runtime: don't let readTrace spin on trace.shutdown

Issue #74045 describes a scenario in which gopark is inlined into
readTrace, such that there are no preemption points. This is only a
problem because readTrace spins if trace.shutdown is set, through
traceReaderAvailable. However, trace.shutdown is almost certainly
overkill for traceReaderAvailable. The first condition, checking whether
the reader gen and the flushed gen match, should be sufficient to ensure
the reader wakes up and finishes flushing all buffers. The first
condition is also safe because it guarantees progress. In the case of
shutdown, all the trace work that will be flushed has been flushed, and
so the trace reader will exit into a regular goroutine context when
it's finished. If not shutting down, then the trace reader will release
doneSema, increase readerGen, and then the gopark unlockf will let it
block until new work actually comes in.

Fixes #74045.

Change-Id: Id9b15c277cb731618488771bd484577341b68675
Reviewed-on: https://go-review.googlesource.com/c/go/+/680738
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nick Ripley <nick.ripley@datadoghq.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

Author: mknyszek

src/runtime/trace.go

@@ -956,7 +956,7 @@ func traceReader() *g {
 // scheduled and should be. Callers should first check that
 // (traceEnabled() || traceShuttingDown()) is true.
 func traceReaderAvailable() *g {
-	// There are three conditions under which we definitely want to schedule
+	// There are two conditions under which we definitely want to schedule
 	// the reader:
 	// - The reader is lagging behind in finishing off the last generation.
 	//   In this case, trace buffers could even be empty, but the trace
@@ -965,12 +965,10 @@ func traceReaderAvailable() *g {
 	// - The reader has pending work to process for it's reader generation
 	//   (assuming readerGen is not lagging behind). Note that we also want
 	//   to be careful *not* to schedule the reader if there's no work to do.
-	// - The trace is shutting down. The trace stopper blocks on the reader
-	//   to finish, much like trace advancement.
 	//
 	// We also want to be careful not to schedule the reader if there's no
 	// reason to.
-	if trace.flushedGen.Load() == trace.readerGen.Load() || trace.workAvailable.Load() || trace.shutdown.Load() {
+	if trace.flushedGen.Load() == trace.readerGen.Load() || trace.workAvailable.Load() {
 		return trace.reader.Load()
 	}
 	return nil