[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:

`thrd_error` and `thrd_success` are C11 thread specific values.  While the implementations of the C11 thread functions will use these values, the pthreads implementation will not.  Since `CndVar` will be shared between the two, I'd prefer not to have C11-threads or pthread specific values in the shared implementation.  Instead, each of the two will need to check the return values from these functions, and may need to pay a cost to translate the return code.


Maybe there's a way with templates to punch out two copies with the specific return values for success and failure.

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