linux-kernel - Re: do_boot_cpu can deadlock?

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Date:	Fri, 24 Oct 2008 12:51:32 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, travis@....com,
	Ingo Molnar <mingo@...e.hu>,
	Vitaly Mayatskikh <vmayatsk@...hat.com>
Subject: Re: do_boot_cpu can deadlock?

On 10/24, Gautham R Shenoy wrote:
>
> On Fri, Oct 24, 2008 at 11:33:42AM +0200, Oleg Nesterov wrote:
>
> > 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?

virtualization/KVM is a black magic to me ;) I don't know how/if it is
possible to add CPU at runtime.

Anyway, booting with maxcpus=1 allows us to bring up another CPU later,
and idle_thread_array[cpu] == NULL in that case, yes?

Perhaps smp_prepare_cpus() can do fork_idle() for_each_possible_cpu() ?
Or we can change do_boot_cpu() to use kthread_run() to create the idle
thread.

Oleg.

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