[libc-commits] [libc] [libc][__support] move CndVar to __support (PR #89329)

[Nick Desaulniers via libc-commits]

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@@ -0,0 +1,107 @@
+//===-- Utility condition variable class ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/threads/CndVar.h"
+#include "src/__support/OSUtil/syscall.h"           // syscall_impl
+#include "src/__support/threads/linux/futex_word.h" // FutexWordType
+#include "src/__support/threads/mutex.h"            // Mutex, MutexLock
+
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace LIBC_NAMESPACE {
+
+int CndVar::wait(Mutex *m) {
+  // The goal is to perform "unlock |m| and wait" in an
+  // atomic operation. However, it is not possible to do it
+  // in the true sense so we do it in spirit. Before unlocking
+  // |m|, a new waiter object is added to the waiter queue with
+  // the waiter queue locked. Iff a signalling thread signals
+  // the waiter before the waiter actually starts waiting, the
+  // wait operation will not begin at all and the waiter immediately
+  // returns.
+
+  CndWaiter waiter;
+  {
+    MutexLock ml(&qmtx);
+    CndWaiter *old_back = nullptr;
+    if (waitq_front == nullptr) {
+      waitq_front = waitq_back = &waiter;
+    } else {
+      old_back = waitq_back;
+      waitq_back->next = &waiter;
+      waitq_back = &waiter;
+    }
+
+    if (m->unlock() != MutexError::NONE) {
+      // If we do not remove the queued up waiter before returning,
+      // then another thread can potentially signal a non-existing
+      // waiter. Note also that we do this with |qmtx| locked. This
+      // ensures that another thread will not signal the withdrawing
+      // waiter.
+      waitq_back = old_back;
+      if (waitq_back == nullptr)
+        waitq_front = nullptr;
+      else
+        waitq_back->next = nullptr;
+
+      return -1;
+    }
+  }
+
+  LIBC_NAMESPACE::syscall_impl<long>(FUTEX_SYSCALL_ID, &waiter.futex_word.val,
+                                     FUTEX_WAIT, WS_Waiting, 0, 0, 0);
+
+  // At this point, if locking |m| fails, we can simply return as the
+  // queued up waiter would have been removed from the queue.
+  auto err = m->lock();
+  return err == MutexError::NONE ? 0 : -1;
----------------
nickdesaulniers wrote:

Yes, not only can clang propagate constants down into function calls, but it can do so back up (when the definition of the callee is in the same translation unit as the caller; that's not the case here though. Maybe it's worth making CndVar a header only library).

https://github.com/llvm/llvm-project/pull/89329