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    SubjectRe: keventd_create_kthread
    Date
    In message <Pine.LNX.4.58.0402190205040.16515@devserv.devel.redhat.com> you write:
    > The current wait_task_inactive() code seems to be OK on x86.
    > Context-switching cannot be preempted. The goal of wait_task_inactive() is
    > to wait for the task to unschedule on a CPU. If that's due to preempt then
    > it's due to preempt.

    No, because it can come back at any time 8(

    > that in any modern interface. Why does keventd_create_kthread() need
    > wait_task_inactive()?

    Um, the code was taken from sched.c to kthread:

    2.6.3 migration_thread():
    set_current_state(TASK_UNINTERRUPTIBLE);
    schedule();

    2.6.3 migration_call():
    kernel_thread(migration_thread, &startup, CLONE_KERNEL);
    wait_for_completion(&startup.startup_done);
    wait_task_inactive(startup.task);

    startup.task->thread_info->cpu = cpu;
    startup.task->cpus_allowed = cpumask_of_cpu(cpu);


    So, if the migration thread has been preempted immediately before
    schedule(), wait_task_inactive returns, but it can come back from
    preempt while we're messing with startup.task->thread_info->cpu.

    Now, the latter part is wrapped in kthread_bind(), which should really
    be doing the wait_task_inactive itself (doing it in kthread_create is
    overzealous). But the race is still there.

    Hope that clarifies,
    Rusty.
    --
    Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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