Date:   Tue, 5 Sep 2017 12:24:15 -0700
From:   Shaohua Li <shli@...nel.org>
To:     Dennis Yang <dennisyang@...p.com>
Cc:     linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] md/raid5: preserve STRIPE_ON_UNPLUG_LIST in
 break_stripe_batch_list

On Fri, Sep 01, 2017 at 05:26:48PM +0800, Dennis Yang wrote:
> >On Mon, Aug 28, 2017 at 08:01:59PM +0800, Dennis Yang wrote:
> >> break_stripe_batch_list() did not preserve STRIPE_ON_UNPLUG_LIST which is
> >> set when a stripe_head gets queued to the stripe_head list maintained by
> >> raid5_plug_cb and waiting for releasing after blk_unplug().
> >> 
> >> In release_stripe_plug(), if a stripe_head has its STRIPE_ON_UNPLUG_LIST
> >> set, it indicates that this stripe_head is already in the raid5_plug_cb
> >> list and release_stripe() would be called instead to drop a reference
> >> count. Otherwise, the STRIPE_ON_UNPLUG_LIST bit would be set for this
> >> stripe_head and it will get queued into the raid5_plug_cb list.
> >> 
> >> Without preserving STRIPE_ON_UNPLUG_LIST in break_stripe_batch_list(),
> >> a stripe_head could be re-added to the raid5_plug_cb list while it is
> >> currently on that list and waiting to be released. This would mess up
> >> the raid5_plug_cb and leads to soft/hard lockup in raid5_unplug() or
> >> kernel crash.
> >
> >Did you observe this? From my understanding, this is impossible. If a stripe is
> >in batch list (that's why we have break_stripe_batch_list), add_stripe_bio will
> >bail out (because sh->batch_head != NULL), so the stripe is never added into
> >plug list, hence has the STRIPE_ON_UNPLUG_LIST bit set. Am I missing anything?
> >
> >Thanks,
> >Shaohua
> 
> It is hard to reproduce. Our storage exports 4 iscsi LUNs which are built on 
> top of a dm-thin pool with 8 x SSD RAID6. Each two of these iscsi LUNs are 
> connected to a windows server which runs 4 fio jobs with 16 threads and 
> iodepth = 16. In this setup, we can get the following call trace after couples 
> of hours. The stack dump basically shows that the raid5_unplug is holding the 
> conf->device_lock for a very long time. After adding some debug traces, we can 
> confirm that the raid5_plug_cb->list is corrupted which sometimes leads to 
> infinite loop in raid5_unplug. 
> 
> I didn't notice that add_stripe_bio will bail out if the stripe_head is in 
> someone's batch list, but I am wondering if the following scenario will still 
> happen. If a stripe_head A is added to another stripe_head B's batch list, in 
> this case A will have its batch_head != NULL and be added into the plug list. 
> After that, stripe_head B gets handled and called break_stripe_batch_list() to 
> reset all the batched stripe_head(including A which is still on the plug list)'s 
> state and reset their batch_head to NULL. Before the plug list gets processed, 
> if there is another write request comes in and get stripe_head A, A will have 
> its batch_head == NULL (cleared by calling break_stripe_batch_list() on B) and 
> be added to plug list once again. 
> 
> Since we have the environment which can reproduce this issue in 6 hours, if 
> there is anything we can do to help resolve this issue, we are more than 
> willing to help. 

yes, I think you are right. If the stripe is on one thread's plug list, the
thread doesn't do unplug till the stripe is handled (because it's part of a
batch), we will have the stripe in plug list but not have the bit set. Can you
resend the patch with description what happens? BTW, does the patch fix your
issue?

