runtime: don't hold trace.lock over semrelease in readTrace0
semrelease may unblock a goroutine, but the act of unblocking a
goroutine may emit an event, which in turn may try to acquire trace.lock
again.
It's safe to release trace.lock in readTrace0 for this because all of
the state (one variable) it uses under the lock will be recomputed when
it reacquires the lock. There's also no other synchronization
requirement to hold trace.lock. This is just a mistake.
Change-Id: Iff6c6b02efa298ebed8e60cdf6539ec161d5ec48
Reviewed-on: https://go-review.googlesource.com/c/go/+/544178
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>