runtime: add missing trace lock edges

We're missing lock edges to trace.lock that happen only rarely. Any
trace event can potentially fill up a trace buffer and acquire
trace.lock in order to flush the buffer, but this happens relatively
rarely, so we simply haven't seen some of these lock edges that could
happen.

With this change, we promote "fin, notifyList < traceStackTab" to
"fin, notifyList < trace" and now everything that emits trace events
with a P enters the tracer lock ranks via "trace", rather than some
things entering at "trace" and others at "traceStackTab".

This was found by inspecting the rank graph for things that didn't
make sense.

Ideally we would add a mayAcquire annotation that any trace event can
potentially acquire trace.lock, but there are actually cases that
violate this ranking right now. This is golang#53979. The chance of a lock
cycle is extremely low given the number of conditions that have to
happen simultaneously.

For golang#53789.

Change-Id: Ic65947d27dee88d2daf639b21b2c9d37552f0ac0
Reviewed-on: https://go-review.googlesource.com/c/go/+/418716
Reviewed-by: Michael Pratt <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/runtime/mklockrank.go

@@ -47,8 +47,8 @@ allg, hchan, reflectOffs, timers, traceBuf < fin;
 traceBuf < traceStrings;
 allg, hchan, notifyList, reflectOffs, timers, traceStrings < mspanSpecial, profInsert, profBlock, profMemActive, gcBitsArenas, spanSetSpine, mheapSpecial;
 profMemActive < profMemFuture;
-hchan, root, sched, traceStrings < trace;
-fin, notifyList, trace < traceStackTab;
+hchan, root, sched, traceStrings, notifyList, fin < trace;
+trace < traceStackTab;
 timers < netpollInit;
 rwmutexW, sysmon < rwmutexR;
 gcBitsArenas, netpollInit, profBlock, profInsert, profMemFuture, spanSetSpine, traceStackTab < gscan;