Thanks,
Shaohua
> 
> Thanks,
> Dennis
> 
> [ 4481.493998] BUG: spinlock lockup suspected on CPU#10, md1_raid6/5497
> [ 4481.501109]  lock: 0xffff88102b242888, .magic: dead4ead, .owner: kworker/u24:2/22322, .owner_cpu: 0
> [ 4481.511231] CPU: 10 PID: 5497 Comm: md1_raid6 Tainted: P        W  O    4.2.8 #7
> [ 4481.519504] Hardware name: Default string Default string/Default string, BIOS QY38AR06 01/24/2017
> [ 4481.529429]  ffff88102b242888 ffff881014967c78 ffffffff81c933b6 0000000000000007
> [ 4481.537739]  ffff8805c9720a00 ffff881014967c98 ffffffff810d2c20 ffff88102b242888
> [ 4481.546048]  0000000082d0f7c8 ffff881014967cc8 ffffffff810d2d9b ffff88102b242888
> [ 4481.554357] Call Trace:
> [ 4481.557093]  [<ffffffff81c933b6>] dump_stack+0x4c/0x65
> [ 4481.562840]  [<ffffffff810d2c20>] spin_dump+0x80/0xd0
> [ 4481.568489]  [<ffffffff810d2d9b>] do_raw_spin_lock+0x9b/0x120
> [ 4481.574916]  [<ffffffff81ca14d1>] _raw_spin_lock_irq+0x41/0x50
> [ 4481.581443]  [<ffffffff816d2204>] ? raid5d+0x64/0x790
> [ 4481.587091]  [<ffffffff816d2204>] raid5d+0x64/0x790
> [ 4481.592546]  [<ffffffff810c35c6>] ? finish_wait+0x46/0x80
> [ 4481.598583]  [<ffffffff810c35e8>] ? finish_wait+0x68/0x80
> [ 4481.604621]  [<ffffffff816d8d2e>] md_thread+0x13e/0x150
> [ 4481.610464]  [<ffffffff810c37e0>] ? wait_woken+0xb0/0xb0
> [ 4481.616405]  [<ffffffff816d8bf0>] ? find_pers+0x80/0x80
> [ 4481.622249]  [<ffffffff8109eed5>] kthread+0x105/0x120
> [ 4481.627899]  [<ffffffff8109edd0>] ? kthread_create_on_node+0x220/0x220
> [ 4481.635201]  [<ffffffff81ca218f>] ret_from_fork+0x3f/0x70
> [ 4481.641239]  [<ffffffff8109edd0>] ? kthread_create_on_node+0x220/0x220
> [ 4481.654638] NMI backtrace for cpu 0
> [ 4481.658536] CPU: 0 PID: 22322 Comm: kworker/u24:2 Tainted: P        W  O    4.2.8 #7
> [ 4481.667198] Hardware name: Default string Default string/Default string, BIOS QY38AR06 01/24/2017
> [ 4481.677124] Workqueue: writeback wb_workfn (flush-252:15)
> [ 4481.683160] task: ffff8805c9720a00 ti: ffff8805c84f8000 task.ti: ffff8805c84f8000
> [ 4481.691531] RIP: 0010:[<ffffffff816cd9f1>]  [<ffffffff816cd9f1>] raid5_unplug+0x71/0x270
> [ 4481.700582] RSP: 0018:ffff8805c84fb938  EFLAGS: 00000046
> [ 4481.706521] RAX: ffff8805cd2ce258 RBX: ffff881038557280 RCX: ffff8805cd2ce258
> [ 4481.714502] RDX: ffff8805cd2ce258 RSI: ffff8805cd2ce248 RDI: ffff88102b242000
> [ 4481.722483] RBP: ffff8805c84fb978 R08: ffffffff81e0a1c0 R09: 0000000000000000
> [ 4481.730464] R10: 0000000000000000 R11: 00000000071775d0 R12: 00000000b921e53c
> [ 4481.738445] R13: ffff88102b242000 R14: ffff8810385572a0 R15: ffff88102feec000
> [ 4481.746427] FS:  0000000000000000(0000) GS:ffff88107fc00000(0000) knlGS:0000000000000000
> [ 4481.755477] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 4481.761904] CR2: 00007fb7bf2d2000 CR3: 0000000fff8df000 CR4: 00000000003406f0
> [ 4481.769886] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 4481.777868] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 4481.785848] Stack:
> [ 4481.788094]  00000000f835a020 ffff88102b242888 ffff8805c84fbb48 ffff8805c84fb9a8
> [ 4481.796366]  0000000000000000 ffff8805c84fba28 ffff8805c84fba48 ffff8805c84fb998
> [ 4481.804640]  ffff8805c84fb9e8 ffffffff81463c8b ffff8805c84fb9d8 ffffffff81461bb2
> [ 4481.812912] Call Trace:
> [ 4481.815644]  [<ffffffff81463c8b>] blk_flush_plug_list+0x9b/0x230
> [ 4481.822361]  [<ffffffff81461bb2>] ? submit_bio+0x62/0x150
