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 > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>