[libc][__support] move CndVar to __support (#89329)

We should be able to reuse this between the implementation of C11 cnd_t
condition variables and POSIX pthread_cond_t condition variables.

The current implementation is hyper linux specific, making use of Futex. That
obviously wont work outside of linux, so split the OS specific functions off
into their own source outside of the header.

Modifies the return values of the to-be-shared impl to return 0 on success and
-1 on error.  This pattern was shamelessly stolen from Bionic's
[__bionic_thrd_error](https://android.googlesource.com/platform/bionic/+/refs/heads/main/libc/include/bits/threads_inlines.h#41).

Fixes: #88580
Link: #88583

Author: nickdesaulniers

libc/src/__support/threads/CMakeLists.txt

@@ -71,3 +71,12 @@ if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.callonce)
       .${LIBC_TARGET_OS}.callonce
   )
 endif()
+
+if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.CndVar)
+  add_object_library(
+    CndVar
+    ALIAS
+    DEPENDS
+    .${LIBC_TARGET_OS}.CndVar
+  )
+endif()

libc/src/__support/threads/CndVar.h

@@ -0,0 +1,52 @@
+//===-- A platform independent abstraction layer for cond vars --*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC___SUPPORT_SRC_THREADS_LINUX_CNDVAR_H
+#define LLVM_LIBC___SUPPORT_SRC_THREADS_LINUX_CNDVAR_H
+
+#include "src/__support/threads/linux/futex_utils.h" // Futex
+#include "src/__support/threads/mutex.h"             // Mutex
+
+#include <stdint.h> // uint32_t
+
+namespace LIBC_NAMESPACE {
+
+struct CndVar {
+  enum CndWaiterStatus : uint32_t {
+    WS_Waiting = 0xE,
+    WS_Signalled = 0x5,
+  };
+
+  struct CndWaiter {
+    Futex futex_word = WS_Waiting;
+    CndWaiter *next = nullptr;
+  };
+
+  CndWaiter *waitq_front;
+  CndWaiter *waitq_back;
+  Mutex qmtx;
+
+  static int init(CndVar *cv) {
+    cv->waitq_front = cv->waitq_back = nullptr;
+    auto err = Mutex::init(&cv->qmtx, false, false, false);
+    return err == MutexError::NONE ? 0 : -1;
+  }
+
+  static void destroy(CndVar *cv) {
+    cv->waitq_front = cv->waitq_back = nullptr;
+  }
+
+  // Returns 0 on success, -1 on error.
+  int wait(Mutex *m);
+  void notify_one();
+  void broadcast();
+};
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_CNDVAR_H

libc/src/__support/threads/linux/CMakeLists.txt

@@ -63,3 +63,16 @@ add_object_library(
   DEPENDS
     .futex_utils
 )
+
+add_object_library(
+  CndVar
+  SRCS
+    CndVar.cpp
+  HDRS
+    ../CndVar.h
+  DEPENDS
+    libc.include.sys_syscall
+    libc.src.__support.OSUtil.osutil
+    libc.src.__support.threads.linux.futex_word_type
+    libc.src.__support.threads.mutex
+)

libc/src/__support/threads/linux/CndVar.cpp

@@ -0,0 +1,103 @@
+//===-- 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;
+    }
+  }
+
+  waiter.futex_word.wait(WS_Waiting, cpp::nullopt, true);
+
+  // 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;
+}
+
+void CndVar::notify_one() {
+  // We don't use an RAII locker in this method as we want to unlock
+  // |qmtx| and signal the waiter using a single FUTEX_WAKE_OP signal.
+  qmtx.lock();
+  if (waitq_front == nullptr)
+    qmtx.unlock();
+
+  CndWaiter *first = waitq_front;
+  waitq_front = waitq_front->next;
+  if (waitq_front == nullptr)
+    waitq_back = nullptr;
+
+  qmtx.futex_word = FutexWordType(Mutex::LockState::Free);
+
+  // this is a special WAKE_OP, so we use syscall directly
+  LIBC_NAMESPACE::syscall_impl<long>(
+      FUTEX_SYSCALL_ID, &qmtx.futex_word.val, FUTEX_WAKE_OP, 1, 1,
+      &first->futex_word.val,
+      FUTEX_OP(FUTEX_OP_SET, WS_Signalled, FUTEX_OP_CMP_EQ, WS_Waiting));
+}
+
+void CndVar::broadcast() {
+  MutexLock ml(&qmtx);
+  uint32_t dummy_futex_word;
+  CndWaiter *waiter = waitq_front;
+  waitq_front = waitq_back = nullptr;
+  while (waiter != nullptr) {
+    // FUTEX_WAKE_OP is used instead of just FUTEX_WAKE as it allows us to
+    // atomically update the waiter status to WS_Signalled before waking
+    // up the waiter. A dummy location is used for the other futex of
+    // FUTEX_WAKE_OP.
+    LIBC_NAMESPACE::syscall_impl<long>(
+        FUTEX_SYSCALL_ID, &dummy_futex_word, FUTEX_WAKE_OP, 1, 1,
+        &waiter->futex_word.val,
+        FUTEX_OP(FUTEX_OP_SET, WS_Signalled, FUTEX_OP_CMP_EQ, WS_Waiting));
+    waiter = waiter->next;
+  }
+}
+
+} // namespace LIBC_NAMESPACE

libc/src/threads/linux/CMakeLists.txt

@@ -1,7 +1,6 @@
 add_header_library(
   threads_utils
   HDRS
-    CndVar.h
     Futex.h
   DEPENDS
     libc.include.sys_syscall
@@ -20,8 +19,8 @@ add_entrypoint_object(
   HDRS
     ../cnd_init.h
   DEPENDS
-    .threads_utils
     libc.include.threads
+    libc.src.__support.threads.CndVar
 )
 
 add_entrypoint_object(
@@ -31,8 +30,8 @@ add_entrypoint_object(
   HDRS
     ../cnd_destroy.h
   DEPENDS
-    .threads_utils
     libc.include.threads
+    libc.src.__support.threads.CndVar
 )
 
 add_entrypoint_object(
@@ -42,9 +41,9 @@ add_entrypoint_object(
   HDRS
     ../cnd_wait.h
   DEPENDS
-    .threads_utils
     libc.include.threads
     libc.src.__support.threads.mutex
+    libc.src.__support.threads.CndVar
 )
 
 add_entrypoint_object(
@@ -54,8 +53,8 @@ add_entrypoint_object(
   HDRS
     ../cnd_signal.h
   DEPENDS
-    .threads_utils
     libc.include.threads
+    libc.src.__support.threads.CndVar
 )
 
 add_entrypoint_object(
@@ -65,6 +64,6 @@ add_entrypoint_object(
   HDRS
     ../cnd_broadcast.h
   DEPENDS
-    .threads_utils
     libc.include.threads
+    libc.src.__support.threads.CndVar
 )