runtime: clear trace map without write barriers

Currently the trace map is cleared with an assignment, but this ends up
invoking write barriers. Theoretically, write barriers could try to
write a trace event and eventually try to acquire the same lock. The
static lock ranking expresses this constraint.

This change replaces the assignment with a call to memclrNoHeapPointer
to clear the map, removing the write barriers.

Note that technically this problem is purely theoretical. The way the
trace maps are used today is such that reset is only ever called when
the tracer is no longer writing events that could emit data into a map.
Furthermore, reset is never called from an event-writing context.

Therefore another way to resolve this is to simply not hold the trace
map lock over the reset operation. However, this makes the trace map
implementation less robust because it needs to be used in a very
specific way. Furthermore, the rest of the trace map code avoids write
barriers already since its internal structures are all notinheap, so
it's actually more consistent to just avoid write barriers in the reset
method.

Fixes #56554.

Change-Id: Icd86472e75e25161b2c10c1c8aaae2c2fed4f67f
Reviewed-on: https://go-review.googlesource.com/c/go/+/560216
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/trace2map.go

@@ -141,5 +141,11 @@ func (tab *traceMap) reset() {
 	assertLockHeld(&tab.lock)
 	tab.mem.drop()
 	tab.seq.Store(0)
-	tab.tab = [1 << 13]atomic.UnsafePointer{}
+	// Clear table without write barriers. The table consists entirely
+	// of notinheap pointers, so this is fine.
+	//
+	// Write barriers may theoretically call into the tracer and acquire
+	// the lock again, and this lock ordering is expressed in the static
+	// lock ranking checker.
+	memclrNoHeapPointers(unsafe.Pointer(&tab.tab), unsafe.Sizeof(tab.tab))
 }