runtime: fix generation skew with trace reentrancy
Currently when performing multiple nested traceAcquires, we re-read
trace.gen on subsequent reads. But this is invalid, since a generation
transition may happen in between a traceAcquire and a nested
traceAcquire. The first one will produce a traceLocker with a gen from
the previous generation, and the second will produce a traceLocker from
the next generation. (Note: generations cannot _complete_ advancement
under traceAcquire, but trace.gen can move forward.) The end result is
earlier events, from the nested traceAcquire, will write to a future
generation, and then previous events will write to a past generation.
This can break the trace.
(There are also a lot of comments left over talking about the
non-reentrancy of the tracer; we should look at those again.)
Change-Id: I08ac8cc86d41ab3e6061c5de58d657b6ad0d19d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/708397
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com>