Linux 4.1 kernel warning in unregister_blkdev

[Vinson Lee]

Hi.

I've hit this warning multiple times from running the Linux Test
Project ltp_block_dev test case.

ltp_block_dev: Test Case 7: unregister_blkdev() with major=0
------------[ cut here ]------------
WARNING: CPU: 10 PID: 33952 at block/genhd.c:352 unregister_blkdev+0x67/0x9c()
Modules linked in: ltp_block_dev(O) veth tun lp parport coretemp
tcp_diag inet_diag iTCO_wdt iTCO_vendor_support i7core_edac edac_core
i2c_i801 dcdbas kvm ipmi_si ipmi_devintf crc32c_intel ipmi_msghandler
acpi_cpufreq ghash_clmulni_intel ioatdma shpchp lpc_ich mfd_core hed
microcode dell_rbu xfs libcrc32c igb ptp pps_core i2c_algo_bit
i2c_core dca ipv6 [last unloaded: ltp_fw_load]
CPU: 10 PID: 33952 Comm: block_dev Tainted: G           O    4.1.3 #1
 0000000000000009 ffff8811fc8f3cd8 ffffffff81538de8 00000000000000da
 0000000000000000 ffff8811fc8f3d18 ffffffff81068171 ffff8811fc8f3d38
 ffffffff8127f73a 0000000000000000 0000000000000000 ffffffff81f01550
Call Trace:
 [<ffffffff81538de8>] dump_stack+0x45/0x57
 [<ffffffff81068171>] warn_slowpath_common+0xa1/0xbb
 [<ffffffff8127f73a>] ? unregister_blkdev+0x67/0x9c
 [<ffffffff8106822e>] warn_slowpath_null+0x1a/0x1c
 [<ffffffff8127f73a>] unregister_blkdev+0x67/0x9c
 [<ffffffffa00986cf>] sys_tcase+0x59b/0x78f [ltp_block_dev]
 [<ffffffff813744e4>] dev_attr_store+0x18/0x22
 [<ffffffff811d3611>] sysfs_kf_write+0x3e/0x40
 [<ffffffff811d2b2a>] kernfs_fop_write+0xeb/0x13b
 [<ffffffff81172a20>] __vfs_write+0x28/0xa7
 [<ffffffff811759c2>] ? __sb_start_write+0xb0/0xe4
 [<ffffffff8123ea0c>] ? security_file_permission+0x2e/0x33
 [<ffffffff81173245>] vfs_write+0x8f/0xe5
 [<ffffffff81173a57>] SyS_write+0x44/0x78
 [<ffffffff8153e2d7>] system_call_fastpath+0x12/0x6a
---[ end trace ea5af5d6119d362a ]---
ltp_block_dev: Test Case Result: PASS
ltp_block_dev: device released

Cheers,
Vinson
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