From ad077d3943f79c0f6481dab929970613c33c31a7 Mon Sep 17 00:00:00 2001
From: Ciro Santilli <ciro.santilli@gmail.com>
Date: Sun, 14 May 2017 13:49:15 +0100
Subject: [PATCH] Workqueue works! QEMU CLI mode with -n for panic

---
 README.md                         |  4 ++++
 kernel_module/workqueue_cheat.c   | 11 ++++++++---
 rootfs_overlay/workqueue_cheat.sh |  9 +++++++++
 run                               | 22 +++++++++++++++++++++-
 4 files changed, 42 insertions(+), 4 deletions(-)
 create mode 100755 rootfs_overlay/workqueue_cheat.sh

diff --git a/README.md b/README.md
index 8341064..3c24bc5 100644
--- a/README.md
+++ b/README.md
@@ -27,6 +27,10 @@ We use `printk` a lot, and it shows on the QEMU terminal by default. If that ann
 
 See also: <https://superuser.com/questions/351387/how-to-stop-kernel-messages-from-flooding-my-console>
 
+When your kernel starts crashing, get the full trace with:
+
+    ./run -n
+
 1.  [Introduction](introduction.md)
 1.  [Build](build.md)
 1.  [kmod](kmod.md)
diff --git a/kernel_module/workqueue_cheat.c b/kernel_module/workqueue_cheat.c
index ed33397..30ce613 100644
--- a/kernel_module/workqueue_cheat.c
+++ b/kernel_module/workqueue_cheat.c
@@ -7,7 +7,7 @@ Usage:
 
 Creates a separate thread. So init_module can return, but some work will still get done.
 
-TODO why can't call this workqueue.ko?
+Can't call this just workqueue.c because there is already a built-in with that name:
 https://unix.stackexchange.com/questions/364956/how-can-insmod-fail-with-kernel-module-is-already-loaded-even-is-lsmod-does-not
 */
 
@@ -19,14 +19,15 @@ MODULE_LICENSE("GPL");
 
 static struct workqueue_struct *queue;
 
-static void worker_func(struct work_struct *work)
+static void work_func(struct work_struct *work)
 {
 	printk(KERN_INFO "worker\n");
 }
 
+DECLARE_WORK(work, work_func);
+
 int init_module(void)
 {
-	DECLARE_WORK(work, worker_func);
 	queue = create_singlethread_workqueue("myworkqueue");
 	queue_work(queue, &work);
 	return 0;
@@ -34,5 +35,9 @@ int init_module(void)
 
 void cleanup_module(void)
 {
+	/* TODO why is this needed? Why flush_workqueue doesn't work? (re-insmod panics)
+	 * http://stackoverflow.com/questions/37216038/whats-the-difference-between-flush-delayed-work-and-cancel-delayed-work-sync */
+	/*flush_workqueue(queue);*/
+	cancel_work_sync(&work);
 	destroy_workqueue(queue);
 }
diff --git a/rootfs_overlay/workqueue_cheat.sh b/rootfs_overlay/workqueue_cheat.sh
new file mode 100755
index 0000000..950cbf7
--- /dev/null
+++ b/rootfs_overlay/workqueue_cheat.sh
@@ -0,0 +1,9 @@
+#!/bin/sh
+
+# Got a few kernel crashes after insert / remove, so let's do this a few times.
+insmod /workqueue_cheat.ko
+rmmod workqueue_cheat
+insmod /workqueue_cheat.ko
+rmmod workqueue_cheat
+insmod /workqueue_cheat.ko
+rmmod workqueue_cheat
diff --git a/run b/run
index 84a8344..dbc2b1f 100755
--- a/run
+++ b/run
@@ -1,5 +1,24 @@
 #!/usr/bin/env bash
 set -e
+
+# CLI handling.
+nographic=false
+while getopts n OPT; do
+  case "$OPT" in
+    n)
+      nographic=true
+      ;;
+  esac
+done
+if $nographic; then
+  extra_append='console=ttyS0'
+  extra_flags='-nographic'
+else
+  extra_append=''
+  extra_flags=''
+fi
+
+# Work.
 cd buildroot
 make BR2_EXTERNAL="$(pwd)/../kernel_module" qemu_x86_64_defconfig
 echo '
@@ -11,9 +30,10 @@ BR2_ROOTFS_OVERLAY="../rootfs_overlay"
 env -u LD_LIBRARY_PATH make BR2_JLEVEL="$(($(nproc) - 2))" kernel_module-rebuild all
 qemu-system-x86_64 \
   -M pc \
-  -append 'root=/dev/vda' \
+  -append "root=/dev/vda $extra_append" \
   -drive file=output/images/rootfs.ext2,if=virtio,format=raw \
   -kernel output/images/bzImage \
   -net nic,model=virtio \
   -net user \
+  $extra_flags \
 ;