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Message-ID: <561E1A20.30305@kyup.com> Date: Wed, 14 Oct 2015 12:02:24 +0300 From: Nikolay Borisov <kernel@...p.com> To: Jan Kara <jack@...e.cz> Cc: Hillf Danton <hillf.zj@...baba-inc.com>, 'linux-kernel' <linux-kernel@...r.kernel.org>, Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, linux-fsdevel@...r.kernel.org, SiteGround Operations <operations@...eground.com>, vbabka@...e.cz, gilad@...yossef.com, mgorman@...e.de, linux-mm@...ck.org, Marian Marinov <mm@...com> Subject: Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts disabled and page-draining IPI On 10/13/2015 04:14 PM, Jan Kara wrote: > On Tue 13-10-15 13:37:16, Nikolay Borisov wrote: >> >> >> 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. > > OK. > >> 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? > > Certainly it's possible to do that - the easiest way is actually to use > > nmi_watchdog=panic > > Then panic will automatically trigger when watchdog fires. > >>>> 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. > > OK, then I have a theory. We can manipulate bh->b_state in a non-atomic > manner in _ext4_get_block(). If we happen to do that on the first buffer in > a page while IO completes on another buffer in the same page, we could in > theory mess up and miss clearing of BH_Uptodate_Lock flag. Can you try > whether the attached patch fixes your problem? I will try the patch, however it might take some time to report back since scheduling reboots on the live servers is going to be tricky and unfortunately at the moment I cannot reproduce the issue on demand. > > Honza > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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