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Date:	Thu, 8 Oct 2015 10:34:26 -0400
From:	"John Stoffel" <>
To:	Nikolay Borisov <>
Cc:	"Linux-Kernel\@Vger. Kernel. Org" <>,
	linux-ext4 <>,
	"Theodore Ts'o" <>, Marian Marinov <>,
	Jan Kara <>
Subject: Re: Hard lockup in ext4_finish_bio

Great bug report, but you're missing the info on which kernel you're
running here...  is this a vendor kernel or self-compiled?  

Nikolay> I've hit a rather strange hard lock up on one of my servers
Nikolay> from the page writeback path, the actual backtrace is:

Nikolay> [427149.717151] ------------[ cut here ]------------
Nikolay> [427149.717553] WARNING: CPU: 23 PID: 4611 at
Nikolay> kernel/watchdog.c:245 watchdog_overflow_callback+0x98/0xc0()
Nikolay> [427149.718216] Watchdog detected hard LOCKUP on cpu 23
Nikolay> [427149.718292] Modules linked in: [427149.718723] tcp_diag
Nikolay> inet_diag netconsole act_police cls_basic sch_ingress
Nikolay> xt_pkttype xt_state veth openvswitch gre vxlan ip_tunnel
Nikolay> xt_owner xt_conntrack iptable_mangle xt_nat iptable_nat
Nikolay> nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_n at xt_CT
Nikolay> nf_conntrack iptable_raw ib_ipoib rdma_ucm ib_ucm ib_uverbs
Nikolay> ib_umad rdma_cm ib_cm iw_cm ib_addr ipv6 ib_sa ext2
Nikolay> dm_thin_pool dm_bio_prison dm_persistent_data dm_bufio
Nikolay> dm_mirror dm_region_hash dm_log i2c_i801 lpc_ich mfd_core
Nikolay> shpchp i oapic ioatdma igb i2c_algo_bit ses enclosure
Nikolay> ipmi_devintf ipmi_si ipmi_msghandler ib_qib dca ib_mad
Nikolay> ib_core [427149.725321] CPU: 23 PID: 4611 Comm: kworker/u98:7
Nikolay> Not tainted 3.12.47-clouder1 #1 [427149.725690] Hardware
Nikolay> name: Supermicro X10DRi/X10DRi, BIOS 1.1 04/14/2015
Nikolay> [427149.726062] Workqueue: writeback bdi_writeback_workfn
Nikolay> (flush-252:148) [427149.726564] 00000000000000f5
Nikolay> ffff883fff366b58 ffffffff81651631 00000000000000f5
Nikolay> [427149.727212] ffff883fff366ba8 ffff883fff366b98
Nikolay> ffffffff81089a6c 0000000000000000 [427149.727860]
Nikolay> ffff883fd2f08000 0000000000000000 ffff883fff366ce8
Nikolay> 0000000000000000 [427149.728490] Call Trace: [427149.728845]
Nikolay> <NMI> [<ffffffff81651631>] dump_stack+0x58/0x7f
Nikolay> [427149.729350] [<ffffffff81089a6c>]
Nikolay> warn_slowpath_common+0x8c/0xc0 [427149.729712]
Nikolay> [<ffffffff81089b56>] warn_slowpath_fmt+0x46/0x50
Nikolay> [427149.730076] [<ffffffff811015f8>]
Nikolay> watchdog_overflow_callback+0x98/0xc0 [427149.730443]
Nikolay> [<ffffffff81132d0c>] __perf_event_overflow+0x9c/0x250
Nikolay> [427149.730810] [<ffffffff81133664>]
Nikolay> perf_event_overflow+0x14/0x20 [427149.731175]
Nikolay> [<ffffffff81061796>] intel_pmu_handle_irq+0x1d6/0x3e0
Nikolay> [427149.739656] [<ffffffff8105b4c4>]
Nikolay> perf_event_nmi_handler+0x34/0x60 [427149.740027]
Nikolay> [<ffffffff8104c152>] nmi_handle+0xa2/0x1a0 [427149.740389]
Nikolay> [<ffffffff8104c3b4>] do_nmi+0x164/0x430 [427149.740754]
Nikolay> [<ffffffff81656e2e>] end_repeat_nmi+0x1a/0x1e [427149.741122]
Nikolay> [<ffffffff8113bfd7>] ? mempool_free_slab+0x17/0x20
Nikolay> [427149.741492] [<ffffffff8125be15>] ?
Nikolay> ext4_finish_bio+0x275/0x2a0 [427149.741854]
Nikolay> [<ffffffff8125be15>] ? ext4_finish_bio+0x275/0x2a0
Nikolay> [427149.742216] [<ffffffff8125be15>] ?
Nikolay> ext4_finish_bio+0x275/0x2a0 [427149.742579] <<EOE>> <IRQ>
Nikolay> [<ffffffff8125c2c8>] ext4_end_bio+0xc8/0x120 [427149.743150]
Nikolay> [<ffffffff811dbf1d>] bio_endio+0x1d/0x40 [427149.743516]
Nikolay> [<ffffffff81546781>] dec_pending+0x1c1/0x360 [427149.743878]
Nikolay> [<ffffffff81546996>] clone_endio+0x76/0xa0 [427149.744239]
Nikolay> [<ffffffff811dbf1d>] bio_endio+0x1d/0x40 [427149.744599]
Nikolay> [<ffffffff81546781>] dec_pending+0x1c1/0x360 [427149.744964]
Nikolay> [<ffffffff81546996>] clone_endio+0x76/0xa0 [427149.745326]
Nikolay> [<ffffffff811dbf1d>] bio_endio+0x1d/0x40 [427149.745686]
Nikolay> [<ffffffff81546781>] dec_pending+0x1c1/0x360 [427149.746048]
Nikolay> [<ffffffff81546996>] clone_endio+0x76/0xa0 [427149.746407]
Nikolay> [<ffffffff811dbf1d>] bio_endio+0x1d/0x40 [427149.746773]
