Futex syscall implementation

so3/include/futex.h

@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2014-2018 Daniel Rossier <daniel.rossier@heig-vd.ch>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ */
+
+#ifndef FUTEX_H
+#define FUTEX_H
+
+#include <timer.h>
+#include <thread.h>
+#include <list.h>
+#include <spinlock.h>
+#include <syscall.h>
+
+/* Commands */
+#define FUTEX_WAIT 0
+#define FUTEX_WAKE 1
+#define FUTEX_FD 2
+#define FUTEX_REQUEUE 3
+#define FUTEX_CMP_REQUEUE 4
+#define FUTEX_WAKE_OP 5
+#define FUTEX_LOCK_PI 6
+#define FUTEX_UNLOCK_PI 7
+#define FUTEX_TRYLOCK_PI 8
+#define FUTEX_WAIT_BITSET 9
+
+#define FUTEX_PRIVATE_FLAG 128
+#define FUTEX_CLOCK_REALTIME 256
+#define FUTEX_CMD_MASK ~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME)
+
+/**
+ * list of thread (waiter) waiting on a futex
+ */
+typedef struct futex_el {
+	struct list_head list;
+	tcb_t *waiter;
+} futex_el_t;
+
+/**
+ * list of futexes
+ */
+typedef struct futex {
+	struct list_head list;
+	struct list_head f_element;
+	uintptr_t key;
+} futex_t;
+
+SYSCALL_DECLARE(futex, u32 *uaddr, int op, u32 val, const struct timespec *utime, u32 *uaddr2, u32 val3)
+
+void futex_init(pcb_t *pcb);
+
+#endif /* FUTEX_H */

so3/include/mutex.h

@@ -59,8 +59,4 @@ void mutex_lock(struct mutex *lock);
 void mutex_unlock(struct mutex *lock);
 void mutex_init(struct mutex *lock);
 
-SYSCALL_DECLARE(mutex_init, void);
-SYSCALL_DECLARE(mutex_lock, unsigned long number);
-SYSCALL_DECLARE(mutex_unlock, unsigned long number);
-
 #endif /* MUTEX_H */

so3/include/process.h

@@ -29,6 +29,8 @@
 #include <memory.h>
 #include <signal.h>
 #include <ptrace.h>
+#include <spinlock.h>
+#include <futex.h>
 #include <mutex.h>
 #include <syscall.h>
 
@@ -82,6 +84,10 @@ struct pcb {
 	/* Number of pages required by this process (including binary image) */
 	size_t page_count;
 
+	/* List of futexes */
+	struct list_head futex;
+	spinlock_t futex_lock;
+
 	/* List of frames (physical pages) belonging to this process */
 	struct list_head page_list;
 

so3/kernel/Makefile

@@ -8,6 +8,7 @@ obj-y += 	main.o \
 		thread.o \
 		schedule.o \
 		mutex.o  \
+		futex.o  \
 		spinlock.o \
 		syscalls.o \
 		softirq.o \

