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SubjectRe: workqueues and percpu (was: [PATCH] dm: remake of the verity target)
On Thu, 8 Mar 2012 17:21:53 -0500 
Mikulas Patocka <mpatocka@redhat.com> wrote:

>
>
> 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.

ugh.

We really don't want to have to avoid using workqueues because of some
daft issue with CPU hot-unplug. And yes, there are assumptions in various
work handlers that they will be pinned to a single CPU. Finding and fixing
those assumptions would be painful.

Heck, even debug_smp_processor_id() can be wrong in the presence of the
cpu-unplug thing.

I'm not sure what we can do about it really, apart from blocking unplug
until all the target CPU's workqueues have been cleared. And/or refusing
to unplug a CPU until all pinned-to-that-cpu kernel threads have been
shut down or pinned elsewhere (which is the same thing, only more
general).

Tejun, is this new behaviour? I do recall that a long time ago we
wrestled with unplug-vs-worker-threads and I ended up OK with the
result, but I forget what it was. IIRC Rusty was involved.


That being said, I don't think it's worth compromising the DM code
because of this workqueue wart: lots of other code has the same wart,
and we should find a centralised fix for it.

> /*
> * 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\n", smp_processor_id());
> msleep(10000);
> printk("finishing work on cpu %d\n", smp_processor_id());
> }
>
> static int __init test_init(void)
> {
> printk("module init\n");
> wq = alloc_workqueue("testd", WQ_MEM_RECLAIM | WQ_CPU_INTENSIVE, 1);
> if (!wq) {
> printk("alloc_workqueue failed\n");
> 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\n");
> }
>
> module_init(test_init)
> module_exit(test_exit)
> MODULE_LICENSE("GPL");


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