[libc] Add implementations of the C standard condition variable functions.

[sivachandra]

I will be adding more tests but sending this out early to get feedback on the actual implementation.

[sivachandra]

Rebase and add another test.

[sivachandra]

Ping

[michaelrj]

LGTM

libc/src/threads/linux/CndVar.h

59

are these braces necessary?

[sivachandra]

libc/src/threads/linux/CndVar.h

59

These are for the RAII MutexLock class instantiated on line 60.

[michaelrj]

libc/src/threads/linux/CndVar.h

59

ah, that makes sense. All good then.

libc/src/threads/linux/CMakeLists.txt

DEPENDS
     libc.config.linux.linux_syscall_h
     libc.include.sys_syscall
     libc.include.threads
 )
 
 add_header_library(
   threads_utils
   HDRS
+    CndVar.h
     Futex.h
     Mutex.h
     Thread.h
   DEPENDS
     .thread_start_args_h
     libc.config.linux.linux_syscall_h
     libc.include.sys_syscall
     libc.include.threads

libc/src/threads/linux/CndVar.h

+//===-- 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 "Futex.h"
+#include "Mutex.h"
+
+#include "config/linux/syscall.h" // For syscall functions.
+#include "include/sys/syscall.h"  // For syscall numbers.
+#include "include/threads.h"      // For values like thrd_success etc.
+
+#include <linux/futex.h> // For futex operations.
+#include <stdatomic.h>   // For atomic operations
+
+namespace __llvm_libc {
+
+struct CndVar {
+  enum CndWaiterStatus : uint32_t {
+    WS_Waiting = 0xE,
+    WS_Signalled = 0x5,
+  };
+
+  struct CndWaiter {
+    FutexWord 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;
+    return Mutex::init(&cv->qmtx, mtx_plain);
+  }
+
+  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;
+    {
+      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() != thrd_success) {
+        // 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;
+      }
+    }
+
+    __llvm_libc::syscall(SYS_futex, &waiter.futex_word, 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.
+    return m->lock();
+  }
+
+  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;
+
+    atomic_store(&qmtx.futex_word, Mutex::MS_Free);
+
+    __llvm_libc::syscall(
+        SYS_futex, &qmtx.futex_word, FUTEX_WAKE_OP, 1, 1, &first->futex_word,
+        FUTEX_OP(FUTEX_OP_SET, WS_Signalled, FUTEX_OP_CMP_EQ, WS_Waiting));
+    return thrd_success;
+  }
+
+  int broadcast() {
+    MutexLock ml(&qmtx);
+    FutexWord 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.
+      __llvm_libc::syscall(
+          SYS_futex, &dummy_futex_word, FUTEX_WAKE_OP, 1, 1,
+          &waiter->futex_word,
+          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 __llvm_libc
+
+#endif // LLVM_LIBC_SRC_THREADS_LINUX_CNDVAR_H

libc/src/threads/linux/cnd_broadcast.cpp

 //===-- Linux implementation of the cnd_broadcast function ----------------===//
 //
 // 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 "CndVar.h"
+
 #include "src/threads/cnd_broadcast.h"
 #include "src/__support/common.h"
 
 namespace __llvm_libc {
 
-LLVM_LIBC_FUNCTION(int, cnd_broadcast, (cnd_t * cond)) { return thrd_success; }
+LLVM_LIBC_FUNCTION(int, cnd_broadcast, (cnd_t * cond)) {
+  CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
+  return cndvar->broadcast();
+}
 
 } // namespace __llvm_libc

libc/src/threads/linux/cnd_destroy.cpp

 //===-- Linux implementation of the cnd_destroy function ------------------===//
 //
 // 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 "CndVar.h"
+
 #include "src/threads/cnd_destroy.h"
 #include "src/__support/common.h"
 
 namespace __llvm_libc {
 
-LLVM_LIBC_FUNCTION(void, cnd_destroy, (cnd_t * cond)) { }
+LLVM_LIBC_FUNCTION(void, cnd_destroy, (cnd_t * cond)) {
+  CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
+  CndVar::destroy(cndvar);
+}
 
 } // namespace __llvm_libc

libc/src/threads/linux/cnd_init.cpp

 //===-- Linux implementation of the cnd_init function ---------------------===//
 //
 // 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 "CndVar.h"
+
 #include "src/threads/cnd_init.h"
 #include "src/__support/common.h"
 
 namespace __llvm_libc {
 
-LLVM_LIBC_FUNCTION(int, cnd_init, (cnd_t * cond)) { return thrd_success; }
+LLVM_LIBC_FUNCTION(int, cnd_init, (cnd_t * cond)) {
+  CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
+  return CndVar::init(cndvar);
+}
 
