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    Subjectworkqueues and percpu (was: [PATCH] dm: remake of the verity target)


    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\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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