runtime: fix leak and locking in BenchmarkMSpanCountAlloc

CL 249917 made the mspan in MSpanCountAlloc no longer stack-allocated
(for good reason), but then allocated an mspan on each call and did not
free it, resulting in a leak. That allocation was also not protected by
the heap lock, which could lead to data corruption of mheap fields and
the spanalloc.

To fix this, export some functions to allocate/free dummy mspans from
spanalloc (with proper locking) and allocate just one up-front for the
benchmark, freeing it at the end. Then, update MSpanCountAlloc to accept
a dummy mspan.

Note that we need to allocate the dummy mspan up-front otherwise we
measure things like heap locking and fixalloc performance instead of
what we actually want to measure: how fast we can do a popcount on the
mark bits.

Fixes #41391.

Change-Id: If6629a6ec1ece639c7fb78532045837a8c872c04
Reviewed-on: https://go-review.googlesource.com/c/go/+/255297
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>

1 files changed, 25 insertions, 3 deletions

diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 929bb35db6..e3d6441c18 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -983,9 +983,31 @@ func MapHashCheck(m interface{}, k interface{}) (uintptr, uintptr) {
 	return x, y
 }
 
-func MSpanCountAlloc(bits []byte) int {
-	s := (*mspan)(mheap_.spanalloc.alloc())
+// mspan wrapper for testing.
+//go:notinheap
+type MSpan mspan
+
+// Allocate an mspan for testing.
+func AllocMSpan() *MSpan {
+	var s *mspan
+	systemstack(func() {
+		s = (*mspan)(mheap_.spanalloc.alloc())
+	})
+	return (*MSpan)(s)
+}
+
+// Free an allocated mspan.
+func FreeMSpan(s *MSpan) {
+	systemstack(func() {
+		mheap_.spanalloc.free(unsafe.Pointer(s))
+	})
+}
+
+func MSpanCountAlloc(ms *MSpan, bits []byte) int {
+	s := (*mspan)(ms)
 	s.nelems = uintptr(len(bits) * 8)
 	s.gcmarkBits = (*gcBits)(unsafe.Pointer(&bits[0]))
-	return s.countAlloc()
+	result := s.countAlloc()
+	s.gcmarkBits = nil
+	return result
 }