From 27c0b6fbc5ffe0979436a6436187d1bf952e9a24 Mon Sep 17 00:00:00 2001
From: Kent Overstreet <kent.overstreet@gmail.com>
Date: Mon, 5 Feb 2018 00:13:40 -0500
Subject: [PATCH] use futex instead of pthread cond variable for schedule

---
 include/linux/sched.h |  5 +----
 linux/kthread.c       |  2 --
 linux/sched.c         | 25 ++++++++++---------------
 3 files changed, 11 insertions(+), 21 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 885cc56..38a5fec 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -74,11 +74,8 @@ struct task_struct {
 	int			(*thread_fn)(void *);
 	void			*thread_data;
 
-	pthread_mutex_t		lock;
-	pthread_cond_t		wait;
-
 	atomic_t		usage;
-	volatile long		state;
+	int			state;
 
 	/* kthread: */
 	unsigned long		kthread_flags;
diff --git a/linux/kthread.c b/linux/kthread.c
index 80a9ac9..eef73fe 100644
--- a/linux/kthread.c
+++ b/linux/kthread.c
@@ -68,8 +68,6 @@ struct task_struct *kthread_create(int (*thread_fn)(void *data),
 	p->thread_fn	= thread_fn;
 	p->thread_data	= thread_data;
 	p->state	= TASK_UNINTERRUPTIBLE;
-	pthread_mutex_init(&p->lock, NULL);
-	pthread_cond_init(&p->wait, NULL);
 	atomic_set(&p->usage, 1);
 	init_completion(&p->exited);
 
diff --git a/linux/sched.c b/linux/sched.c
index cc614b1..2d61c48 100644
--- a/linux/sched.c
+++ b/linux/sched.c
@@ -1,4 +1,5 @@
 
+#include <linux/futex.h>
 #include <string.h>
 #include <sys/mman.h>
 
@@ -6,6 +7,7 @@
 #include <linux/printk.h>
 #include <linux/rcupdate.h>
 #include <linux/sched.h>
+#include <linux/slab.h>
 #include <linux/timer.h>
 
 __thread struct task_struct *current;
@@ -19,28 +21,23 @@ void __put_task_struct(struct task_struct *t)
 /* returns true if process was woken up, false if it was already running */
 int wake_up_process(struct task_struct *p)
 {
-	int ret;
+	int ret = p->state != TASK_RUNNING;
 
-	pthread_mutex_lock(&p->lock);
-	ret = p->state != TASK_RUNNING;
 	p->state = TASK_RUNNING;
-
-	pthread_cond_signal(&p->wait);
-	pthread_mutex_unlock(&p->lock);
-
+	futex(&p->state, FUTEX_WAKE|FUTEX_PRIVATE_FLAG,
+	      INT_MAX, NULL, NULL, 0);
 	return ret;
 }
 
 void schedule(void)
 {
-	rcu_quiescent_state();
+	int v;
 
-	pthread_mutex_lock(&current->lock);
-
-	while (current->state != TASK_RUNNING)
-		pthread_cond_wait(&current->wait, &current->lock);
+	rcu_quiescent_state();
 
-	pthread_mutex_unlock(&current->lock);
+	while ((v = current->state) != TASK_RUNNING)
+		futex(&current->state, FUTEX_WAIT|FUTEX_PRIVATE_FLAG,
+		      v, NULL, NULL, 0);
 }
 
 static void process_timeout(unsigned long __data)
@@ -169,8 +166,6 @@ static void sched_init(void)
 	memset(p, 0, sizeof(*p));
 
 	p->state	= TASK_RUNNING;
-	pthread_mutex_init(&p->lock, NULL);
-	pthread_cond_init(&p->wait, NULL);
 	atomic_set(&p->usage, 1);
 	init_completion(&p->exited);
 
-- 
2.39.2