From 82151cf1d2198204950a078ae37f767a72038fb7 Mon Sep 17 00:00:00 2001
From: Matheus <matheusuky@gmail.com>
Date: Sun, 14 Apr 2024 19:09:49 -0300
Subject: [PATCH] syscalls: futex

---
 kernel-src/sys/syscalls/futex.c | 165 ++++++++++++++++++++++++++++++++
 1 file changed, 165 insertions(+)
 create mode 100644 kernel-src/sys/syscalls/futex.c

diff --git a/kernel-src/sys/syscalls/futex.c b/kernel-src/sys/syscalls/futex.c
new file mode 100644
index 00000000..9ec1f1a1
--- /dev/null
+++ b/kernel-src/sys/syscalls/futex.c
@@ -0,0 +1,165 @@
+#include <kernel/syscalls.h>
+#include <kernel/slab.h>
+#include <kernel/vmm.h>
+#include <time.h>
+#include <hashtable.h>
+#include <kernel/poll.h>
+#include <kernel/alloc.h>
+
+static scache_t *futexcache;
+static mutex_t futexmutex;
+static hashtable_t hashtable;
+static bool hashtableinit;
+
+typedef struct {
+	pollheader_t pollheader;
+	int waiting;
+	int waking;
+} futex_t;
+
+#define FUTEX_WAIT 0
+#define FUTEX_WAKE 1
+
+static futex_t *getfutex(void *physical) {
+	void *tmp;
+	if (hashtable_get(&hashtable, &tmp, &physical, sizeof(physical)))
+		tmp = NULL;
+
+	return tmp;
+}
+
+static int setfutex(futex_t *futex, void *physical) {
+	return hashtable_set(&hashtable, futex, &physical, sizeof(physical), true);
+}
+
+static void removefutex(void *physical) {
+	hashtable_remove(&hashtable, &physical, sizeof(physical));
+}
+
+static void ctor(scache_t *cache, void *obj) {
+	futex_t *futex = obj;
+	POLL_INITHEADER(&futex->pollheader);
+	futex->waiting = 0;
+}
+
+syscallret_t syscall_futex(context_t *, uint32_t *futexp, int op, uint32_t value, timespec_t *tm) {
+	syscallret_t ret = {
+		.ret = -1
+	};
+
+	if (futexcache == NULL) {
+		futexcache = slab_newcache(sizeof(thread_t), 0, ctor, ctor);
+
+		if (futexcache == NULL) {
+			ret.errno = ENOMEM;
+			return ret;
+		}
+	}
+
+	if (hashtableinit == false) {
+		if (hashtable_init(&hashtable, 256)) {
+			ret.errno = ENOMEM;
+			return ret;
+		}
+
+		hashtableinit = true;
+		MUTEX_INIT(&futexmutex);
+	}
+
+	timespec_t timespec = tm ? *tm : (timespec_t){0};
+	polldesc_t desc = {0};
+	ret.errno = poll_initdesc(&desc, 1);
+	if (ret.errno)
+		return ret;
+
+	MUTEX_ACQUIRE(&futexmutex, false);
+
+	uint32_t word;
+	__atomic_load(futexp, &word, __ATOMIC_RELAXED);
+
+	uint32_t *physical = vmm_getphysical(futexp);
+	futex_t *futex = getfutex(physical);
+
+	switch (op) {
+		case FUTEX_WAKE:
+			ret.errno = 0;
+			if (futex == NULL) {
+				ret.ret = 0;
+				break;
+			}
+
+			int delta = futex->waiting - futex->waking;
+			ret.ret = value > delta ? delta : value;
+
+			futex->waking += ret.ret;
+
+			poll_event(&futex->pollheader, POLLOUT);
+
+			break;
+		case FUTEX_WAIT:
+			if (word != value) {
+				ret.errno = EAGAIN;
+				break;
+			}
+
+			if (futex == NULL) {
+				futex = alloc(sizeof(futex_t));
+				if (futex == NULL) {
+					ret.errno = ENOMEM;
+					break;
+				}
+
+				ret.errno = setfutex(futex, physical);
+				if (ret.errno)
+					break;
+			}
+
+			uintmax_t us = timespec.s * 1000000 + timespec.ns / 1000;
+			++futex->waiting;
+			
+			for (;;) {
+				poll_add(&futex->pollheader, &desc.data[0], POLLOUT);
+
+				MUTEX_RELEASE(&futexmutex);
+
+				ret.errno = poll_dowait(&desc, us);
+				int revents = desc.event->revents;
+
+				MUTEX_ACQUIRE(&futexmutex, false);
+
+				if (revents == 0 || ret.errno == EINTR) {
+					// timed out or interrupted
+					--futex->waiting;
+					futex->waking = futex->waking > futex->waiting ? futex->waiting : futex->waking; // if it happened during a wakeup, just to make sure nothing bad happens
+					ret.errno = ret.errno == EINTR ? EINTR : 0;
+				} else if (futex->waking == 0) {
+					// should go back to sleep
+					poll_leave(&desc);
+					continue;
+				} else {
+					// can leave normally!
+					--futex->waiting;
+					--futex->waking;
+					ret.errno = 0;
+				}
+
+				// clean up if needed
+				if (futex->waiting == 0) {
+					free(futex);
+					removefutex(physical);
+				}
+
+				ret.ret = 0;
+				break;
+			}
+			break;
+			default:
+			ret.errno = ENOSYS;
+	}
+
+	poll_leave(&desc);
+	poll_destroydesc(&desc);
+
+	MUTEX_RELEASE(&futexmutex);
+	return ret;
+}