kernel: add condition variables

Introduce condition variables similar to how they are done in POSIX with
a mutex.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

include/kernel.h

@@ -3168,6 +3168,84 @@ __syscall int k_mutex_unlock(struct k_mutex *mutex);
  * @}
  */
 
+
+struct k_condvar {
+	_wait_q_t wait_q;
+};
+
+#define Z_CONDVAR_INITIALIZER(obj)                                             \
+	{                                                                      \
+		.wait_q = Z_WAIT_Q_INIT(&obj.wait_q),                          \
+	}
+
+/**
+ * @defgroup condvar_apis Condition Variables APIs
+ * @ingroup kernel_apis
+ * @{
+ */
+
+/**
+ * @brief Initialize a condition variable
+ *
+ * @param condvar pointer to a @p k_condvar structure
+ * @retval 0 Condition variable created successfully
+ */
+__syscall int k_condvar_init(struct k_condvar *condvar);
+
+/**
+ * @brief Signals one thread that is pending on the condition variable
+ *
+ * @param condvar pointer to a @p k_condvar structure
+ * @retval 0 On success
+ */
+__syscall int k_condvar_signal(struct k_condvar *condvar);
+
+/**
+ * @brief Unblock all threads that are pending on the condition
+ * variable
+ *
+ * @param condvar pointer to a @p k_condvar structure
+ * @return An integer with number of woken threads on success
+ */
+__syscall int k_condvar_broadcast(struct k_condvar *condvar);
+
+/**
+ * @brief Waits on the condition variable releasing the mutex lock
+ *
+ * Automically releases the currently owned mutex, blocks the current thread
+ * waiting on the condition variable specified by @a condvar,
+ * and finally acquires the mutex again.
+ *
+ * The waiting thread unblocks only after another thread calls
+ * k_condvar_signal, or k_condvar_broadcast with the same condition variable.
+ *
+ * @param condvar pointer to a @p k_condvar structure
+ * @param mutex Address of the mutex.
+ * @param timeout Waiting period for the condition variable
+ *                or one of the special values K_NO_WAIT and K_FOREVER.
+ * @retval 0 On success
+ * @retval -EAGAIN Waiting period timed out.
+ */
+__syscall int k_condvar_wait(struct k_condvar *condvar, struct k_mutex *mutex,
+			     k_timeout_t timeout);
+
+/**
+ * @brief Statically define and initialize a condition variable.
+ *
+ * The condition variable can be accessed outside the module where it is
+ * defined using:
+ *
+ * @code extern struct k_condvar <name>; @endcode
+ *
+ * @param name Name of the condition variable.
+ */
+#define K_CONDVAR_DEFINE(name)                                                 \
+	Z_STRUCT_SECTION_ITERABLE(k_condvar, name) =                           \
+		Z_CONDVAR_INITIALIZER(name)
+/**
+ * @}
+ */
+
 /**
  * @cond INTERNAL_HIDDEN
  */

include/linker/common-ram.ld

@@ -102,6 +102,7 @@
 	Z_ITERABLE_SECTION_RAM_GC_ALLOWED(k_pipe, 4)
 	Z_ITERABLE_SECTION_RAM_GC_ALLOWED(k_sem, 4)
 	Z_ITERABLE_SECTION_RAM_GC_ALLOWED(k_queue, 4)
+	Z_ITERABLE_SECTION_RAM_GC_ALLOWED(k_condvar, 4)
 
 	SECTION_DATA_PROLOGUE(_net_buf_pool_area,,SUBALIGN(4))
 	{

kernel/CMakeLists.txt

@@ -24,6 +24,7 @@ list(APPEND kernel_files
   thread_abort.c
   version.c
   work_q.c
+  condvar.c
   smp.c
   banner.c
   )

kernel/condvar.c

@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2020 Intel Corporation.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <kernel.h>
+#include <kernel_structs.h>
+#include <toolchain.h>
+#include <ksched.h>
+#include <wait_q.h>
+
+static struct k_spinlock lock;
+
+int z_impl_k_condvar_init(struct k_condvar *condvar)
+{
+	z_waitq_init(&condvar->wait_q);
+	z_object_init(condvar);
+	return 0;
+}
+
+int z_impl_k_condvar_signal(struct k_condvar *condvar)
+{
+	k_spinlock_key_t key = k_spin_lock(&lock);
+	struct k_thread *thread = z_unpend_first_thread(&condvar->wait_q);
+
+	if (thread != NULL) {
+		arch_thread_return_value_set(thread, 0);
+		z_ready_thread(thread);
+		z_reschedule(&lock, key);
+	} else {
+		k_spin_unlock(&lock, key);
+	}
+	return 0;
+}
+
+int z_impl_k_condvar_broadcast(struct k_condvar *condvar)
+{
+	struct k_thread *pending_thread;
+	k_spinlock_key_t key;
+	int woken = 0;
+
+	key = k_spin_lock(&lock);
+
+	/* wake up any threads that are waiting to write */
+	while ((pending_thread = z_unpend_first_thread(&condvar->wait_q)) !=
+	       NULL) {
+		arch_thread_return_value_set(pending_thread, 0);
+		z_ready_thread(pending_thread);
+		woken++;
+	}
+
+	z_reschedule(&lock, key);
+
+	return woken;
+}
+
+int z_impl_k_condvar_wait(struct k_condvar *condvar, struct k_mutex *mutex,
+			  k_timeout_t timeout)
+{
+	k_spinlock_key_t key;
+	int ret;
+
+	key = k_spin_lock(&lock);
+	k_mutex_unlock(mutex);
+
+	ret = z_pend_curr(&lock, key, &condvar->wait_q, timeout);
+	k_mutex_lock(mutex, K_FOREVER);
+
+	return ret;
+}