Create a work that will busy wait.

Create a workqueue and queue two works, the first one being a work that busy
waits for a number of seconds, delaying the execution of the second one.

diff --git a/Makefile b/Makefile
index f2266eb..dbcf910 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,6 @@
 ifneq ($(KERNELRELEASE),)
        obj-m := wq.o
+       obj-m += block_wq.o
 else
        KERNELDIR ?= /lib/modules/$(shell uname -r)/build
        PWD := $(shell pwd)

diff --git a/block_wq.c b/block_wq.c
new file mode 100644 (file)
index 0000000..ae6d8ab
--- /dev/null
+++ b/block_wq.c
@@ -0,0 +1,68 @@
+/*
+ *  Copyright (C) 2010  Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ *
+ *  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 Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+/*
+ * This code demonstrates why a work should not block for too long.
+ * This is worse in the case that the work is scheduled in keventd.
+ */
+
+#include <linux/module.h>
+#include <linux/workqueue.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Thadeu Lima de Souza Cascardo");
+
+static void work_block(struct work_struct *work)
+{
+       unsigned long tmp = jiffies + 5 * HZ;
+       printk(KERN_INFO "Blocking other works in the queue\n");
+       printk(KERN_INFO "Executing task in CPU %d\n", smp_processor_id());
+       /* We also kick the CPU high */
+       while (!time_after(jiffies, tmp))
+               cpu_relax();
+}
+
+static void work_print(struct work_struct *work)
+{
+       printk(KERN_INFO "Phew! That was waiting!\n");
+       printk(KERN_INFO "Task done in CPU %d\n", smp_processor_id());
+}
+
+static struct workqueue_struct *block_wq;
+static DECLARE_WORK(block_work, work_block);
+static DECLARE_WORK(print_work, work_print);
+
+static int block_wq_init(void)
+{
+       block_wq = create_workqueue("block_wq");
+       if (!block_wq)
+               return -ENOMEM;
+       printk(KERN_INFO "Queueing task in CPU %d\n", get_cpu());
+       put_cpu();
+       queue_work(block_wq, &block_work);
+       queue_work(block_wq, &print_work);
+       return 0;
+}
+
+static void block_wq_exit(void)
+{
+       destroy_workqueue(block_wq);
+}
+
+module_init(block_wq_init);
+module_exit(block_wq_exit);