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    SubjectModule/kthread/printk question/problem
    I'm writing a kernel module which creates a substantial amount of
    kernel threads. After dropping some real stuff, the module is:

    #include <linux/kernel.h>
    #include <linux/sched.h>
    #include <linux/kthread.h>
    #include <linux/module.h>
    #include <linux/slab.h>

    MODULE_LICENSE("GPL");

    static int nrthreads = 128;
    module_param(nrthreads, int, 0644);

    static int loopcount = 1024;
    module_param(loopcount, int, 0644);

    static int usehrtime = 0;
    module_param(usehrtime, int, 0644);

    static int slack = 50000;
    module_param(slack, int, 0644);

    static int msecs = 1;
    module_param(msecs, int, 0644);

    static DECLARE_COMPLETION(done);
    static struct task_struct **threads;
    static atomic_t nrunning;

    static int test(void *unused)
    {
    int i;
    ktime_t expires = ktime_set(0, msecs * NSEC_PER_MSEC);

    for (i = 0; !kthread_should_stop() && i < loopcount; i++) {
    if (usehrtime) {
    set_current_state(TASK_UNINTERRUPTIBLE);
    schedule_hrtimeout_range(&expires, slack, HRTIMER_MODE_REL);
    }
    else
    schedule_timeout_uninterruptible(msecs_to_jiffies(msecs));
    }

    if (atomic_dec_and_test(&nrunning)) {
    printk("last thread done\n");
    complete(&done);
    }
    return 0;
    }

    static int __init testmod_init(void)
    {
    int i;

    printk("test begin\n");

    atomic_set(&nrunning, nrthreads);

    threads = kmalloc(nrthreads * sizeof(struct task_struct *), GFP_KERNEL);
    if (!threads)
    return -ENOMEM;

    for (i = 0; i < nrthreads; i++) {
    threads[i] = kthread_run(test, NULL, "test/%d", i);
    if (IS_ERR(threads[i])) {
    int j, err = PTR_ERR(threads[i]);

    for (j = 0; j < i; j++)
    kthread_stop(threads[j]);
    kfree(threads);
    return err;
    }
    }
    return 0;
    }

    static void __exit testmod_exit(void)
    {
    wait_for_completion(&done);
    kfree(threads);
    }

    module_init(testmod_init);
    module_exit(testmod_exit);

    Usually it works as expected, at least from 8 to 128 threads.
    But when I'm trying to run it a loop like:

    while true; do insmod testmod.ko && rmmod testmod.ko; sleep 1; done

    it's also possible to catch a very rare crash (ARM example):

    Unable to handle kernel paging request at virtual address 7f1200c4
    pgd = 80004000
    [7f1200c4] *pgd=bdc28811, *pte=00000000, *ppte=00000000
    Internal error: Oops: 80000007 [#1] PREEMPT SMP
    Modules linked in: [last unloaded: testmod]
    CPU: 1 Tainted: G O (3.3.0-rc2 #3)
    PC is at 0x7f1200c4
    LR is at __schedule+0x684/0x6e4
    pc : [<7f1200c4>] lr : [<802c053c>] psr: 600f0113
    sp : bf115f88 ip : 00000000 fp : 00000000
    r10: 00000000 r9 : 00000000 r8 : 7f120394
    r7 : 00000002 r6 : 00000400 r5 : 7f120204 r4 : bf114000
    r3 : 00000000 r2 : bf115ec0 r1 : bf9220c0 r0 : 00000001
    Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment kernel
    Control: 10c5387d Table: bfbcc04a DAC: 00000015
    Process test/126 (pid: 10918, stack limit = 0xbf1142f8)
    Stack: (0xbf115f88 to 0xbf116000)
    5f80: 000f4240 00000000 bf213e4c 00000000 7f120000 00000013
    5fa0: 00000000 80049228 00000000 00000000 00000000 00000000 00000000 00000000
    5fc0: dead4ead ffffffff ffffffff 8048b2b8 00000000 00000000 8036a3f9 bf115fdc
    5fe0: bf115fdc 271aee1c bf213e4c 8004919c 8000eabc 8000eabc bfefc811 bfefcc11
    Code: bad PC value

    Note the bad PC, and stack is just a nonsense. I suspect that the kernel calls
    testmod_exit() and frees module memory _before_ all test/X threads are really
    dead - i.e. the module memory is freed when at least one of the test/X threads
    is somewhere in do_exit() or nearby. Is that possible? If yes, what's the better
    way to ensure that all test/X threads are really gone at some point of
    testmod_exit()?

    An interesting thing is that I can't reproduce this fault with both printk()s
    commented out. No ideas why.

    Dmitry


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