sync/atomic: disable GC during TestHammerStoreLoad

TestHammerStoreLoad involves a stress test of StorePointer, which has a
write barrier. The "pointer" that is being written is not a real value,
which is generally fine (though not *really* safe) on 64-bit systems
because they never point to an actual object.

On 32-bit systems, however, this is much more likely. Because I can't
figure out how to rewrite the test such that it still is testing the
same conditions but is also using real pointers, just disable the GC
during the test, and make sure there isn't one currently in progress.

Fixes #49362.

Change-Id: If81883fedf06568132e6484f40c820aa69027a9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/361455
Trust: Michael Knyszek <[email protected]>
Run-TryBot: Michael Knyszek <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: mknyszek

src/sync/atomic/atomic_test.go

@@ -7,6 +7,7 @@ package atomic_test
 import (
 	"fmt"
 	"runtime"
+	"runtime/debug"
 	"strings"
 	. "sync/atomic"
 	"testing"
@@ -1196,6 +1197,11 @@ func TestHammerStoreLoad(t *testing.T) {
 	}
 	const procs = 8
 	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(procs))
+	// Disable the GC because hammerStoreLoadPointer invokes
+	// write barriers on values that aren't real pointers.
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
+	// Ensure any in-progress GC is finished.
+	runtime.GC()
 	for _, tt := range tests {
 		c := make(chan int)
 		var val uint64