runtime: refactor runtime->tracer API to appear more like a lock

Currently the execution tracer synchronizes with itself using very
heavyweight operations. As a result, it's totally fine for most of the
tracer code to look like:

    if traceEnabled() {
	traceXXX(...)
    }

However, if we want to make that synchronization more lightweight (as
issue #60773 proposes), then this is insufficient. In particular, we
need to make sure the tracer can't observe an inconsistency between g
atomicstatus and the event that would be emitted for a particular
g transition. This means making the g status change appear to happen
atomically with the corresponding trace event being written out from the
perspective of the tracer.

This requires a change in API to something more like a lock. While we're
here, we might as well make sure that trace events can *only* be emitted
while this lock is held. This change introduces such an API:
traceAcquire, which returns a value that can emit events, and
traceRelease, which requires the value that was returned by
traceAcquire. In practice, this won't be a real lock, it'll be more like
a seqlock.

For the current tracer, this API is completely overkill and the value
returned by traceAcquire basically just checks trace.enabled. But it's
necessary for the tracer described in #60773 and we can implement that
more cleanly if we do this refactoring now instead of later.

For #60773.

Change-Id: Ibb9ff5958376339fafc2b5180aef65cf2ba18646
Reviewed-on: https://go-review.googlesource.com/c/go/+/515635
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 12 insertions, 6 deletions

diff --git a/src/runtime/mgcsweep.go b/src/runtime/mgcsweep.go
index 03772d22bf..2d84c0d07c 100644
--- a/src/runtime/mgcsweep.go
+++ b/src/runtime/mgcsweep.go
@@ -516,8 +516,10 @@ func (sl *sweepLocked) sweep(preserve bool) bool {
 		throw("mspan.sweep: bad span state")
 	}
 
-	if traceEnabled() {
-		traceGCSweepSpan(s.npages * _PageSize)
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.GCSweepSpan(s.npages * _PageSize)
+		traceRelease(trace)
 	}
 
 	mheap_.pagesSwept.Add(int64(s.npages))
@@ -889,8 +891,10 @@ func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) {
 		return
 	}
 
-	if traceEnabled() {
-		traceGCSweepStart()
+	trace := traceAcquire()
+	if trace.ok() {
+		trace.GCSweepStart()
+		traceRelease(trace)
 	}
 
 	// Fix debt if necessary.
@@ -929,8 +933,10 @@ retry:
 		}
 	}
 
-	if traceEnabled() {
-		traceGCSweepDone()
+	trace = traceAcquire()
+	if trace.ok() {
+		trace.GCSweepDone()
+		traceRelease(trace)
 	}
 }