> [ 4481.828398]  [<ffffffff81464364>] blk_finish_plug+0x24/0x40
> [ 4481.834628]  [<ffffffff812292c3>] mpage_writepages+0x73/0x90
> [ 4481.840956]  [<ffffffff81222f90>] ? I_BDEV+0x10/0x10
> [ 4481.846506]  [<ffffffff81225380>] qnap_blkdev_writepages+0x10/0x20
> [ 4481.853418]  [<ffffffff81188af9>] do_writepages+0x59/0x70
> [ 4481.859455]  [<ffffffff81213874>] __writeback_single_inode+0x74/0xa50
> [ 4481.866658]  [<ffffffff81214a41>] writeback_sb_inodes+0x2f1/0x680
> [ 4481.873472]  [<ffffffff81214e54>] __writeback_inodes_wb+0x84/0xb0
> [ 4481.880287]  [<ffffffff812151a9>] wb_writeback+0x329/0x6e0
> [ 4482.914006] NMI backtrace for cpu 5
> [ 4482.917906] CPU: 5 PID: 26333 Comm: kworker/u24:5 Tainted: P        W  O    4.2.8 #7
> [ 4482.926567] Hardware name: Default string Default string/Default string, BIOS QY38AR06 01/24/2017
> [ 4482.936492] Workqueue: writeback wb_workfn (flush-252:14)
> [ 4482.942529] task: ffff8805c7a74340 ti: ffff8805c6228000 task.ti: ffff8805c6228000
> [ 4482.950898] RIP: 0010:[<ffffffff8149cfd7>]  [<ffffffff8149cfd7>] delay_tsc+0x17/0x70
> [ 4482.959558] RSP: 0018:ffff8805c622b368  EFLAGS: 00000082
> [ 4482.965498] RAX: 00000000dc0ef28f RBX: ffff88102b242888 RCX: 00000000dc0ef259
> [ 4482.973479] RDX: 0000000000003c68 RSI: 0000000000000005 RDI: 0000000000000001
> [ 4482.981460] RBP: ffff8805c622b368 R08: 0000000000000001 R09: 0000000000000001
> [ 4482.989441] R10: 0000000000000000 R11: 0000000000000007 R12: 00000000763815f7
> [ 4482.997422] R13: 0000000000000001 R14: 0000000082d0f7c8 R15: ffff88102feec000
> [ 4483.005403] FS:  0000000000000000(0000) GS:ffff88107fca0000(0000) knlGS:0000000000000000
> [ 4483.014459] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 4483.020876] CR2: 00007f54d805a030 CR3: 0000001006c20000 CR4: 00000000003406e0
> [ 4483.028858] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [ 4483.036840] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [ 4483.044822] Stack:
> [ 4483.047069]  ffff8805c622b378 ffffffff8149cf2a ffff8805c622b3a8 ffffffff810d2d87
> [ 4483.055341]  ffff88102b242888 0000000000000000 ffff88102b242000 ffff881007d702a0
> [ 4483.063613]  ffff8805c622b3d8 ffffffff81ca14d1 ffffffff816cd9ca 0000000000000000
> [ 4483.071885] Call Trace:
> [ 4483.074617]  [<ffffffff8149cf2a>] __delay+0xa/0x10
> [ 4483.079973]  [<ffffffff810d2d87>] do_raw_spin_lock+0x87/0x120
> [ 4483.086399]  [<ffffffff81ca14d1>] _raw_spin_lock_irq+0x41/0x50
> [ 4483.092920]  [<ffffffff816cd9ca>] ? raid5_unplug+0x4a/0x270
> [ 4483.099151]  [<ffffffff816cd9ca>] raid5_unplug+0x4a/0x270
> [ 4483.105188]  [<ffffffff81463c8b>] blk_flush_plug_list+0x9b/0x230
> [ 4483.111907]  [<ffffffff81c9bf9d>] schedule+0x6d/0x90
> [ 4483.117459]  [<ffffffff816c9910>] __get_active_stripe.constprop.61+0x750/0x970
> [ 4483.125539]  [<ffffffff810c37e0>] ? wait_woken+0xb0/0xb0
> [ 4483.131480]  [<ffffffff816c9d2a>] make_request+0x1fa/0xe30
> [ 4483.137616]  [<ffffffff816ddc8e>] ? md_make_request+0x1fe/0x4b0
> [ 4483.144238]  [<ffffffff810c37e0>] ? wait_woken+0xb0/0xb0
> [ 4483.150179]  [<ffffffff816ddc52>] ? md_make_request+0x1c2/0x4b0
> [ 4483.156800]  [<ffffffff816ddc8e>] md_make_request+0x1fe/0x4b0
> [ 4483.163227]  [<ffffffff816ddade>] ? md_make_request+0x4e/0x4b0
> [ 4483.169750]  [<ffffffff816edd43>] ? dm_make_request+0x73/0x110
> [ 4483.176273]  [<ffffffff81461b0e>] generic_make_request+0xce/0x110
> 
> 

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