waitqueue example works, kthread_uniterruptible completely wrong and renamed to schedule, params added

README.md

@@ -309,6 +309,7 @@ But TODO I don't think you can see where you are in the kernel source code and l
             1.  [hello2](kernel_module/hello2.c)
             1.  [debugfs](kernel_module/debugfs.c)
             1.  [panic](kernel_module/panic.c)
+        1.  [params](kernel_module/params.c)
         1.  [fops](kernel_module/fops.c)
             1. [poll](poll.c)
         1.  Asynchronous
@@ -316,7 +317,7 @@ But TODO I don't think you can see where you are in the kernel source code and l
             1. [sleep](kernel_module/sleep.c)
             1. [kthread](kernel_module/kthread.c)
             1. [kthreads](kernel_module/kthreads.c)
-            1. [kthread_uninterruptible](kernel_module/kthread_uninterruptible.c)
+            1. [schedule](kernel_module/schedule.c)
             1. [timer](kernel_module/timer.c)
             1. [work_from_work](kernel_module/work_from_work.c)
         1.  [irq](irq.c)

kernel_module/kthread_uninterruptible.c

@@ -1,38 +0,0 @@
-/*
-Let's block the entire kernel! Yay!
-
-Also try after dmesg -n 1 to become convinced of the full blockage.
-*/
-
-#include <linux/kernel.h>
-#include <linux/kthread.h>
-#include <linux/module.h>
-
-MODULE_LICENSE("GPL");
-
-static struct task_struct *kthread;
-
-static int work_func(void *data)
-{
-	set_current_state(TASK_UNINTERRUPTIBLE);
-	int i = 0;
-	while (!kthread_should_stop()) {
-		printk(KERN_INFO "%d\n", i);
-		i++;
-		if (i == 10)
-			i = 0;
-	}
-	return 0;
-}
-
-int init_module(void)
-{
-	kthread = kthread_create(work_func, NULL, "mykthread");
-	wake_up_process(kthread);
-	return 0;
-}
-
-void cleanup_module(void)
-{
-	kthread_stop(kthread);
-}

kernel_module/params.c

@@ -0,0 +1,51 @@
+/*
+Allows passing parameters at insertion time.
+
+Those parameters can also be read and modified at runtime from /sys.
+
+	insmod /params.ko
+	# dmesg => 0
+	cd /sys/module/params/parameters
+	cat i
+	# => 0
+	printf 1 >i
+	# dmesg => 1
+	rmmod params
+	insmod /params.ko i=1
+	# dmesg => 1
+*/
+
+#include <linux/delay.h> /* usleep_range */
+#include <linux/kernel.h>
+#include <linux/kthread.h>
+#include <linux/module.h>
+#include <uapi/linux/stat.h> /* S_IRUSR | S_IWUSR */
+
+MODULE_LICENSE("GPL");
+
+static int i = 0;
+module_param(i, int, S_IRUSR | S_IWUSR);
+MODULE_PARM_DESC(i, "my favorite int");
+
+static struct task_struct *kthread;
+
+static int work_func(void *data)
+{
+	while (!kthread_should_stop()) {
+		pr_info("%d\n", i);
+		usleep_range(1000000, 1000001);
+	}
+	return 0;
+}
+
+int init_module(void)
+{
+	kthread = kthread_create(work_func, NULL, "mykthread");
+	wake_up_process(kthread);
+	return 0;
+}
+
+void cleanup_module(void)
+{
+	kthread_stop(kthread);
+}

kernel_module/schedule.c

@@ -0,0 +1,58 @@
+/*
+Let's block the entire kernel! Yay!
+
+kthreads only allow interrupting if you call schedule.
+
+If you don't, they just run forever, and you have to kill the VM.
+
+Test with:
+
+	dmesg -n 1
+	insmod /schedule.ko yn=[01]
+	dmesg | tail
+
+Then:
+
+- 	yn=0:
+	- `qemu -smp 1`: everything blocks!
+	- `qemu -smp 2`: you can still use the board, but is it noticeably slow
+- 	yn=1: all good
+*/
+
+#include <linux/delay.h> /* usleep_range */
+#include <linux/kernel.h>
+#include <linux/kthread.h>
+#include <linux/module.h>
+#include <uapi/linux/stat.h> /* S_IRUSR | S_IWUSR */
+
+MODULE_LICENSE("GPL");
+
+static int yn = 1;
+module_param(yn, int,  S_IRUSR | S_IWUSR);
+MODULE_PARM_DESC(yn, "A short integer");
+
+static struct task_struct *kthread;
+
+static int work_func(void *data)
+{
+	unsigned int i = 0;
+	while (!kthread_should_stop()) {
+		pr_info("%u\n", i);
+		i++;
+		if (yn)
+			schedule();
+	}
+	return 0;
+}
+
+int init_module(void)
+{
+	kthread = kthread_create(work_func, NULL, "mykthread");
+	wake_up_process(kthread);
+	return 0;
+}
+
+void cleanup_module(void)
+{
+	kthread_stop(kthread);
+}

kernel_module/wait_queue.c

@@ -1,5 +1,8 @@
 /*
-TODO get working. Thread 2 only wakes up once! Wake thread 2 up every 2 seconds from thread 1.
+"wait_event" works a bit like:
+
+	if (!cond)
+		sleep_until_event
 */
 
 #include <linux/delay.h> /* usleep_range */
@@ -12,20 +15,17 @@ MODULE_LICENSE("GPL");
 
 static struct task_struct *kthread1, *kthread2;
 static wait_queue_head_t queue;
-static int awake = 0;
+static atomic_t awake = ATOMIC_INIT(0);
 
 static int kthread_func1(void *data)
 {
-	int i = 0;
+	unsigned int i = 0;
 	while (!kthread_should_stop()) {
-		awake = !awake;
-		pr_info("1 %d\n", i);
-		wake_up_interruptible(&queue);
-		schedule();
+		pr_info("1 %u\n", i);
 		usleep_range(1000000, 1000001);
+		atomic_set(&awake, 1);
+		wake_up(&queue);
 		i++;
-		if (i == 10)
-			i = 0;
 	}
 	i = !i;
 	wake_up_interruptible(&queue);
@@ -35,15 +35,13 @@ static int kthread_func1(void *data)
 static int kthread_func2(void *data)
 {
 	set_current_state(TASK_INTERRUPTIBLE);
-	int i = 0;
+	unsigned int i = 0;
 	while (!kthread_should_stop()) {
-		wait_event(queue, awake);
-		pr_info("2 %d\n", i);
-		set_current_state(TASK_INTERRUPTIBLE);
-		schedule();
+		pr_info("2 %u\n", i);
 		i++;
-		if (i == 10)
-			i = 0;
+		wait_event(queue, atomic_read(&awake));
+		atomic_set(&awake, 0);
+		schedule();
 	}
 	return 0;
 }