Export of internal Abseil changes

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7b6a68aa92dcc7247236d1a1813914e035383bf8 by Abseil Team <absl-team@google.com>:

Use atomic exchange to mark completion in absl::once_flag

This prevents a potential for a missed wakeup if one thread marks
itself as a waiter while another thread is completing the invocation.

PiperOrigin-RevId: 344946791

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ddff21d1dde08d1368d8be5fca81b154e78be2fc by Abseil Team <absl-team@google.com>:

Add missing string_view include. This is currently used transitively through the cord header.

PiperOrigin-RevId: 344845266
GitOrigin-RevId: 7b6a68aa92dcc7247236d1a1813914e035383bf8
Change-Id: Ia24e98a1df832fc4cb491d888fdf21182b5954f4

1 files changed, 2 insertions, 9 deletions

diff --git a/absl/base/call_once.h b/absl/base/call_once.h
index 5b468af8..96109f53 100644
--- a/absl/base/call_once.h
+++ b/absl/base/call_once.h
@@ -177,15 +177,8 @@ void CallOnceImpl(std::atomic<uint32_t>* control,
                                   scheduling_mode) == kOnceInit) {
     base_internal::invoke(std::forward<Callable>(fn),
                           std::forward<Args>(args)...);
-    // The call to SpinLockWake below is an optimization, because the waiter
-    // in SpinLockWait is waiting with a short timeout. The atomic load/store
-    // sequence is slightly faster than an atomic exchange:
-    //   old_control = control->exchange(base_internal::kOnceDone,
-    //                                   std::memory_order_release);
-    // We opt for a slightly faster case when there are no waiters, in spite
-    // of longer tail latency when there are waiters.
-    old_control = control->load(std::memory_order_relaxed);
-    control->store(base_internal::kOnceDone, std::memory_order_release);
+    old_control =
+        control->exchange(base_internal::kOnceDone, std::memory_order_release);
     if (old_control == base_internal::kOnceWaiter) {
       base_internal::SpinLockWake(control, true);
     }