runtime: use racereleasemerge for godebugInc

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>

diff --git a/src/runtime/runtime.go b/src/runtime/runtime.go
index 92cdfc310ec91cca0f666d2e964f09a8e4062a15..c70a76e40905710479f4bf26a5855348f5eba806 100644 (file)
--- a/src/runtime/runtime.go
+++ b/src/runtime/runtime.go
@@ -167,13 +167,10 @@ func (g *godebugInc) IncNonDefault() {
                if newInc == nil {
                        return
                }
-               // If other goroutines are racing here, no big deal. One will win,
-               // and all the inc functions will be using the same underlying
-               // *godebug.Setting.
                inc = new(func())
                *inc = (*newInc)(g.name)
                if raceenabled {
-                       racerelease(unsafe.Pointer(&g.inc))
+                       racereleasemerge(unsafe.Pointer(&g.inc))
                }
                if !g.inc.CompareAndSwap(nil, inc) {
                        inc = g.inc.Load()