CL 549796 adds race annotations to godebugInc. It uses racerelease
to model a CompareAndSwap. However, a CompareAndSwap is
essentially a load and a store. Modeling it as just racerelease
makes it not synchronized with other racerelease, i.e. other CAS.
For the following execution
thread A B
load, got nil
load, got nil
set *inc
set *inc
racerelease
CAS success
racerelease
CAS fail
load
raceacquire
use *inc (from A)
On thread B, the raceacquire synchronizes with the previous
racerelease, which is not synchronized with racerelease on thread
A, so it doesn't know that the use of *inc on thread B is after
the set on thread A, and will report a race.
Change it to use racereleasemerge, which synchronizes with
previous racerelease and racereleasemerge. So in the case above it
knows thread B's CAS is after thread A's.
Also remove stale comment that was more relevant when the code
used atomic store, where CL 549796 changed to CAS.
Updates #64649.
Change-Id: I17671090a19c0699fcb4e6481e2abd98ef2e5542
Reviewed-on: https://go-review.googlesource.com/c/go/+/551856 Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com>