1 files changed, 39 insertions, 0 deletions
diff --git a/vdso/seqlock.h b/vdso/seqlock.h
new file mode 100644
index 000000000..b527bdbca
--- /dev/null
+++ b/vdso/seqlock.h
@@ -0,0 +1,39 @@
+// Copyright 2018 Google Inc.
+//
+// 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.
+
+// Low level raw interfaces to the sequence counter used by the VDSO.
+#ifndef VDSO_SEQLOCK_H_
+#define VDSO_SEQLOCK_H_
+
+#include <stdint.h>
+
+#include "vdso/barrier.h"
+#include "vdso/compiler.h"
+
+namespace vdso {
+
+inline int32_t read_seqcount_begin(const uint64_t* s) {
+  uint64_t seq = *s;
+  read_barrier();
+  return seq & ~1;
+}
+
+inline int read_seqcount_retry(const uint64_t* s, uint64_t seq) {
+  read_barrier();
+  return unlikely(*s != seq);
+}
+
+}  // namespace vdso
+
+#endif  // VDSO_SEQLOCK_H_