 } // namespace __llvm_libc

libc/src/threads/linux/cnd_signal.cpp

 //===-- Linux implementation of the cnd_signal function -------------------===//
 //
 // 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 "CndVar.h"
+
 #include "src/threads/cnd_signal.h"
 #include "src/__support/common.h"
 
 namespace __llvm_libc {
 
-LLVM_LIBC_FUNCTION(int, cnd_signal, (cnd_t * cond)) { return thrd_success; }
+LLVM_LIBC_FUNCTION(int, cnd_signal, (cnd_t * cond)) {
+  CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
+  return cndvar->notify_one();
+}
 
 } // namespace __llvm_libc

libc/src/threads/linux/cnd_wait.cpp

 //===-- Linux implementation of the cnd_wait function ---------------------===//
 //
 // 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 "CndVar.h"
+#include "Mutex.h"
+
 #include "src/threads/cnd_wait.h"
 #include "src/__support/common.h"
 
 namespace __llvm_libc {
 
-LLVM_LIBC_FUNCTION(int, cnd_wait, (cnd_t * cond, mtx_t *mutex)) {
-  return thrd_success;
+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);
 }
 
 } // namespace __llvm_libc

libc/test/src/threads/CMakeLists.txt

DEPENDS
     libc.src.errno.__errno_location
     libc.src.threads.mtx_destroy
     libc.src.threads.mtx_init
     libc.src.threads.mtx_lock
     libc.src.threads.mtx_unlock
     libc.src.threads.thrd_create
     libc.src.threads.thrd_join
 )
+
+add_libc_unittest(
+  cnd_test
+  SUITE
+    libc_threads_unittests
+  SRCS
+    cnd_test.cpp
+  DEPENDS
+    libc.include.threads
+    libc.src.threads.cnd_init
+    libc.src.threads.cnd_broadcast
+    libc.src.threads.cnd_signal
+    libc.src.threads.cnd_destroy
+    libc.src.threads.cnd_wait
+    libc.src.threads.mtx_destroy
+    libc.src.threads.mtx_init
+    libc.src.threads.mtx_lock
+    libc.src.threads.mtx_unlock
+    libc.src.threads.thrd_create
+    libc.src.threads.thrd_join
+    libc.src.threads.linux.threads_utils
+)

