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

[Nikolay Borisov <kernel@kyup.com>]

On 10/13/2015 11:15 AM, Jan Kara wrote:
> On Mon 12-10-15 17:51:07, Nikolay Borisov wrote:
>> 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.
> 
> Actually ext4_bio_write_page() also sets BH_Async_Write so that seems like
> a more likely place where that flag got set since ext4_finish_bio() was
> then handling IO completion.
> 
>> 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...
> 
> Yup. Possible but a long shot. Is the problem reproducible in any way?

Okay, I rule out hardware issue since a different server today 
experienced the same hard lockup. One thing which looks 
suspicious to me are the repetitions of bio_endio/clone_endio: 

Oct 13 03:16:54 10.80.5.48 Call Trace:
Oct 13 03:16:54 10.80.5.48 <NMI>
Oct 13 03:16:54 10.80.5.48 [<ffffffff81651631>] dump_stack+0x58/0x7f
Oct 13 03:16:54 10.80.5.48 [<ffffffff81089a6c>] warn_slowpath_common+0x8c/0xc0
Oct 13 03:16:54 10.80.5.48 [<ffffffff81089b56>] warn_slowpath_fmt+0x46/0x50
Oct 13 03:16:54 10.80.5.48 [<ffffffff811015f8>] watchdog_overflow_callback+0x98/0xc0
Oct 13 03:16:54 10.80.5.48 [<ffffffff81132d0c>] __perf_event_overflow+0x9c/0x250
Oct 13 03:16:54 10.80.5.48 [<ffffffff81133664>] perf_event_overflow+0x14/0x20
Oct 13 03:16:54 10.80.5.48 [<ffffffff81061796>] intel_pmu_handle_irq+0x1d6/0x3e0
Oct 13 03:16:54 10.80.5.48 [<ffffffff8105b4c4>] perf_event_nmi_handler+0x34/0x60
Oct 13 03:16:54 10.80.5.48 [<ffffffff8104c152>] nmi_handle+0xa2/0x1a0
Oct 13 03:16:54 10.80.5.48 [<ffffffff8104c3b4>] do_nmi+0x164/0x430
Oct 13 03:16:54 10.80.5.48 [<ffffffff81656e2e>] end_repeat_nmi+0x1a/0x1e
Oct 13 03:16:54 10.80.5.48 [<ffffffff8125be19>] ? ext4_finish_bio+0x279/0x2a0
Oct 13 03:16:54 10.80.5.48 [<ffffffff8125be19>] ? ext4_finish_bio+0x279/0x2a0
Oct 13 03:16:54 10.80.5.48 [<ffffffff8125be19>] ? ext4_finish_bio+0x279/0x2a0
Oct 13 03:16:54 10.80.5.48 <<EOE>> 
Oct 13 03:16:54 10.80.5.48 <IRQ> 
Oct 13 03:16:54 10.80.5.48 [<ffffffff8125c2c8>] ext4_end_bio+0xc8/0x120
Oct 13 03:16:54 10.80.5.48 [<ffffffff811dbf1d>] bio_endio+0x1d/0x40
Oct 13 03:16:54 10.80.5.48 [<ffffffff81546781>] dec_pending+0x1c1/0x360
Oct 13 03:16:54 10.80.5.48 [<ffffffff81546996>] clone_endio+0x76/0xa0
Oct 13 03:16:54 10.80.5.48 [<ffffffff811dbf1d>] bio_endio+0x1d/0x40
Oct 13 03:16:54 10.80.5.48 [<ffffffff81546781>] dec_pending+0x1c1/0x360
Oct 13 03:16:54 10.80.5.48 [<ffffffff81546996>] clone_endio+0x76/0xa0
Oct 13 03:16:54 10.80.5.48 [<ffffffff811dbf1d>] bio_endio+0x1d/0x40
Oct 13 03:16:54 10.80.5.48 [<ffffffff81546781>] dec_pending+0x1c1/0x360
Oct 13 03:16:54 10.80.5.48 [<ffffffff81546996>] clone_endio+0x76/0xa0
Oct 13 03:16:54 10.80.5.48 [<ffffffff811dbf1d>] bio_endio+0x1d/0x40
Oct 13 03:16:54 10.80.5.48 [<ffffffff812fad2b>] blk_update_request+0x21b/0x450
Oct 13 03:16:54 10.80.5.48 [<ffffffff810e7797>] ? generic_exec_single+0xa7/0xb0
Oct 13 03:16:54 10.80.5.48 [<ffffffff812faf87>] blk_update_bidi_request+0x27/0xb0
Oct 13 03:16:54 10.80.5.48 [<ffffffff810e7817>] ? __smp_call_function_single+0x77/0x120
Oct 13 03:16:54 10.80.5.48 [<ffffffff812fcc7f>] blk_end_bidi_request+0x2f/0x80
Oct 13 03:16:54 10.80.5.48 [<ffffffff812fcd20>] blk_end_request+0x10/0x20
Oct 13 03:16:54 10.80.5.48 [<ffffffff813fdc1c>] scsi_io_completion+0xbc/0x620
Oct 13 03:16:54 10.80.5.48 [<ffffffff813f57f9>] scsi_finish_command+0xc9/0x130
Oct 13 03:16:54 10.80.5.48 [<ffffffff813fe2e7>] scsi_softirq_done+0x147/0x170
Oct 13 03:16:54 10.80.5.48 [<ffffffff813035ad>] blk_done_softirq+0x7d/0x90
Oct 13 03:16:54 10.80.5.48 [<ffffffff8108ed87>] __do_softirq+0x137/0x2e0
Oct 13 03:16:54 10.80.5.48 [<ffffffff81658a0c>] call_softirq+0x1c/0x30
Oct 13 03:16:54 10.80.5.48 [<ffffffff8104a35d>] do_softirq+0x8d/0xc0
Oct 13 03:16:54 10.80.5.48 [<ffffffff8108e925>] irq_exit+0x95/0xa0
Oct 13 03:16:54 10.80.5.48 [<ffffffff81658f76>] do_IRQ+0x66/0xe0
Oct 13 03:16:54 10.80.5.48 [<ffffffff816567ef>] common_interrupt+0x6f/0x6f
Oct 13 03:16:54 10.80.5.48 <EOI> 
Oct 13 03:16:54 10.80.5.48 [<ffffffff81656836>] ? retint_swapgs+0xe/0x13
Oct 13 03:16:54 10.80.5.48 ---[ end trace 4a0584a583c66b92 ]---

Doing addr2line on ffffffff8125c2c8 shows: /home/projects/linux-stable/fs/ext4/page-io.c:335
which for me is the last bio_put in ext4_end_bio. However, the ? addresses, 
right at the beginning of the NMI stack (ffffffff8125be19) map to inner loop in
bit_spin_lock:

} while (test_bit(bitnum, addr));

and this is in line with my initial bug report. 

Unfortunately I wasn't able to acquire a crashdump since the machine hard-locked
way too fast. On a slightly different note is it possible to panic the machine 
via NMIs? Since if all the CPUs are hard lockedup they cannot process sysrq interrupts?

> 
>> 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.
> 
> Agreed, that patch is definitely wrong.
> 
>>> 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.
> 
> Well, if you have a crashdump, then bh->b_size is the block size. So just
> check that for the bh we are spinning on.

Turns out in my original email the bh->b_size was shown : 
b_size = 0x400 == 1k. So the filesystem is not 4k but 1k. 


> 
> 								Honza
> 

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