Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts disabled and page-draining IPI

[Nikolay Borisov]

Hello and thanks for the reply,

On 10/12/2015 04:40 PM, Jan Kara wrote:
> On Fri 09-10-15 11:03:30, Nikolay Borisov wrote:
>> On 10/09/2015 10:37 AM, Hillf Danton wrote:
>>>>>> @@ -109,8 +109,8 @@ static void ext4_finish_bio(struct bio *bio)
>>>>>>  			if (bio->bi_error)
>>>>>>  				buffer_io_error(bh);
>>>>>>  		} while ((bh = bh->b_this_page) != head);
>>>>>> -		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
>>>>>>  		local_irq_restore(flags);
>>>>>
>>>>> What if it takes 100ms to unlock after IRQ restored?
>>>>
>>>> I'm not sure I understand in what direction you are going? Care to
>>>> elaborate?
>>>>
>>> Your change introduces extra time cost the lock waiter has to pay in
>>> the case that irq happens before the lock is released.
>>
>> [CC filesystem and mm people. For reference the thread starts here:
>>  http://thread.gmane.org/gmane.linux.kernel/2056996 ]
>>
>> Right, I see what you mean and it's a good point but when doing the
>> patches I was striving for correctness and starting a discussion, hence
>> the RFC. In any case I'd personally choose correctness over performance
>> always ;).
>>
>> As I'm not an fs/ext4 expert and have added the relevant parties (please
>> use reply-all from now on so that the thread is not being cut in the
>> middle) who will be able to say whether it impact is going to be that
>> big. I guess in this particular code path worrying about this is prudent
>> as writeback sounds like a heavily used path.
>>
>> Maybe the problem should be approached from a different angle e.g.
>> drain_all_pages and its reliance on the fact that the IPI will always be
>> delivered in some finite amount of time? But what if a cpu with disabled
>> interrupts is waiting on the task issuing the IPI?
> 
> So I have looked through your patch and also original report (thread starts
> here: https://lkml.org/lkml/2015/10/8/341) and IMHO one question hasn't
> been properly answered yet: Who is holding BH_Uptodate_Lock we are spinning
> on? You have suggested in https://lkml.org/lkml/2015/10/8/464 that it was
> __block_write_full_page_endio() call but that cannot really be the case.
> BH_Uptodate_Lock is used only in IO completion handlers -
> end_buffer_async_read, end_buffer_async_write, ext4_finish_bio. So there
> really should be some end_io function running on some other CPU which holds
> BH_Uptodate_Lock for that buffer.

I did check all the call traces of the current processes on the machine
at the time of the hard lockup and none of the 3 functions you mentioned
were in any of the call chains. But while I was looking the code of
end_buffer_async_write and in the comments I saw it was mentioned that
those completion handler were called from __block_write_full_page_endio
so that's what pointed my attention to that function. But you are right
that it doesn't take the BH lock.

Furthermore the fact that the BH_Async_Write flag is set points me in
the direction that end_buffer_async_write should have been executing but
as I said issuing "bt" for all the tasks didn't show this function.

I'm beginning to wonder if it's possible that a single bit memory error
has crept up, but this still seems like a long shot...

Btw I think in any case the spin_lock patch is wrong as this code can be
called from within softirq context and we do want to be interrupt safe
at that point.

> 
> BTW: I suppose the filesystem uses 4k blocksize, doesn't it?

Unfortunately I cannot tell you with 100% certainty, since on this
server there are multiple block devices with blocksize either 1k or 4k.
So it is one of these. If you know a way to extract this information
from a vmcore file I'd be happy to do it.

> 
> 								Honza
> 
>>>>>> +		bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
>>>>>>  		if (!under_io) {
>>>>>>  #ifdef CONFIG_EXT4_FS_ENCRYPTION
>>>>>>  			if (ctx)
>>>>>> --
>>>>>> 2.5.0
>>>>>
>>>
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