
 
+	// enabled indicates whether tracing is enabled, but it is only an optimization,
+	// NOT the source of truth on whether tracing is enabled. Tracing is only truly
+	// enabled if gen != 0. This is used as an optimistic fast path check.
+	//
+	// Transitioning this value from true -> false is easy (once gen is 0)
+	// because it's OK for enabled to have a stale "true" value. traceAcquire will
+	// always double-check gen.
+	//
+	// Transitioning this value from false -> true is harder. We need to make sure
+	// this is observable as true strictly before gen != 0. To maintain this invariant
+	// we only make this transition with the world stopped and use the store to gen
+	// as a publication barrier.
+	enabled bool
+
 	// Trace generation counter.
 	gen            atomic.Uintptr
 	lastNonZeroGen uintptr // last non-zero value of gen
@@ -211,8 +225,19 @@ func StartTrace() error {
 
 	// Start tracing.
 	//
-	// After this executes, other Ms may start creating trace buffers and emitting
+	// Set trace.enabled. This is *very* subtle. We need to maintain the invariant that if
+	// trace.gen != 0, then trace.enabled is always observed as true. Simultaneously, for
+	// performance, we need trace.enabled to be read without any synchronization.
+	//
+	// We ensure this is safe by stopping the world, which acts a global barrier on almost
+	// every M, and explicitly synchronize with any other Ms that could be running concurrently
+	// with us. Today, there are only two such cases:
+	// - sysmon, which we synchronized with by acquiring sysmonlock.
+	// - goroutines exiting syscalls, which we synchronize with via trace.exitingSyscall.
+	//
+	// After trace.gen is updated, other Ms may start creating trace buffers and emitting
 	// data into them.
+	trace.enabled = true
 	trace.gen.Store(firstGen)
 
 	// Wait for exitingSyscall to drain.
@@ -222,8 +247,9 @@ func StartTrace() error {
 	// goroutines to run on.
 	//
 	// Because we set gen before checking this, and because exitingSyscall is always incremented
-	// *after* traceAcquire (which checks gen), we can be certain that when exitingSyscall is zero
-	// that any goroutine that goes to exit a syscall from then on *must* observe the new gen.
+	// *before* traceAcquire (which checks gen), we can be certain that when exitingSyscall is zero
+	// that any goroutine that goes to exit a syscall from then on *must* observe the new gen as
+	// well as trace.enabled being set to true.
 	//
 	// The critical section on each goroutine here is going to be quite short, so the likelihood
 	// that we observe a zero value is high.
@@ -376,6 +402,10 @@ func traceAdvance(stopTrace bool) {
 			trace.shutdown.Store(true)
 			trace.gen.Store(0)
 			unlock(&trace.lock)
+
+			// Clear trace.enabled. It is totally OK for this value to be stale,
+			// because traceAcquire will always double-check gen.
+			trace.enabled = false
 		})
 	} else {
 		trace.gen.Store(traceNextGen(gen))

src/runtime/trace2runtime.go

@@ -141,7 +141,7 @@ var traceGoStopReasonStrings = [...]string{
 //
 //go:nosplit
 func traceEnabled() bool {
-	return trace.gen.Load() != 0
+	return trace.enabled
 }
 
 // traceShuttingDown returns true if the trace is currently shutting down.