libc/test/src/threads/cnd_test.cpp

+//===-- Unittests for condition variable broadcast fucntionality ----------===//
+//
+// 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 <stdatomic.h>
+
+#include "include/threads.h"
+#include "src/threads/cnd_broadcast.h"
+#include "src/threads/cnd_destroy.h"
+#include "src/threads/cnd_init.h"
+#include "src/threads/cnd_signal.h"
+#include "src/threads/cnd_wait.h"
+#include "src/threads/mtx_destroy.h"
+#include "src/threads/mtx_init.h"
+#include "src/threads/mtx_lock.h"
+#include "src/threads/mtx_unlock.h"
+#include "src/threads/thrd_create.h"
+#include "src/threads/thrd_join.h"
+#include "utils/UnitTest/Test.h"
+
+namespace wait_notify_broadcast_test {
+
+// The test in this namespace tests all condition variable operations. The
+// main thread spawns THRD_COUNT threads, each of which wait on a condition
+// variable |broadcast_cnd|. After spawing the threads, it waits on another
+// condition variable |threads_ready_cnd| which will be signalled by the last
+// thread before it starts waiting on |broadcast_cnd|. On signalled by the
+// last thread, the main thread then wakes up to broadcast to all waiting
+// threads to wake up. Each of the THRD_COUNT child threads increment
+// |broadcast_count| by 1 before they start waiting on |broadcast_cnd|, and
+// decrement it by 1 after getting signalled on |broadcast_cnd|.
+
+constexpr int THRD_COUNT = 10000;
+
+static atomic_uint broadcast_count = 0;
+static cnd_t broadcast_cnd, threads_ready_cnd;
+static mtx_t broadcast_mtx, threads_ready_mtx;
+
+int broadcast_thread_func(void *) {
+  __llvm_libc::mtx_lock(&broadcast_mtx);
+  int oldval = atomic_fetch_add(&broadcast_count, 1);
+  if (oldval == THRD_COUNT - 1) {
+    __llvm_libc::mtx_lock(&threads_ready_mtx);
+    __llvm_libc::cnd_signal(&threads_ready_cnd);
+    __llvm_libc::mtx_unlock(&threads_ready_mtx);
+  }
+
+  __llvm_libc::cnd_wait(&broadcast_cnd, &broadcast_mtx);
+  __llvm_libc::mtx_unlock(&broadcast_mtx);
+  atomic_fetch_sub(&broadcast_count, 1);
+  return 0;
+}
+
+TEST(LlvmLibcCndVarTest, WaitNotifyBroadcastTest) {
+  __llvm_libc::cnd_init(&broadcast_cnd);
+  __llvm_libc::cnd_init(&threads_ready_cnd);
+  __llvm_libc::mtx_init(&broadcast_mtx, mtx_plain);
+  __llvm_libc::mtx_init(&threads_ready_mtx, mtx_plain);
+
+  __llvm_libc::mtx_lock(&threads_ready_mtx);
+  thrd_t threads[THRD_COUNT];
+  for (unsigned int i = 0; i < THRD_COUNT; ++i)
+    __llvm_libc::thrd_create(&threads[i], broadcast_thread_func, nullptr);
+
+  __llvm_libc::cnd_wait(&threads_ready_cnd, &threads_ready_mtx);
+  __llvm_libc::mtx_unlock(&threads_ready_mtx);
+
+  __llvm_libc::mtx_lock(&broadcast_mtx);
+  ASSERT_EQ(int(broadcast_count), THRD_COUNT);
+  __llvm_libc::cnd_broadcast(&broadcast_cnd);
+  __llvm_libc::mtx_unlock(&broadcast_mtx);
+
+  for (unsigned int i = 0; i < THRD_COUNT; ++i) {
+    int retval = 0xBAD;
+    __llvm_libc::thrd_join(&threads[i], &retval);
+    ASSERT_EQ(retval, 0);
+  }
+
+  ASSERT_EQ(int(broadcast_count), 0);
+
+  __llvm_libc::cnd_destroy(&broadcast_cnd);
+  __llvm_libc::cnd_destroy(&threads_ready_cnd);
+  __llvm_libc::mtx_destroy(&broadcast_mtx);
+  __llvm_libc::mtx_destroy(&threads_ready_mtx);
+}
+
+} // namespace wait_notify_broadcast_test
+
+namespace single_waiter_test {
+
+// In this namespace we set up test with two threads, one the main thread
+// and the other a waiter thread. They wait on each other using condition
+// variables and mutexes before proceeding to completion.
+
+mtx_t waiter_mtx, main_thread_mtx;
+cnd_t waiter_cnd, main_thread_cnd;
+
+int waiter_thread_func(void *unused) {
+  __llvm_libc::mtx_lock(&waiter_mtx);
+
+  __llvm_libc::mtx_lock(&main_thread_mtx);
+  __llvm_libc::cnd_signal(&main_thread_cnd);
+  __llvm_libc::mtx_unlock(&main_thread_mtx);
+
+  __llvm_libc::cnd_wait(&waiter_cnd, &waiter_mtx);
+  __llvm_libc::mtx_unlock(&waiter_mtx);
+
+  return 0x600D;
+}
+
+TEST(LlvmLibcCndVarTest, SingleWaiterTest) {
+  ASSERT_EQ(__llvm_libc::mtx_init(&waiter_mtx, mtx_plain), int(thrd_success));
+  ASSERT_EQ(__llvm_libc::mtx_init(&main_thread_mtx, mtx_plain),
+            int(thrd_success));
+  ASSERT_EQ(__llvm_libc::cnd_init(&waiter_cnd), int(thrd_success));
+  ASSERT_EQ(__llvm_libc::cnd_init(&main_thread_cnd), int(thrd_success));
+
+  ASSERT_EQ(__llvm_libc::mtx_lock(&main_thread_mtx), int(thrd_success));
+
+  thrd_t waiter_thread;
+  __llvm_libc::thrd_create(&waiter_thread, waiter_thread_func, nullptr);
+
+  ASSERT_EQ(__llvm_libc::cnd_wait(&main_thread_cnd, &main_thread_mtx),
+            int(thrd_success));
+  ASSERT_EQ(__llvm_libc::mtx_unlock(&main_thread_mtx), int(thrd_success));
+
+  ASSERT_EQ(__llvm_libc::mtx_lock(&waiter_mtx), int(thrd_success));
+  ASSERT_EQ(__llvm_libc::cnd_signal(&waiter_cnd), int(thrd_success));
+  ASSERT_EQ(__llvm_libc::mtx_unlock(&waiter_mtx), int(thrd_success));
+
+  int retval;
+  __llvm_libc::thrd_join(&waiter_thread, &retval);
+  ASSERT_EQ(retval, 0x600D);
+
+  __llvm_libc::mtx_destroy(&waiter_mtx);
+  __llvm_libc::mtx_destroy(&main_thread_mtx);
+  __llvm_libc::cnd_destroy(&waiter_cnd);
+  __llvm_libc::cnd_destroy(&main_thread_cnd);
+}
+
+} // namespace single_waiter_test