runtime: remove write barriers from traceRegionAlloc

The memory managed by traceRegionAlloc is off-heap by design. However,
stores to the "current" pointer currently have a write barrier. This CL
switches the stores to their write barrier-free equivalents. If the
traceMap data structure gets extended and used elsewhere in the future,
such as for runtime lock contention profiling, we might not have a P and
thus won't be able to use a write barrier in this code. But for now this
is just a cleanup and minor efficiency improvment.

Change-Id: I3081c9d48a8471fd87534e5c4951823d6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/748120
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/src/runtime/traceregion.go b/src/runtime/traceregion.go
index eb19294f1b..e3a024c458 100644
--- a/src/runtime/traceregion.go
+++ b/src/runtime/traceregion.go
@@ -92,8 +92,8 @@ func (a *traceRegionAlloc) alloc(n uintptr) *notInHeap {
 		block.off.Store(n)
 		x = (*notInHeap)(unsafe.Pointer(&block.data[0]))
 
-		// Publish the new block.
-		a.current.Store(unsafe.Pointer(block))
+		// Publish the new block. No write barrier as the memory is off heap.
+		a.current.StoreNoWB(unsafe.Pointer(block))
 		unlock(&a.lock)
 	})
 	return x
@@ -112,7 +112,7 @@ func (a *traceRegionAlloc) drop() {
 	}
 	if current := a.current.Load(); current != nil {
 		sysFree(current, unsafe.Sizeof(traceRegionAllocBlock{}), &memstats.other_sys)
-		a.current.Store(nil)
+		a.current.StoreNoWB(nil)
 	}
 	a.dropping.Store(false)
 }