On Fri, Sep 04, 2009 at 09:37:49AM +0200, Ingo Molnar wrote: > > * Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > > On 09/01, Eric W. Biederman wrote: > > > > > > Oleg Nesterov <oleg@xxxxxxxxxx> writes: > > > > > > > On 09/01, Oleg Nesterov wrote: > > > >> > > > >> On 09/01, Ingo Molnar wrote: > > > >> > > > > >> > * Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > > >> > > > > >> > > On 09/01, Ingo Molnar wrote: > > > >> > > > > > > >> > > > * Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > > >> > > > > > > >> > > > > Yes, this should work. But I _think_ we can make the better fix... > > > >> > > > > > > > >> > > > > I'll try to make the patch soon. Afaics we don't need > > > >> > > > > kthreadd_task_init_done. > > > >> > > > > > > >> > > > ok. > > > >> > > > > > >> > > Just in case, the patch is ready. [...] > > > >> > > > > >> > yes - that's roughly the cleanup i referred to in the commit log. > > > >> > > > > >> > way too late for -rc8 though - the minimal fix i did _might_ be > > > >> > eligible. > > > >> > > > > >> > agreed? > > > >> > > > >> Agreed. Then I will sent the patch on top of this change. > > > > > > > > OK, I am sending the patch on top of your fix. Not sure how to really > > > > test it, but at least the kernel works when I apply the debugging patch > > > > below on top. > > > > > > Stupid question. How is it that we wind up trying to start kernel threads > > > before it is safe to do so? > > > > > > Races should be impossible because the scheduler isn't running until a few > > > lines later. > > > > Yes, I am confused too. > > > > At first I thought I understand the race, now I don't. Please see > > the whole thread: http://marc.info/?t=125180592500005 > > I dont understand it either - and the .config being !SMP excludes > any sort of SMP race as well. > > Lets delay this until i can debug it more fully. > CONFIG_PREEMPT_VOLUNTARY looks like the key of this problem, might_resched becomes _cond_resched, and since d86ee480 changes it not to check SYSTEM_RUNNING, this function may call schedule(). And might_resched may be called even from do_fork(), so this is a scenario: kernel_thread(kernel_init, ...) pid = kernel_thread(kthreadd, ...) -- switch to thread kernel_init, which refers to kthreadd_task and NULL reference happens. Note, because of !CONFIG_LOCK_KERNEL, lock_kernel() is nop. What about just creating kthreadd before kernel_init as the following, it works for me? diff --git a/init/main.c b/init/main.c index b34fd8e..ae86699 100644 --- a/init/main.c +++ b/init/main.c @@ -452,10 +452,10 @@ static noinline void __init_refok rest_init(void) int pid; rcu_scheduler_starting(); - kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND); - numa_default_policy(); pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES); kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns); + kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND); + numa_default_policy(); unlock_kernel(); /* -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html