runtime: encapsulate access to allgs

Correctly accessing allgs is a bit hairy. Some paths need to lock
allglock, some don't. Those that don't are safest using atomicAllG, but
usage is not consistent.

Rather than doing this ad-hoc, move all access* through forEachG /
forEachGRace, the locking and atomic versions, respectively. This will
make it easier to ensure safe access.

* markroot is the only exception, as it has a far-removed guarantee of
safe access via an atomic load of allglen far before actual use.

Change-Id: Ie1c7a8243e155ae2b4bc3143577380c695680e89
Reviewed-on: https://go-review.googlesource.com/c/go/+/279994
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 3 insertions, 5 deletions

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 7c7239beb8..6927e90daa 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -2229,14 +2229,12 @@ func gcSweep(mode gcMode) {
 //
 //go:systemstack
 func gcResetMarkState() {
-	// This may be called during a concurrent phase, so make sure
+	// This may be called during a concurrent phase, so lock to make sure
 	// allgs doesn't change.
-	lock(&allglock)
-	for _, gp := range allgs {
+	forEachG(func(gp *g) {
 		gp.gcscandone = false // set to true in gcphasework
 		gp.gcAssistBytes = 0
-	}
-	unlock(&allglock)
+	})
 
 	// Clear page marks. This is just 1MB per 64GB of heap, so the
 	// time here is pretty trivial.