runtime: formalize and fix gcPercent synchronization
Currently gcController.gcPercent is read non-atomically by
gcControllerState.revise and gcTrigger.test, but these users may
execute concurrently with an update to gcPercent.
Although revise's results are best-effort, reading it directly in this
way is, generally speaking, unsafe.
This change makes gcPercent atomically updated for concurrent readers
and documents the complete synchronization semantics.
Because gcPercent otherwise only updated with the heap lock held or the
world stopped, all other reads can remain unsynchronized.
For #44167.
Change-Id: If09af103aae84a1e133e2d4fed8ab888d4b8f457
Reviewed-on: https://go-review.googlesource.com/c/go/+/308690
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Michael Pratt <mpratt@google.com>