mutex: add wait queue and spinlocks

arch/at91sam3x8e/include/arch/serial.h

@@ -7,10 +7,6 @@
 #include <ardix/serial.h>
 #include <ardix/util.h>
 
-#ifndef CONFIG_ARCH_SERIAL_BUFSZ
-#define CONFIG_ARCH_SERIAL_BUFSZ 32
-#endif /* CONFIG_ARCH_SERIAL_BUFSZ */
-
 /** Architecture-specific extension of `struct serial_device` */
 struct arch_serial_device {
 	/** should always match REG_UART_PDC_TPR */

arch/at91sam3x8e/include/asm.S

@@ -19,7 +19,8 @@
 	 * Hardware automatically saves r0-r3, r12, pc, lr, and psr onto the
 	 * stack that was used before exception entry.  Bit 2 in lr is 1 if the
 	 * psp was used, and 0 if msp was used.  We store the appropriate stack
-	 * pointer into r12 and push it onto the stack
+	 * pointer into r12 and push it onto the stack along with the remaining
+	 * registers.
 	 */
 	tst	lr,	#(1 << 2)
 	ite	ne

arch/at91sam3x8e/mutex.S

@@ -2,10 +2,10 @@
 
 .include "asm.S"
 
-.section .text.shared
+.text
 
-/* void _mutex_lock(uint8_t *lock); */
-func_begin _mutex_lock
+/* void _spin_lock(uint8_t *lock); */
+func_begin _spin_lock
 
 	mov		r1,	#1		/* uint8_t newval = 1; */
 
@@ -21,10 +21,12 @@ func_begin _mutex_lock
 	dmb
 	bx		lr
 
-func_end _mutex_lock
+func_end _spin_lock
 
 /* int _mutex_trylock(uint8_t *lock); */
 func_begin _mutex_trylock
+/* int _spin_trylock(uint8_t *lock); */
+func_begin _spin_trylock
 
 	mov		r1,	#1		/* uint8_t newval = 1; */
 	/* move lock to r2 to make room in r0 for the return value */
@@ -40,16 +42,20 @@ func_begin _mutex_trylock
 
 	bx		lr			/* return tmp; */
 
+func_end _spin_trylock
 func_end _mutex_trylock
 
 /* void _mutex_unlock(uint8_t *lock); */
 func_begin _mutex_unlock
+/* void _spin_unlock(uint8_t *lock); */
+func_begin _spin_unlock
 
 	mov	r1,	#0
 	strb	r1,	[r0]
 	dmb
 	bx	lr
 
+func_end _spin_unlock
 func_end _mutex_unlock
 
 /*

include/ardix/mutex.h

@@ -2,6 +2,8 @@
 
 #pragma once
 
+#include <ardix/atomic.h>
+#include <ardix/list.h>
 #include <ardix/types.h>
 
 #include <errno.h>
@@ -13,23 +15,72 @@
  * @{
  */
 
+/**
+ * @brief A spinning mutex.
+ * This is the most primitive type of mutex.  It is locked by looping (spinning)
+ * so long until the lock could be acquired, thus blocking the CPU from doing
+ * anything else.  For this reason, tasks that hold a spinlock must not sleep
+ * while holding such a lock.  This behavior is enforced by calling
+ * `atomic_enter()` in the `spin_lock()` function and `atomic_leave()` in
+ * `spin_unlock()`.
+ */
+typedef struct {
+	uint8_t lock;
+} spin_t;
+
+extern void _spin_lock(uint8_t *lock);
+extern void _spin_unlock(uint8_t *lock);
+extern int _spin_trylock(uint8_t *lock);
+
+__always_inline void spin_init(spin_t *spin)
+{
+	spin->lock = 0;
+}
+
+__always_inline void spin_lock(spin_t *spin)
+{
+	atomic_enter();
+	_spin_lock(&spin->lock);
+}
+
+__always_inline void spin_unlock(spin_t *spin)
+{
+	_spin_unlock(&spin->lock);
+	atomic_leave();
+}
+
+__always_inline int spin_trylock(spin_t *spin)
+{
+	if (_spin_trylock(&spin->lock) == 0)
+		return 0;
+	else
+		return -EAGAIN;
+}
+
+__always_inline bool spin_is_locked(spin_t *spin)
+{
+	return spin->lock != 0;
+}
+
 /**
  * @brief A simple mutex.
  *
- * Mutexes can be locked using the `mutex_lock()` and `mutex_unlock()` methods
- * respectively.  The former will block until the lock is acquired and thus
- * should never be used from interrupt context.  Use `mutex_trylock()` if you
- * don't want blocking.
+ * Mutexes can be locked and unlocked using the `mutex_lock()` and
+ * `mutex_unlock()` methods respectively.  The former will block until the lock
+ * is acquired and thus should never be used from interrupt context.
+ * Use `mutex_trylock()` if you don't want blocking.
  */
 struct mutex {
 	uint8_t lock;	/**< Current lock value, don't read directly */
+	spin_t wait_queue_lock;
+	struct list_head wait_queue; /**< -> mutex_wait::link */
+};
+
+struct mutex_wait {
+	struct list_head link;
+	struct task *task;
 };
 
