[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

7 files changed, 38 insertions, 169 deletions

diff --git a/libc/src/threads/linux/CMakeLists.txt b/libc/src/threads/linux/CMakeLists.txt
index 68b7106c2052..a5a02e47aab3 100644
--- a/libc/src/threads/linux/CMakeLists.txt
+++ b/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
 )
diff --git a/libc/src/threads/linux/CndVar.h b/libc/src/threads/linux/CndVar.h
deleted file mode 100644
index c08ffa393856..000000000000
--- a/libc/src/threads/linux/CndVar.h
+++ /dev/null
@@ -1,148 +0,0 @@
-//===-- 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
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef LLVM_LIBC_SRC_THREADS_LINUX_CNDVAR_H
-#define LLVM_LIBC_SRC_THREADS_LINUX_CNDVAR_H
-
-#include "src/__support/CPP/atomic.h"
-#include "src/__support/CPP/mutex.h" // lock_guard
-#include "src/__support/CPP/optional.h"
-#include "src/__support/OSUtil/syscall.h" // For syscall functions.
-#include "src/__support/threads/linux/futex_utils.h"
-#include "src/__support/threads/mutex.h"
-
-#include <linux/futex.h> // For futex operations.
-#include <stdint.h>
-#include <sys/syscall.h> // For syscall numbers.
-#include <threads.h>     // For values like thrd_success etc.
-
-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 ? thrd_success : thrd_error;
-  }
-
-  static void destroy(CndVar *cv) {
-    cv->waitq_front = cv->waitq_back = nullptr;
-  }
-
-  int 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;
-    {
-      cpp::lock_guard 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 thrd_error;
-      }
-    }
-
-    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 ? thrd_success : thrd_error;
-  }
-
-  int 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();
-      return thrd_success;
-    }
-
-    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));
-    return thrd_success;
-  }
-
-  int broadcast() {
-    cpp::lock_guard 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;
-    }
-    return thrd_success;
-  }
-};
-
-static_assert(sizeof(CndVar) == sizeof(cnd_t),
-              "Mismatch in the size of the "
-              "internal representation of condition variable and the public "
-              "cnd_t type.");
-
-} // namespace LIBC_NAMESPACE
-
-#endif // LLVM_LIBC_SRC_THREADS_LINUX_CNDVAR_H
diff --git a/libc/src/threads/linux/cnd_broadcast.cpp b/libc/src/threads/linux/cnd_broadcast.cpp
index 180ac6d68ee8..a56aaa21ee12 100644
--- a/libc/src/threads/linux/cnd_broadcast.cpp
+++ b/libc/src/threads/linux/cnd_broadcast.cpp
@@ -6,16 +6,21 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "CndVar.h"
-
 #include "src/threads/cnd_broadcast.h"
 #include "src/__support/common.h"
+#include "src/__support/threads/CndVar.h"
+
+// TODO: https://github.com/llvm/llvm-project/issues/92968
+#include <threads.h> // cnd_t, thrd_error, thrd_success
 
 namespace LIBC_NAMESPACE {
 
+static_assert(sizeof(CndVar) == sizeof(cnd_t));
+
 LLVM_LIBC_FUNCTION(int, cnd_broadcast, (cnd_t * cond)) {
   CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
-  return cndvar->broadcast();
+  cndvar->broadcast();
+  return thrd_success;
 }
 
 } // namespace LIBC_NAMESPACE
diff --git a/libc/src/threads/linux/cnd_destroy.cpp b/libc/src/threads/linux/cnd_destroy.cpp
index 08eb3a1057b1..2b03b18c48e4 100644
--- a/libc/src/threads/linux/cnd_destroy.cpp
+++ b/libc/src/threads/linux/cnd_destroy.cpp
@@ -6,13 +6,16 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "CndVar.h"
-
 #include "src/threads/cnd_destroy.h"
 #include "src/__support/common.h"
+#include "src/__support/threads/CndVar.h"
+
+#include <threads.h> // cnd_t
 
 namespace LIBC_NAMESPACE {
 
+static_assert(sizeof(CndVar) == sizeof(cnd_t));
+
 LLVM_LIBC_FUNCTION(void, cnd_destroy, (cnd_t * cond)) {
   CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
   CndVar::destroy(cndvar);
diff --git a/libc/src/threads/linux/cnd_init.cpp b/libc/src/threads/linux/cnd_init.cpp
index 5e3f360b1d2b..d3d2c8a57d82 100644
--- a/libc/src/threads/linux/cnd_init.cpp
+++ b/libc/src/threads/linux/cnd_init.cpp
@@ -6,16 +6,19 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "CndVar.h"
-
 #include "src/threads/cnd_init.h"
 #include "src/__support/common.h"
+#include "src/__support/threads/CndVar.h"
+
+#include <threads.h> // cnd_t, thrd_error, thrd_success
 
 namespace LIBC_NAMESPACE {
 
+static_assert(sizeof(CndVar) == sizeof(cnd_t));
+
 LLVM_LIBC_FUNCTION(int, cnd_init, (cnd_t * cond)) {
   CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
-  return CndVar::init(cndvar);
+  return CndVar::init(cndvar) ? thrd_error : thrd_success;
 }
 
 } // namespace LIBC_NAMESPACE
diff --git a/libc/src/threads/linux/cnd_signal.cpp b/libc/src/threads/linux/cnd_signal.cpp
index dba01abdefbc..f144013e0882 100644
--- a/libc/src/threads/linux/cnd_signal.cpp
+++ b/libc/src/threads/linux/cnd_signal.cpp
@@ -6,16 +6,20 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "CndVar.h"
-
 #include "src/threads/cnd_signal.h"
 #include "src/__support/common.h"
+#include "src/__support/threads/CndVar.h"
+
+#include <threads.h> // cnd_t, thrd_error, thrd_success
 
 namespace LIBC_NAMESPACE {
 
+static_assert(sizeof(CndVar) == sizeof(cnd_t));
+
 LLVM_LIBC_FUNCTION(int, cnd_signal, (cnd_t * cond)) {
   CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
-  return cndvar->notify_one();
+  cndvar->notify_one();
+  return thrd_success;
 }
 
 } // namespace LIBC_NAMESPACE
diff --git a/libc/src/threads/linux/cnd_wait.cpp b/libc/src/threads/linux/cnd_wait.cpp
index db3d7f1436eb..97cade3f231d 100644
--- a/libc/src/threads/linux/cnd_wait.cpp
+++ b/libc/src/threads/linux/cnd_wait.cpp
@@ -6,18 +6,21 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "CndVar.h"
-
+#include "src/threads/cnd_wait.h"
 #include "src/__support/common.h"
+#include "src/__support/threads/CndVar.h"
 #include "src/__support/threads/mutex.h"
-#include "src/threads/cnd_wait.h"
+
+#include <threads.h> // cnd_t, mtx_t, thrd_error, thrd_success
 
 namespace LIBC_NAMESPACE {
 
+static_assert(sizeof(CndVar) == sizeof(cnd_t));
+
 LLVM_LIBC_FUNCTION(int, cnd_wait, (cnd_t * cond, mtx_t *mtx)) {
   CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
   Mutex *mutex = reinterpret_cast<Mutex *>(mtx);
-  return cndvar->wait(mutex);
+  return cndvar->wait(mutex) ? thrd_error : thrd_success;
 }
 
 } // namespace LIBC_NAMESPACE