Messages in this thread | | | Date | Fri, 24 Oct 2008 15:23:36 +0530 | From | Gautham R Shenoy <> | Subject | Re: do_boot_cpu can deadlock? |
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On Fri, Oct 24, 2008 at 11:33:42AM +0200, Oleg Nesterov wrote: > On 10/23, Cyrill Gorcunov wrote: > > > > [Gautham R Shenoy - Thu, Oct 23, 2008 at 11:51:19PM +0530] > > | On Thu, Oct 23, 2008 at 07:02:12PM +0200, Oleg Nesterov wrote: > > | > Hmm. arch/x86/kernel/smpboot.c:do_boot_cpu() can deadlock ? > > | > > > | > It is called from _cpu_up() under cpu_hotplug_begin(), and it > > | > waits for c_idle.work. Again, if we have the pending work which > > | > needs get_online_cpus() we seem to have problems. > > | > > | Good point. Though this code gets triggered mostly during boot time when > > | the CPUs are being brought online for the first time. If we have some > > | work-item pending at that time, which needs get_online_cpus(), we could > > | possibly see this deadlock. > > | > > | > > > | > Oleg. > > | > > | -- > > | Thanks and Regards > > | gautham > > | > > > > May I ask? If I understand right we do use this part of do_boot_cpu > > > > if (!keventd_up() || current_is_keventd()) > > c_idle.work.func(&c_idle.work); > > else { > > schedule_work(&c_idle.work); > > wait_for_completion(&c_idle.done); > > } > > > > if only we've been called the first time after power on. And all > > subsequent call of this do_boot_cpu would lead to > > > > if (c_idle.idle) { > > c_idle.idle->thread.sp = (unsigned long) (((struct pt_regs *) > > (THREAD_SIZE + task_stack_page(c_idle.idle))) - 1); > > init_idle(c_idle.idle, cpu); > > goto do_rest; > > } > > > > ie go to do_rest and no wait_for_completion/schedule_work at all. > > Did I miss something? *Sorry* in advance if the question is quite > > not related. This work-pending situation is in 'possible' scenario > > only (ie we don't have such a callers for now... yet)? > > There are no problems during boot time, afaics. > > kernel_init() calls smp_init() before do_basic_setup()->init_workqueues(). > This means that do_boot_cpu() won't use workqueues due to !keventd_up(). > > But let's suppose we boot with maxcpus=1, and then bring up another CPU. > Or we really add the new physical CPU (I don't really know if this is > possible on x86).
Even I am not sure if physical hotplug is possible.
But think about the virtualization case when we want to add additional CPU to a KVM guest at runtime? This is not such a rare use-case. It could dead-lock that time, No?
> > Oleg.
-- Thanks and Regards gautham
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