Nikolay> [<ffffffff812fad2b>] blk_update_request+0x21b/0x450
Nikolay> [427149.747138] [<ffffffff812faf87>]
Nikolay> blk_update_bidi_request+0x27/0xb0 [427149.747513]
Nikolay> [<ffffffff812fcc7f>] blk_end_bidi_request+0x2f/0x80
Nikolay> [427149.748101] [<ffffffff812fcd20>]
Nikolay> blk_end_request+0x10/0x20 [427149.748705]
Nikolay> [<ffffffff813fdc1c>] scsi_io_completion+0xbc/0x620
Nikolay> [427149.749297] [<ffffffff813f57f9>]
Nikolay> scsi_finish_command+0xc9/0x130 [427149.749891]
Nikolay> [<ffffffff813fe2e7>] scsi_softirq_done+0x147/0x170
Nikolay> [427149.750491] [<ffffffff813035ad>]
Nikolay> blk_done_softirq+0x7d/0x90 [427149.751089]
Nikolay> [<ffffffff8108ed87>] __do_softirq+0x137/0x2e0 [427149.751694]
Nikolay> [<ffffffff81658a0c>] call_softirq+0x1c/0x30 [427149.752284]
Nikolay> [<ffffffff8104a35d>] do_softirq+0x8d/0xc0 [427149.752892]
Nikolay> [<ffffffff8108e925>] irq_exit+0x95/0xa0 [427149.753526]
Nikolay> [<ffffffff8106f755>]
Nikolay> smp_call_function_single_interrupt+0x35/0x40 [427149.754149]
Nikolay> [<ffffffff8165826f>] call_function_single_interrupt+0x6f/0x80
Nikolay> [427149.754750] <EOI> [<ffffffff813276e6>] ? memcpy+0x6/0x110
Nikolay> [427149.755572] [<ffffffff811dc6d6>] ? __bio_clone+0x26/0x70
Nikolay> [427149.756179] [<ffffffff81546db9>]
Nikolay> __clone_and_map_data_bio+0x139/0x160 [427149.756814]
Nikolay> [<ffffffff815471cd>] __split_and_process_bio+0x3ed/0x490
Nikolay> [427149.757444] [<ffffffff815473a6>] dm_request+0x136/0x1e0
Nikolay> [427149.758041] [<ffffffff812fbe0a>]
Nikolay> generic_make_request+0xca/0x100 [427149.758641]
Nikolay> [<ffffffff812fbeb9>] submit_bio+0x79/0x160 [427149.759035]
Nikolay> [<ffffffff81144c3d>] ? account_page_writeback+0x2d/0x40
Nikolay> [427149.759406] [<ffffffff81144dbd>] ?
Nikolay> __test_set_page_writeback+0x16d/0x1f0 [427149.759781]
Nikolay> [<ffffffff8125b7a9>] ext4_io_submit+0x29/0x50 [427149.760151]
Nikolay> [<ffffffff8125b8fb>] ext4_bio_write_page+0x12b/0x2f0
Nikolay> [427149.760519] [<ffffffff81252fe8>]
Nikolay> mpage_submit_page+0x68/0x90 [427149.760887]
Nikolay> [<ffffffff81253100>] mpage_process_page_bufs+0xf0/0x110
Nikolay> [427149.761257] [<ffffffff81254a80>]
Nikolay> mpage_prepare_extent_to_map+0x210/0x310 [427149.761624]
Nikolay> [<ffffffff8125a911>] ? ext4_writepages+0x361/0xc60
Nikolay> [427149.761998] [<ffffffff81283c09>] ?
Nikolay> __ext4_journal_start_sb+0x79/0x110 [427149.762362]
Nikolay> [<ffffffff8125a948>] ext4_writepages+0x398/0xc60
Nikolay> [427149.762726] [<ffffffff812fd358>] ?
Nikolay> blk_finish_plug+0x18/0x50 [427149.763089]
Nikolay> [<ffffffff81146b40>] do_writepages+0x20/0x40 [427149.763454]
Nikolay> [<ffffffff811cec79>] __writeback_single_inode+0x49/0x2b0
Nikolay> [427149.763819] [<ffffffff810aeeef>] ? wake_up_bit+0x2f/0x40
Nikolay> [427149.764182] [<ffffffff811cfdee>]
Nikolay> writeback_sb_inodes+0x2de/0x540 [427149.764544]
Nikolay> [<ffffffff811a6e65>] ? put_super+0x25/0x50 [427149.764903]
Nikolay> [<ffffffff811d00ee>] __writeback_inodes_wb+0x9e/0xd0
Nikolay> [427149.765265] [<ffffffff811d035b>] wb_writeback+0x23b/0x340
Nikolay> [427149.765628] [<ffffffff811d04f9>]
Nikolay> wb_do_writeback+0x99/0x230 [427149.765996]
Nikolay> [<ffffffff810a40f1>] ? set_worker_desc+0x81/0x90
Nikolay> [427149.766364] [<ffffffff810c7a6a>] ?
Nikolay> dequeue_task_fair+0x36a/0x4c0 [427149.766734]
Nikolay> [<ffffffff811d0bf8>] bdi_writeback_workfn+0x88/0x260
Nikolay> [427149.767101] [<ffffffff810bbb3e>] ?
Nikolay> finish_task_switch+0x4e/0xe0 [427149.767473]
Nikolay> [<ffffffff81653dac>] ? __schedule+0x2dc/0x760 [427149.767845]
Nikolay> [<ffffffff810a61e5>] process_one_work+0x195/0x550
Nikolay> [427149.768212] [<ffffffff810a848a>]
Nikolay> worker_thread+0x13a/0x430 [427149.768575]
Nikolay> [<ffffffff810a8350>] ? manage_workers+0x2c0/0x2c0
Nikolay> [427149.768940] [<ffffffff810ae48e>] kthread+0xce/0xe0
Nikolay> [427149.769303] [<ffffffff810ae3c0>] ?
Nikolay> kthread_freezable_should_stop+0x80/0x80 [427149.769668]
Nikolay> [<ffffffff816571c8>] ret_from_fork+0x58/0x90 [427149.770030]
Nikolay> [<ffffffff810ae3c0>] ?
Nikolay> kthread_freezable_should_stop+0x80/0x80 [427149.770394] ---[
Nikolay> end trace 76566eb23cf1b028 ]---

