linux-kernel - Re: linux-next: BUG: unable to handle kernel NULL pointer dereference in cfq_print_leaf

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Date:	Fri, 19 Jun 2015 00:09:02 +0300
From:	Andrey Wagin <avagin@...il.com>
To:	Arianna Avanzini <avanzini.arianna@...il.com>
Cc:	Jens Axboe <axboe@...nel.dk>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: BUG: unable to handle kernel NULL pointer dereference
 in cfq_print_leaf_weight()

2015-06-18 0:55 GMT+03:00 Andrey Wagin <avagin@...il.com>:
> Hi All,
>
> I executed CRIU tests on the 4.1.0-rc8-next-20150617 kernel and met
> this bug. Maybe it will be interested for someone to look at it.

This bug isn't reproduced if one of devices uses the cfq scheduler:

[root@...gin-fc19-cr ~]# cat /sys/block/sda/queue/scheduler
noop deadline [cfq]
[root@...gin-fc19-cr ~]# cat /sys/fs/cgroup/blkio/blkio.leaf_weight
1000

[root@...gin-fc19-cr ~]# echo noop > /sys/block/sda/queue/scheduler
[root@...gin-fc19-cr ~]# cat /sys/block/sda/queue/scheduler
[noop] deadline cfq
[root@...gin-fc19-cr ~]# cat /sys/fs/cgroup/blkio/blkio.leaf_weight
Killed

blkcg_to_cfqgd(blkcg) returns NULL.

>
> [   30.130897] BUG: unable to handle kernel NULL pointer dereference
> at 000000000000001c
> [   30.130970] IP: [<ffffffff813bc7e1>] cfq_print_leaf_weight+0x31/0x50
> [   30.131013] PGD 3b77a067 PUD 39883067 PMD 0
> [   30.131013] Oops: 0000 [#1] SMP
> [   30.131013] Modules linked in: tun netlink_diag af_packet_diag
> udp_diag tcp_diag inet_diag unix_diag ppdev crct10dif_pclmul
> crc32_pclmul crc32c_intel ghash_clmulni_intel virtio_balloon serio_raw
> i2c_piix4 parport_pc parport virtio_blk virtio_net cirrus
> drm_kms_helper ttm drm virtio_pci virtio_ring virtio ata_generic
> pata_acpi
> [   30.131013] CPU: 0 PID: 613 Comm: criu Not tainted 4.1.0-rc8-next-20150617 #1
> [   30.131013] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> [   30.131013] task: ffff880039eb0000 ti: ffff880039db0000 task.ti:
> ffff880039db0000
> [   30.131013] RIP: 0010:[<ffffffff813bc7e1>]  [<ffffffff813bc7e1>]
> cfq_print_leaf_weight+0x31/0x50
> [   30.131013] RSP: 0018:ffff880039db3d08  EFLAGS: 00010286
> [   30.131013] RAX: 0000000000000000 RBX: ffff88003a524200 RCX: 0000000000000001
> [   30.131013] RDX: 0000000000000001 RSI: 0000000000000001 RDI: ffff88003a524800
> [   30.131013] RBP: ffff880039db3d18 R08: 0000000000000001 R09: 0000000000000000
> [   30.131013] R10: 0000000000000080 R11: 0000000000000000 R12: ffffffff81cba6c8
> [   30.131013] R13: 0000000000000001 R14: ffff880039db3f18 R15: ffff88003a524200
> [   30.131013] FS:  00007f2b6c495740(0000) GS:ffff88003d200000(0000)
> knlGS:0000000000000000
> [   30.131013] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   30.131013] CR2: 000000000000001c CR3: 000000003de3c000 CR4: 00000000000407f0
> [   30.131013] Stack:
> [   30.131013]  ffff880039db3d28 ffff88003a524200 ffff880039db3d48
> ffffffff8113c9d3
> [   30.131013]  ffff880039db3d58 0000000000000000 ffff880039773380
> 0000000000000001
> [   30.131013]  ffff880039db3d58 ffffffff812b9df6 ffff880039db3dd8
> ffffffff81257b7d
> [   30.131013] Call Trace:
> [   30.131013]  [<ffffffff8113c9d3>] cgroup_seqfile_show+0x43/0xc0
> [   30.131013]  [<ffffffff812b9df6>] kernfs_seq_show+0x26/0x30
> [   30.131013]  [<ffffffff81257b7d>] seq_read+0x10d/0x3c0
> [   30.131013]  [<ffffffff812ba7c9>] kernfs_fop_read+0x129/0x180
> [   30.131013]  [<ffffffff8122f9d7>] __vfs_read+0x37/0x100
> [   30.131013]  [<ffffffff81349db3>] ? security_file_permission+0xa3/0xc0
> [   30.131013]  [<ffffffff8122ff86>] ? rw_verify_area+0x56/0xe0
> [   30.131013]  [<ffffffff81230096>] vfs_read+0x86/0x130
> [   30.131013]  [<ffffffff81230f28>] SyS_read+0x58/0xd0
> [   30.131013]  [<ffffffff817c386e>] entry_SYSCALL_64_fastpath+0x12/0x76
> [   30.131013] Code: 48 89 e5 53 48 89 fb 48 83 ec 08 48 8b bf d8 00
> 00 00 e8 93 fe d7 ff 48 85 c0 74 0f 48 63 15 27 f0 8f 00 48 8b 84 d0
> 28 01 00 00 <8b> 50 1c 48 89 df 31 c0 48 c7 c6 f8 f8 ad 81 e8 1b b9 e9
> ff 48
> [   30.131013] RIP  [<ffffffff813bc7e1>] cfq_print_leaf_weight+0x31/0x50
> [   30.131013]  RSP <ffff880039db3d08>
> [   30.131013] CR2: 000000000000001c
> [   30.142367] ---[ end trace bad3e020b932bbb1 ]---
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