Export of internal Abseil changes.

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c508ee8bcc38dad1019c0ad744c47ac70efd36b7 by Derek Mauro <dmauro@google.com>:

Opensource the Linux specific SpinLock Wait/Wake implementation.

PiperOrigin-RevId: 209420888

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c13113e19973d496f3e798c0e6d5569c78e41466 by Derek Mauro <dmauro@google.com>:

Fix build error from https://github.com/abseil/abseil-cpp/issues/159

PiperOrigin-RevId: 209152118
GitOrigin-RevId: c508ee8bcc38dad1019c0ad744c47ac70efd36b7
Change-Id: I1a287df973198f9e26936d8f91031bb354db0c71

1 files changed, 72 insertions, 0 deletions

diff --git a/absl/base/internal/spinlock_linux.inc b/absl/base/internal/spinlock_linux.inc
new file mode 100644
index 000000000000..94c861dc6ca2
--- /dev/null
+++ b/absl/base/internal/spinlock_linux.inc
@@ -0,0 +1,72 @@
+// Copyright 2018 The Abseil 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.
+//
+// This file is a Linux-specific part of spinlock_wait.cc
+
+#include <linux/futex.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include <atomic>
+#include <cerrno>
+#include <climits>
+#include <cstdint>
+#include <ctime>
+
+#include "absl/base/attributes.h"
+
+// The SpinLock lockword is `std::atomic<uint32_t>`. Here we assert that
+// `std::atomic<uint32_t>` is bitwise equivalent of the `int` expected
+// by SYS_futex. We also assume that reads/writes done to the lockword
+// by SYS_futex have rational semantics with regard to the
+// std::atomic<> API. C++ provides no guarantees of these assumptions,
+// but they are believed to hold in practice.
+static_assert(sizeof(std::atomic<uint32_t>) == sizeof(int),
+              "SpinLock lockword has the wrong size for a futex");
+
+// Some Android headers are missing these definitions even though they
+// support these futex operations.
+#ifdef __BIONIC__
+#ifndef SYS_futex
+#define SYS_futex __NR_futex
+#endif
+#ifndef FUTEX_PRIVATE_FLAG
+#define FUTEX_PRIVATE_FLAG 128
+#endif
+#endif
+
+extern "C" {
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
+    std::atomic<uint32_t> *w, uint32_t value, int loop,
+    absl::base_internal::SchedulingMode) {
+  if (loop != 0) {
+    int save_errno = errno;
+    struct timespec tm;
+    tm.tv_sec = 0;
+    // Increase the delay; we expect (but do not rely on) explicit wakeups.
+    // We don't rely on explicit wakeups because we intentionally allow for
+    // a race on the kSpinLockSleeper bit.
+    tm.tv_nsec = 16 * absl::base_internal::SpinLockSuggestedDelayNS(loop);
+    syscall(SYS_futex, w, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, value, &tm);
+    errno = save_errno;
+  }
+}
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(std::atomic<uint32_t> *w,
+                                                  bool all) {
+  syscall(SYS_futex, w, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, all ? INT_MAX : 1, 0);
+}
+
+}  // extern "C"