Nikolay> What's not evident here is that in ext4_end_bio, the CPU has
Nikolay> actually been executing ext4_finish_bio as seen from the
Nikolay> following NMI backtrace:

Nikolay> [427160.405277] NMI backtrace for cpu 23 [427160.405279] CPU:
Nikolay> 23 PID: 4611 Comm: kworker/u98:7 Tainted: G W
Nikolay> 3.12.47-clouder1 #1 [427160.405281] Hardware name: Supermicro
Nikolay> X10DRi/X10DRi, BIOS 1.1 04/14/2015 [427160.405285] Workqueue:
Nikolay> writeback bdi_writeback_workfn (flush-252:148)
Nikolay> [427160.405286] task: ffff8825aa819830 ti: ffff882b19180000
Nikolay> task.ti: ffff882b19180000 [427160.405290] RIP:
Nikolay> 0010:[<ffffffff8125be13>] [<ffffffff8125be13>]
Nikolay> ext4_finish_bio+0x273/0x2a0 <-- Important bit [427160.405291]
Nikolay> RSP: 0000:ffff883fff3639b0 EFLAGS: 00000002 [427160.405292]
Nikolay> RAX: ffff882b19180000 RBX: ffff883f67480a80 RCX:
Nikolay> 0000000000000110 [427160.405292] RDX: ffff882b19180000 RSI:
Nikolay> 0000000000000000 RDI: ffff883f67480a80 [427160.405293] RBP:
Nikolay> ffff883fff363a70 R08: 0000000000014b80 R09: ffff881fff454f00
Nikolay> [427160.405294] R10: ffffea00473214c0 R11: ffffffff8113bfd7
Nikolay> R12: ffff880826272138 [427160.405295] R13: 0000000000000000
Nikolay> R14: 0000000000000000 R15: ffffea00aeaea400 [427160.405296]
Nikolay> FS: 0000000000000000(0000) GS:ffff883fff360000(0000)
Nikolay> knlGS:0000000000000000 [427160.405296] CS: 0010 DS: 0000 ES:
Nikolay> 0000 CR0: 0000000080050033 [427160.405297] CR2:
Nikolay> 0000003c5b009c24 CR3: 0000000001c0b000 CR4: 00000000001407e0
Nikolay> [427160.405297] Stack: [427160.405305] 0000000000000000
Nikolay> ffffffff8203f230 ffff883fff363a00 ffff882b19180000
Nikolay> [427160.405312] ffff882b19180000 ffff882b19180000
Nikolay> 00000400018e0af8 ffff882b19180000 [427160.405319]
Nikolay> ffff883f67480a80 0000000000000000 0000000000000202
Nikolay> 00000000d219e720 [427160.405320] Call Trace: [427160.405324]
Nikolay> <IRQ> //SAME AS THE FIRST STACKTRACE [427160.405436] <EOI>
Nikolay> //SAME AS THE FIRST STACKTRACE [427160.405540] Code: e8 a3 38
Nikolay> ec ff ff 75 90 9d e9 2a ff ff ff 0f 0b 0f 1f 84 00 00 00 00
Nikolay> 00 eb f6 48 8b 85 60 ff ff ff 48 8b 95 58 ff ff ff ff 48 1c
Nikolay> <f3> 90 49 8b 0c 24 80 e1 10 75 f5 ff 42 1c f0 41 0f ba 2c 24
Nikolay> 04

