From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f171.google.com (mail-wi0-f171.google.com [209.85.212.171]) by kanga.kvack.org (Postfix) with ESMTP id 790316B0253 for ; Tue, 13 Oct 2015 09:15:21 -0400 (EDT) Received: by wicge5 with SMTP id ge5so57755777wic.0 for ; Tue, 13 Oct 2015 06:15:20 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id u11si3596039wiv.13.2015.10.13.06.14.57 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 13 Oct 2015 06:14:57 -0700 (PDT) Date: Tue, 13 Oct 2015 15:14:53 +0200 From: Jan Kara Subject: Re: [RFC PATCH 1/2] ext4: Fix possible deadlock with local interrupts disabled and page-draining IPI Message-ID: <20151013131453.GA1332@quack.suse.cz> References: <062501d10262$d40d0a50$7c271ef0$@alibaba-inc.com> <56176C10.8040709@kyup.com> <062801d10265$5a749fc0$0f5ddf40$@alibaba-inc.com> <561774D2.3050002@kyup.com> <20151012134020.GA21302@quack.suse.cz> <561BC8DB.6070600@kyup.com> <20151013081512.GJ17050@quack.suse.cz> <561CDEDC.30707@kyup.com> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="Q68bSM7Ycu6FN28Q" Content-Disposition: inline In-Reply-To: <561CDEDC.30707@kyup.com> Sender: owner-linux-mm@kvack.org List-ID: To: Nikolay Borisov Cc: Jan Kara , Hillf Danton , 'linux-kernel' , Theodore Ts'o , Andreas Dilger , linux-fsdevel@vger.kernel.org, SiteGround Operations , vbabka@suse.cz, gilad@benyossef.com, mgorman@suse.de, linux-mm@kvack.org, Marian Marinov --Q68bSM7Ycu6FN28Q Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 > Oct 13 03:16:54 10.80.5.48 [] dump_stack+0x58/0x7f > Oct 13 03:16:54 10.80.5.48 [] warn_slowpath_common+0x8c/0xc0 > Oct 13 03:16:54 10.80.5.48 [] warn_slowpath_fmt+0x46/0x50 > Oct 13 03:16:54 10.80.5.48 [] watchdog_overflow_callback+0x98/0xc0 > Oct 13 03:16:54 10.80.5.48 [] __perf_event_overflow+0x9c/0x250 > Oct 13 03:16:54 10.80.5.48 [] perf_event_overflow+0x14/0x20 > Oct 13 03:16:54 10.80.5.48 [] intel_pmu_handle_irq+0x1d6/0x3e0 > Oct 13 03:16:54 10.80.5.48 [] perf_event_nmi_handler+0x34/0x60 > Oct 13 03:16:54 10.80.5.48 [] nmi_handle+0xa2/0x1a0 > Oct 13 03:16:54 10.80.5.48 [] do_nmi+0x164/0x430 > Oct 13 03:16:54 10.80.5.48 [] end_repeat_nmi+0x1a/0x1e > Oct 13 03:16:54 10.80.5.48 [] ? ext4_finish_bio+0x279/0x2a0 > Oct 13 03:16:54 10.80.5.48 [] ? ext4_finish_bio+0x279/0x2a0 > Oct 13 03:16:54 10.80.5.48 [] ? ext4_finish_bio+0x279/0x2a0 > Oct 13 03:16:54 10.80.5.48 <> > Oct 13 03:16:54 10.80.5.48 > Oct 13 03:16:54 10.80.5.48 [] ext4_end_bio+0xc8/0x120 > Oct 13 03:16:54 10.80.5.48 [] bio_endio+0x1d/0x40 > Oct 13 03:16:54 10.80.5.48 [] dec_pending+0x1c1/0x360 > Oct 13 03:16:54 10.80.5.48 [] clone_endio+0x76/0xa0 > Oct 13 03:16:54 10.80.5.48 [] bio_endio+0x1d/0x40 > Oct 13 03:16:54 10.80.5.48 [] dec_pending+0x1c1/0x360 > Oct 13 03:16:54 10.80.5.48 [] clone_endio+0x76/0xa0 > Oct 13 03:16:54 10.80.5.48 [] bio_endio+0x1d/0x40 > Oct 13 03:16:54 10.80.5.48 [] dec_pending+0x1c1/0x360 > Oct 13 03:16:54 10.80.5.48 [] clone_endio+0x76/0xa0 > Oct 13 03:16:54 10.80.5.48 [] bio_endio+0x1d/0x40 > Oct 13 03:16:54 10.80.5.48 [] blk_update_request+0x21b/0x450 > Oct 13 03:16:54 10.80.5.48 [] ? generic_exec_single+0xa7/0xb0 > Oct 13 03:16:54 10.80.5.48 [] blk_update_bidi_request+0x27/0xb0 > Oct 13 03:16:54 10.80.5.48 [] ? __smp_call_function_single+0x77/0x120 > Oct 13 03:16:54 10.80.5.48 [] blk_end_bidi_request+0x2f/0x80 > Oct 13 03:16:54 10.80.5.48 [] blk_end_request+0x10/0x20 > Oct 13 03:16:54 10.80.5.48 [] scsi_io_completion+0xbc/0x620 > Oct 13 03:16:54 10.80.5.48 [] scsi_finish_command+0xc9/0x130 > Oct 13 03:16:54 10.80.5.48 [] scsi_softirq_done+0x147/0x170 > Oct 13 03:16:54 10.80.5.48 [] blk_done_softirq+0x7d/0x90 > Oct 13 03:16:54 10.80.5.48 [] __do_softirq+0x137/0x2e0 > Oct 13 03:16:54 10.80.5.48 [] call_softirq+0x1c/0x30 > Oct 13 03:16:54 10.80.5.48 [] do_softirq+0x8d/0xc0 > Oct 13 03:16:54 10.80.5.48 [] irq_exit+0x95/0xa0 > Oct 13 03:16:54 10.80.5.48 [] do_IRQ+0x66/0xe0 > Oct 13 03:16:54 10.80.5.48 [] common_interrupt+0x6f/0x6f > Oct 13 03:16:54 10.80.5.48 > Oct 13 03:16:54 10.80.5.48 [] ? 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? Honza -- Jan Kara SUSE Labs, CR --Q68bSM7Ycu6FN28Q Content-Type: text/x-patch; charset=us-ascii Content-Disposition: attachment; filename="0001-ext4-Fix-bh-b_state-corruption.patch" --Q68bSM7Ycu6FN28Q--