so3/kernel/futex.c

@@ -0,0 +1,163 @@
+/*
+ * Copyright (C) 2025 Jean-Pierre Miceli <jean-pierre.miceli@heig-vd.ch>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ */
+
+#include <process.h>
+#include <heap.h>
+#include <errno.h>
+#include <futex.h>
+
+/**
+ * do_futex_wait - block on futex_w
+ *
+ * @param futex_w address of the futex word
+ * @param val expected value of the futex word
+ * @return 0 on success or error value
+ */
+static int do_futex_wait(uint32_t *futex_w, uint32_t val)
+{
+	unsigned long flags;
+	pcb_t *pcb = current()->pcb;
+	struct list_head *pos;
+	futex_t *futex;
+	futex_el_t *f_element;
+
+	flags = spin_lock_irqsave(&pcb->futex_lock);
+
+	if (*futex_w != val)
+		return -EAGAIN;
+
+	/* look if a futex_w already exists */
+	list_for_each(pos, &pcb->futex) {
+		futex = list_entry(pos, futex_t, list);
+
+		if ((uintptr_t) futex_w == futex->key)
+			break;
+	}
+
+	if (pos == &pcb->futex) {
+		/* no futex on futex_w */
+		futex = (futex_t *) calloc(1, sizeof(futex_t));
+		if (futex == NULL)
+			BUG();
+
+		futex->key = (uintptr_t) futex_w;
+
+		INIT_LIST_HEAD(&futex->f_element);
+		list_add_tail(&futex->list, &pcb->futex);
+	}
+
+	/* Add the thread in the futex_element list */
+	f_element = (futex_el_t *) calloc(1, sizeof(futex_el_t));
+	if (f_element == NULL)
+		BUG();
+
+	f_element->waiter = current();
+
+	list_add_tail(&f_element->list, &futex->f_element);
+
+	/* go to sleep. */
+	spin_unlock(&pcb->futex_lock);
+	waiting();
+
+	BUG_ON(local_irq_is_enabled());
+
+	spin_unlock_irqrestore(&pcb->futex_lock, flags);
+
+	return 0;
+}
+
+/**
+ * do_futex_wake - wake one or more tasks blocked on uaddr
+ *
+ * @nr_wake wake up to this many tasks
+ * @return the number of waiters that were woken up
+ */
+static int do_futex_wake(uint32_t *futex_w, uint32_t nr_wake)
+{
+	unsigned long flags;
+	pcb_t *pcb = current()->pcb;
+	struct list_head *pos, *p;
+	futex_t *futex;
+	futex_el_t *f_element;
+	unsigned idx = 0;
+
+	flags = spin_lock_irqsave(&pcb->futex_lock);
+
+	/* Search for the futex element with futex_w as key */
+	list_for_each(pos, &pcb->futex) {
+		futex = list_entry(pos, futex_t, list);
+
+		if ((uintptr_t) futex_w == futex->key)
+			break;
+	}
+
+	if (pos == &pcb->futex)
+		/* key does not exists in futex - Error */
+		BUG();
+
+	/* wakes at most nr_wake of the waiters that are waiting */
+	list_for_each_safe(pos, p, &futex->list) {
+		f_element = list_entry(pos, futex_el_t, list);
+
+		if (idx == nr_wake)
+			break;
+
+		list_del(&f_element->list);
+		ready(f_element->waiter);
+
+		idx++;
+	}
+
+	spin_unlock_irqrestore(&pcb->futex_lock, flags);
+
+	return 0;
+}
+
+SYSCALL_DEFINE6(futex, uint32_t *, uaddr, int, op, uint32_t, val, const struct timespec *, utime, uint32_t *, uaddr2, uint32_t,
+		val3)
+{
+	int cmd = op & FUTEX_CMD_MASK;
+
+	switch (cmd) {
+	case FUTEX_WAIT:
+		return do_futex_wait(uaddr, val);
+	case FUTEX_WAKE:
+		return do_futex_wake(uaddr, val);
+	case FUTEX_FD:
+	case FUTEX_REQUEUE:
+	case FUTEX_CMP_REQUEUE:
+	case FUTEX_WAKE_OP:
+	case FUTEX_LOCK_PI:
+	case FUTEX_UNLOCK_PI:
+	case FUTEX_TRYLOCK_PI:
+	case FUTEX_WAIT_BITSET:
+		printk("Futex cmd '%d' not supported !\n");
+		return -EINVAL;
+	}
+
+	return -ENOSYS;
+}
+
+/**
+ * futex_init - Futex initialization
+ */
+void futex_init(pcb_t *pcb)
+{
+	INIT_LIST_HEAD(&pcb->futex);
+	spin_lock_init(&pcb->futex_lock);
+}

so3/kernel/mutex.c

@@ -146,27 +146,6 @@ void mutex_unlock(struct mutex *lock)
 		schedule();
 }
 
-/*
- * The following syscall implementation are a first attempt, mainly used for debugging kernel mutexes.
- */
-SYSCALL_DEFINE1(mutex_lock, unsigned long, number)
-{
-	if (number >= N_MUTEX) {
-		return -EINVAL;
-	}
-	mutex_lock(&current()->pcb->lock[number]);
-	return 0;
-}
-
-SYSCALL_DEFINE1(mutex_unlock, unsigned long, number)
-{
-	if (number >= N_MUTEX) {
-		return -EINVAL;
-	}
-	mutex_unlock(&current()->pcb->lock[number]);
-	return 0;
-}
-
 void mutex_init(struct mutex *lock)
 {
 	memset(lock, 0, sizeof(struct mutex));

so3/kernel/process.c

@@ -205,6 +205,9 @@ pcb_t *new_process(void)
 	/* Init the list of pages */
 	INIT_LIST_HEAD(&pcb->page_list);
 
+	/* Init the futex */
+	futex_init(pcb);
+
 	pcb->pid = pid_current++;
 
 	for (i = 0; i < PROC_MAX_THREADS; i++)

so3/kernel/syscalls.c

@@ -28,6 +28,7 @@
 #include <signal.h>
 #include <timer.h>
 #include <net.h>
+#include <futex.h>
 #include <syscall.h>
 
 extern void __get_syscall_args_ext(uint32_t *syscall_no);

so3/syscall.tbl

@@ -37,6 +37,7 @@ newfstatat
 mmap
 mmap2
 nanosleep
+futex
 pipe                    IPC_PIPE
 pipe2                   IPC_PIPE
 rt_sigaction            IPC_SIGNAL