linux-kernel - Re: do_boot_cpu can deadlock?

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Message-ID: <20081024093342.GA4583@redhat.com>
Date:	Fri, 24 Oct 2008 11:33:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, travis@....com,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: do_boot_cpu can deadlock?

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).

Oleg.

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