-/**
- * @brief Internal assembly routine for `mutex_lock()`.
- * @private
- */
-extern void _mutex_lock(uint8_t *lock);
 /**
  * @brief Internal assembly routine for `mutex_trylock()`.
  * @private
@@ -49,28 +100,23 @@ extern void _mutex_unlock(uint8_t *lock);
 __always_inline void mutex_init(struct mutex *mutex)
 {
 	mutex->lock = 0;
+	list_init(&mutex->wait_queue);
 }
 
 /**
  * @brief Acquire an exclusive lock on a mutex.
- * This call will block until the lock was acquired and therefore cannot fail.
+ * This call may sleep if the lock cannot be acquired instantly.
  *
  * @param mutex Mutex to lock
  */
-__always_inline void mutex_lock(struct mutex *mutex)
-{
-	_mutex_lock(&mutex->lock);
-}
+void mutex_lock(struct mutex *mutex);
 
 /**
  * @brief Release an exclusive lock on a mutex.
  *
  * @param mutex Mutex to unlock
  */
-__always_inline void mutex_unlock(struct mutex *mutex)
-{
-	_mutex_unlock(&mutex->lock);
-}
+void mutex_unlock(struct mutex *mutex);
 
 /**
  * @brief Attempt to acquire an exclusive lock on a mutex.
@@ -91,11 +137,11 @@ __always_inline int mutex_trylock(struct mutex *mutex)
  * @brief Determine whether a mutex is locked.
  *
  * @param mutex Mutex to get the lock value of
- * @returns Nonzero if the mutex is locked, zero otherwise
+ * @returns true if the mutex is locked
  */
-__always_inline int mutex_is_locked(struct mutex *mutex)
+__always_inline bool mutex_is_locked(struct mutex *mutex)
 {
-	return mutex->lock;
+	return mutex->lock != 0;
 }
 
 /**

include/ardix/sched.h

@@ -25,6 +25,8 @@ enum task_state {
 	TASK_SLEEP,
 	/** Task is waiting for I/O to flush buffers. */
 	TASK_IOWAIT,
+	/** Task is waiting for a mutex to be unlocked. */
+	TASK_LOCKWAIT,
 };
 
 /** @brief Core structure holding information about a task. */

kernel/CMakeLists.txt

@@ -14,6 +14,7 @@ target_sources(ardix_kernel PRIVATE
 	io.c
 	kent.c
 	kevent.c
+	mutex.c
 	ringbuf.c
 	sched.c
 	serial.c

kernel/device.c

@@ -59,7 +59,7 @@ static void device_kevent_destroy(struct kent *kent)
 
 struct device_kevent *device_kevent_create(struct device *device, enum device_channel channel)
 {
-	struct device_kevent *event = malloc(sizeof(*event));
+	struct device_kevent *event = atomic_malloc(sizeof(*event));
 	if (event == NULL)
 		return NULL;
 

kernel/fs/file.c

@@ -77,19 +77,12 @@ struct file *file_get(int fd)
 
 	return f;
 }
-#include <arch/debug.h>
 
 void file_put(struct file *f)
 {
 	kent_put(&f->kent);
 }
 
-struct io_file_kevent_extra {
-	struct file *file;
-	struct task *task;
-	enum file_kevent_flags flags;
-};
-
 struct io_device_kevent_extra {
 	struct file *file;
 	struct task *task;
@@ -115,43 +108,6 @@ static int io_device_kevent_listener(struct kevent *event, void *_extra)
 	return KEVENT_CB_LISTENER_DEL | KEVENT_CB_STOP;
 }
 
-static int io_file_kevent_listener(struct kevent *event, void *_extra)
-{
-	struct io_file_kevent_extra *extra = _extra;
-
-	struct file *file = kevent_to_file(event);
-	if (file != extra->file)
-		return KEVENT_CB_NONE;
-
-	struct file_kevent *file_kevent = kevent_to_file_kevent(event);
-	if ((file_kevent->flags & extra->flags) == 0)
-		return KEVENT_CB_NONE;
-
-	extra->task->state = TASK_QUEUE;
-	free(extra);
-	file_put(extra->file);
-	kent_put(&extra->task->kent);
-	return KEVENT_CB_LISTENER_DEL | KEVENT_CB_STOP;
-}
-
-static int iowait_file(struct file *file, enum file_kevent_flags flags)
-{
-	file_get(file->fd);
-	kent_get(&current->kent);
-
-	struct io_file_kevent_extra *extra = malloc(sizeof(*extra));
-	if (extra == NULL)
-		return -ENOMEM;
-
-	extra->file = file;
-	extra->task = current;
-	extra->flags = flags;
-
-	kevent_listener_add(KEVENT_FILE, io_file_kevent_listener, extra);
-	yield(TASK_IOWAIT);
-	return 0;
-}
-
 static int iowait_device(struct file *file, enum device_channel channel)
 {
 	file_get(file->fd);
@@ -177,11 +133,7 @@ ssize_t file_read(void *buf, struct file *file, size_t len)
 
 	ssize_t ret = 0;
 
