Date:	Tue, 31 Mar 2009 21:56:16 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Bjorn Helgaas <bjorn.helgaas@...com>,
	andreas.herrmann3@....com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: 2.6.29 boot hang

Randy Dunlap wrote:
> Rusty Russell wrote:
>> On Wednesday 01 April 2009 07:15:35 Randy Dunlap wrote:
>>> On a 4-proc x86_64 (HP BladeCenter, AMD CPUs) system, booting 2.6.29
>>> (or earlier, back to 2.6.28-6921-g873392c) hangs during boot.
>>>
>>> git bisect says:
>>> 873392ca514f87eae39f53b6944caf85b1a047cb is first bad commit
>>> commit 873392ca514f87eae39f53b6944caf85b1a047cb
>>> Author: Rusty Russell <rusty@...tcorp.com.au>
>>> Date:   Wed Dec 31 23:54:56 2008 +1030
>>>
>>>     PCI: work_on_cpu: use in drivers/pci/pci-driver.c
>> ...
>>
>>> If I change CONFIG_MICROCODE_AMD=y to CONFIG_MICROCODE_AMD=n & rebuild,
>>> the kernel boots successfully.
>> How very very odd.  My first thought was a deadlock with keventd used
>> by work_on_cpu (changed in latest Linus tree), but the microcode code at
>> that version doesn't use work_on_cpu.
> 
> Yep, I thought it a bit odd also.
> 
>> So I don't think that's it, but this patch should canonically eliminate it:
>>
>> Subject: work_on_cpu(): rewrite it to create a kernel thread on demand
>> From: Andrew Morton <akpm@...ux-foundation.org>
> 
> This patch doesn't apply to 2.6.29-final, but it does apply to 2.6.29-git8,
> so I applied/tested it there.  with surprising results (at least to me).
> 
> 2.6.29-git8 works for me without any patches applied.  After applying
> this patch, I get the same boot hang that I was seeing with 2.6.29-final.

That's incorrect.  Sorry about that.

2.6.29-git8 with or without this patch applied work for me.


> Make sense to you??
> 
> Thanks for your help.
> 
>> The various implemetnations and proposed implemetnations of work_on_cpu()
>> are vulnerable to various deadlocks because they all used queues of some
>> form.
>>
>> Unrelated pieces of kernel code thus gained dependencies wherein if one
>> work_on_cpu() caller holds a lock which some other work_on_cpu() callback
>> also takes, the kernel could rarely deadlock.
>>
>> Fix this by creating a short-lived kernel thread for each work_on_cpu()
>> invokation.
>>
>> This is not terribly fast, but the only current caller of work_on_cpu() is
>> pci_call_probe().
>>
>> It would be nice to find some other way of doing the node-local
>> allocations in the PCI probe code so that we can zap work_on_cpu()
>> altogether.  The code there is rather nasty.  I can't think of anything
>> simple at this time...
>>
>> Cc: Rusty Russell <rusty@...tcorp.com.au>
>> Cc: Ingo Molnar <mingo@...e.hu>
>> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
>> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
>> ---
>>  kernel/workqueue.c |   36 +++++++++++++++++++-----------------
>>  1 file changed, 19 insertions(+), 17 deletions(-)
>>
>> diff -puN kernel/workqueue.c~work_on_cpu-rewrite-it-to-create-a-kernel-thread-on-demand kernel/workqueue.c
>> --- a/kernel/workqueue.c~work_on_cpu-rewrite-it-to-create-a-kernel-thread-on-demand
>> +++ a/kernel/workqueue.c
>> @@ -985,20 +985,20 @@ undo:
>>  }
>>  
>>  #ifdef CONFIG_SMP
>> -static struct workqueue_struct *work_on_cpu_wq __read_mostly;
>>  
>>  struct work_for_cpu {
>> -	struct work_struct work;
>> +	struct completion completion;
>>  	long (*fn)(void *);
>>  	void *arg;
>>  	long ret;
>>  };
>>  
>> -static void do_work_for_cpu(struct work_struct *w)
>> +static int do_work_for_cpu(void *_wfc)
>>  {
>> -	struct work_for_cpu *wfc = container_of(w, struct work_for_cpu, work);
>> -
>> +	struct work_for_cpu *wfc = _wfc;
>>  	wfc->ret = wfc->fn(wfc->arg);
>> +	complete(&wfc->completion);
>> +	return 0;
>>  }
>>  
>>  /**
>> @@ -1009,17 +1009,23 @@ static void do_work_for_cpu(struct work_
>>   *
>>   * This will return the value @fn returns.
>>   * It is up to the caller to ensure that the cpu doesn't go offline.
>> + * The caller must not hold any locks which would prevent @fn from completing.
>>   */
>>  long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
>>  {
>> -	struct work_for_cpu wfc;
>> -
>> -	INIT_WORK(&wfc.work, do_work_for_cpu);
>> -	wfc.fn = fn;
>> -	wfc.arg = arg;
>> -	queue_work_on(cpu, work_on_cpu_wq, &wfc.work);
>> -	flush_work(&wfc.work);
>> -
>> +	struct task_struct *sub_thread;
>> +	struct work_for_cpu wfc = {
>> +		.completion = COMPLETION_INITIALIZER_ONSTACK(wfc.completion),
>> +		.fn = fn,
>> +		.arg = arg,
>> +	};
>> +
>> +	sub_thread = kthread_create(do_work_for_cpu, &wfc, "work_for_cpu");
>> +	if (IS_ERR(sub_thread))
>> +		return PTR_ERR(sub_thread);
>> +	kthread_bind(sub_thread, cpu);
>> +	wake_up_process(sub_thread);
>> +	wait_for_completion(&wfc.completion);
>>  	return wfc.ret;
>>  }
>>  EXPORT_SYMBOL_GPL(work_on_cpu);
>> @@ -1035,8 +1041,4 @@ void __init init_workqueues(void)
>>  	hotcpu_notifier(workqueue_cpu_callback, 0);
>>  	keventd_wq = create_workqueue("events");
>>  	BUG_ON(!keventd_wq);
>> -#ifdef CONFIG_SMP
>> -	work_on_cpu_wq = create_workqueue("work_on_cpu");
>> -	BUG_ON(!work_on_cpu_wq);
>> -#endif
>>  }
>> _


-- 
~Randy
--
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