Nikolay> After sampling from several different NMI prints it turns out
Nikolay> the CPU has looped on the following bit lock acquire:

Nikolay> local_irq_save(flags); bit_spin_lock(BH_Uptodate_Lock,
Nikolay> &head->b_state); do { if (bh_offset(bh) < bio_start ||
Nikolay> bh_offset(bh) + bh->b_size > bio_end) { if
Nikolay> (buffer_async_write(bh)) under_io++; continue; }
Nikolay> clear_buffer_async_write(bh); if (error) buffer_io_error(bh);
Nikolay> } while ((bh = bh->b_this_page) != head);
Nikolay> bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
Nikolay> local_irq_restore(flags);

Nikolay> So far everything seems to check out - the code is spinning
Nikolay> on acquiring the BH_Uptodate_Lock with interrupts disabled
Nikolay> and this causes the deadlock. Now, the pertinent question is
Nikolay> why can't it acquire the spinlock. Grepping for the users of
Nikolay> BH_Uptodate_Lock it is revealed that the only other places
Nikolay> where they are used are in end_buffer_async_read and
Nikolay> end_buffer_async_write. Looking at the NMI backtraces for the
Nikolay> rest of the CPUs on the system I didn't find it in any
Nikolay> execution path. Most of the cores were in acpi_idle_enter and
Nikolay> one was in mem cgroup reclaim path. However I think those are
Nikolay> unrelated.

Nikolay> Ignoring the NMI call stack, this is by far the deepest
Nikolay> callchain I've ever seen, I wonder whether some sort of
Nikolay> memory corruption has happened which confuses the code into
Nikolay> thinking the lock is held. Here is the struct buffer_head:

crash> struct buffer_head ffff880826272138 -x

Nikolay> struct buffer_head { b_state = 0x131, b_this_page =
Nikolay> 0xffff8808262728f0, b_page = 0xffffea00aeaea400, b_blocknr =
Nikolay> 0x36bea69, b_size = 0x400, b_data = 0xffff882baba90000
Nikolay> struct: page excluded: kernel virtual address:
Nikolay> ffff882baba90000 type: "gdb_readmem_callback" struct: page
Nikolay> excluded: kernel virtual address: ffff882baba90000 type:
Nikolay> "gdb_readmem_callback" <Address 0xffff882baba90000 out of
Nikolay> bounds>, b_bdev = 0xffff883fcfb6db00, b_end_io = 0x0,
Nikolay> b_private = 0x0, b_assoc_buffers = { next =
Nikolay> 0xffff880826272180, prev = 0xffff880826272180 }, b_assoc_map
Nikolay> = 0x0, b_count = { counter = 0x0 } }

Nikolay> b_state = 0x131 = (0001 0011 0001) = BH_Uptodate,
Nikolay> BG_Uptodate_Lock, BH_Mapped and BH_Async_Write, which implies
Nikolay> that end_buffer_async_write should have been executing
Nikolay> somewhere but I do not see this.  And in any case it hasn't
Nikolay> completed in reasonable time since we've hit the deadlock. At
Nikolay> this point I'm inclined to believe that some sort of hardware
Nikolay> issue might have reared its head?

Nikolay> What do you guys think?

Nikolay> Regards, Nikolay

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