systrap: don't call Goyield() if the race detector is enabled

Before this change:
BM_Getpid      290875 ns        80000 ns         7000
After this change:
BM_Getpid       20114 ns         5400 ns       116667

Reported-by: syzbot+b65fd5695fdb97a59033@syzkaller.appspotmail.com
PiperOrigin-RevId: 532262444

pkg/sentry/platform/systrap/BUILD

@@ -53,6 +53,8 @@ go_library(
         "lib_amd64.s",
         "lib_arm64.s",
         "shared_context.go",
+        "shared_context_norace.go",
+        "shared_context_race.go",
         "stub_amd64.s",
         "stub_arm64.s",
         "stub_defs.go",

pkg/sentry/platform/systrap/shared_context.go

@@ -23,7 +23,6 @@ import (
 	"golang.org/x/sys/unix"
 	"gvisor.dev/gvisor/pkg/sentry/platform"
 	"gvisor.dev/gvisor/pkg/sentry/platform/systrap/sysmsg"
-	gsync "gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/syncevent"
 )
 
@@ -351,7 +350,7 @@ func (q *fastPathDispatcher) loop(target *sharedContext) {
 				// q.list has to be empty at the end.
 				continue
 			}
-			gsync.Goyield()
+			yield()
 		}
 	}
 }

pkg/sentry/platform/systrap/shared_context_norace.go

@@ -0,0 +1,26 @@
+// Copyright 2023 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !race
+// +build !race
+
+package systrap
+
+import (
+	"gvisor.dev/gvisor/pkg/sync"
+)
+
+func yield() {
+	sync.Goyield()
+}

pkg/sentry/platform/systrap/shared_context_race.go

@@ -0,0 +1,22 @@
+// Copyright 2023 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build race
+// +build race
+
+package systrap
+
+// yield is just a stub because sync.Goyield() is very expensive with the race
+// detector.
+func yield() {}