On Tue, 6 Mar 2012, Mandeep Singh Baines wrote:

> You are
> allocated a complete shash_desc per I/O. We only allocate one per CPU.

I looked at it --- and using percpu variables in workqueues isn't safe
because the workqueue can change CPU if the CPU is hot-unplugged.

dm-crypt has the same bug --- it also uses workqueue with per-cpu
variables and assumes that the CPU doesn't change for a single work item.

This program shows that work executed in a workqueue can be switched to a
different CPU.

I'm wondering how much other kernel code assumes that workqueues are bound
to a specific CPU, which isn't true if we unplug that CPU.

Mikulas

---

/*
* A proof of concept that a work item executed on a workqueue may change CPU
* when CPU hot-unplugging is used.
* Compile this as a module and run:
* insmod test.ko; sleep 1; echo 0 >/sys/devices/system/cpu/cpu1/online
* You see that the work item starts executing on CPU 1 and ends up executing
* on different CPU, usually 0.
*/

#include <linux/module.h>
#include <linux/delay.h>

static struct workqueue_struct *wq;
static struct work_struct work;

static void do_work(struct work_struct *w)
{
printk("starting work on cpu %d
", smp_processor_id());
msleep(10000);
printk("finishing work on cpu %d
", smp_processor_id());
}

static int __init test_init(void)
{
printk("module init
");
wq = alloc_workqueue("testd", WQ_MEM_RECLAIM | WQ_CPU_INTENSIVE, 1);
if (!wq) {
printk("alloc_workqueue failed
");
return -ENOMEM;
}
INIT_WORK(&work, do_work);
queue_work_on(1, wq, &work);
return 0;
}

static void __exit test_exit(void)
{
destroy_workqueue(wq);
printk("module exit
");
}

module_init(test_init)
module_exit(test_exit)
MODULE_LICENSE("GPL");

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