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Date: Thu, 23 Oct 2008 22:49:06 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Gautham R Shenoy <ego@in.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>, Rusty Russell <rusty@rustcorp.com.au>,
       linux-kernel@vger.kernel.org, travis@sgi.com,
       Ingo Molnar <mingo@elte.hu>
Subject: Re: do_boot_cpu can deadlock?
Message-ID: <20081023184906.GA2612@localhost>
References: <20081023005751.53973DDEFE@ozlabs.org> <20081023094036.GA7593@redhat.com> <20081023143605.GN5255@in.ibm.com> <20081023163517.GB21008@redhat.com> <20081023170212.GA22599@redhat.com> <20081023182119.GA1480@in.ibm.com>
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[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)?

		- Cyrill -
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