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>