-	while (mutex_trylock(&file->lock) != 0) {
-		ret = iowait_file(file, FILE_KEVENT_UNLOCK);
-		if (ret != 0)
-			return ret;
-	}
+	mutex_lock(&file->lock);
 
 	while (ret < (ssize_t)len) {
 		ssize_t tmp = file->device->read(buf, file->device, len, file->pos);
@@ -214,23 +166,17 @@ ssize_t file_write(struct file *file, const void *buf, size_t len)
 
 	ssize_t ret = 0;
 
-	while (mutex_trylock(&file->lock) != 0) {
-		ret = iowait_file(file, FILE_KEVENT_UNLOCK);
-		if (ret != 0)
-			return ret;
-	}
+	mutex_lock(&file->lock);
 
 	while (ret < (ssize_t)len) {
 		ssize_t tmp = file->device->write(file->device, buf, len, file->pos);
 		if (tmp < 0) {
 			if (tmp == -EBUSY) {
-				__breakpoint;
 				tmp = iowait_device(file, DEVICE_CHANNEL_OUT);
 				if (tmp < 0) {
 					ret = tmp;
 					break;
 				}
-				__breakpoint;
 			} else {
 				ret = tmp;
 				break;
@@ -259,7 +205,7 @@ static void file_kevent_destroy(struct kent *kent)
 
 struct file_kevent *file_kevent_create(struct file *f, enum file_kevent_flags flags)
 {
-	struct file_kevent *event = malloc(sizeof(*event));
+	struct file_kevent *event = atomic_malloc(sizeof(*event));
 	if (event == NULL)
 		return NULL;
 

kernel/mutex.c

@@ -0,0 +1,63 @@
+/* See the end of this file for copyright, license, and warranty information. */
+
+#include <arch-generic/do_switch.h>
+#include <ardix/list.h>
+#include <ardix/malloc.h>
+#include <ardix/mutex.h>
+#include <ardix/sched.h>
+#include <ardix/util.h>
+
+#include <errno.h>
+#include <stddef.h>
+
+void mutex_lock(struct mutex *mutex)
+{
+	if (mutex_trylock(mutex) != 0) {
+		struct mutex_wait *entry = malloc(sizeof(*entry));
+		if (entry == NULL) {
+			_spin_lock(&mutex->lock); /* fall back to spinning */
+			return;
+		}
+
+		spin_lock(&mutex->wait_queue_lock);
+		entry->task = current;
+		list_insert(&mutex->wait_queue, &entry->link);
+		spin_unlock(&mutex->wait_queue_lock);
+
+		yield(TASK_LOCKWAIT);
+	}
+}
+
+void mutex_unlock(struct mutex *mutex)
+{
+	struct mutex_wait *waiter = NULL;
+
+	spin_lock(&mutex->wait_queue_lock);
+	if (!list_is_empty(&mutex->wait_queue)) {
+		waiter = list_first_entry(&mutex->wait_queue, struct mutex_wait, link);
+		list_delete(&waiter->link);
+	}
+	spin_unlock(&mutex->wait_queue_lock);
+
+	if (waiter != NULL) {
+		struct task *task = waiter->task;
+		free(waiter);
+		current->state = TASK_QUEUE;
+		do_switch(current, task);
+	} else {
+		_mutex_unlock(&mutex->lock);
+	}
+}
+
+/*
+ * This file is part of Ardix.
+ * Copyright (c) 2021 Felix Kopp <owo@fef.moe>.
+ *
+ * Ardix is non-violent software: you may only use, redistribute,
+ * and/or modify it under the terms of the CNPLv6+ as found in
+ * the LICENSE file in the source code root directory or at
+ * <https://git.pixie.town/thufie/CNPL>.
+ *
+ * Ardix comes with ABSOLUTELY NO WARRANTY, to the extent
+ * permitted by applicable law.  See the CNPLv6+ for details.
+ */

kernel/sched.c

@@ -90,6 +90,7 @@ static inline bool can_run(const struct task *task)
 		return true;
 	case TASK_DEAD:
 	case TASK_IOWAIT:
+	case TASK_LOCKWAIT:
 		return false;
 	}
 

kernel/serial.c

@@ -85,9 +85,7 @@ ssize_t serial_read(void *dest, struct serial_device *dev, size_t len)
 {
 	ssize_t ret;
 
-	atomic_enter();
 	ret = (ssize_t)ringbuf_read(dest, dev->rx, len);
-	atomic_leave();
 
 	return ret;
 }
@@ -96,9 +94,7 @@ ssize_t serial_write(struct serial_device *dev, const void *data, size_t len)
 {
 	ssize_t ret;
 
-	atomic_enter();
 	ret = arch_serial_write(dev, data, len);
-	atomic_leave();
 
 	return ret;
 }