[dm-devel] [bug?] "vgs" command hanging after running "targetctl clear"

[Chris Friesen]

On 06/15/2016 12:27 PM, Chris Friesen wrote:
> I'm running a CentOS-7 based system, so if that disqualifies me due to the
> amount of kernel patches please let me know. :)
>
> Anyways, I've run into some weird behaviour.  I have a single system.  I'm
> exporting an ISCSI target using targetctl.  The backing store is a
> thinly-provisioned LVM volume, where the underlying PV is a single drbd device,
> which in turn is backed by /dev/sdb1.  The LVM/drbd setup (as well as other
> configuration) is done by scripts and I'm not aware of all the exact config
> details.
>
> I'm using iscsiadm to discover and then login to the target, so that "ls -l
> /dev/disk/by-path" shows this:
>
> lrwxrwxrwx 1 root root  9 Jun 15 16:36
> ip-127.0.0.1:3260-iscsi-iqn.2014-10.com.example.server1:iscsi-1-lun-0 -> ../../sdc
>
>
> Now here's where it gets a bit odd.  If I run "targetctl clear", then run "vgs",
> the vgs command hangs.  /proc/<pid>/stack for the hung process looks like this:
>
> controller-0:/home/wrsroot# cat /proc/15379/stack
> [<ffffffff81081ae5>] flush_work+0x105/0x1d0
> [<ffffffff81081c39>] __cancel_work_timer+0x89/0x120
> [<ffffffff81081d03>] cancel_delayed_work_sync+0x13/0x20
> [<ffffffff812dba60>] disk_block_events+0x80/0x90
> [<ffffffff811dee0e>] __blkdev_get+0x6e/0x4d0
> [<ffffffff811df445>] blkdev_get+0x1d5/0x360
> [<ffffffff811df67b>] blkdev_open+0x5b/0x80
> [<ffffffff811a1cc7>] do_dentry_open+0x1a7/0x2e0
> [<ffffffff811a1ef9>] vfs_open+0x39/0x70
> [<ffffffff811b131d>] do_last+0x1ed/0x1270
> [<ffffffff811b4082>] path_openat+0xc2/0x490
> [<ffffffff811b584b>] do_filp_open+0x4b/0xb0
> [<ffffffff811a33c3>] do_sys_open+0xf3/0x1f0
> [<ffffffff811a34de>] SyS_open+0x1e/0x20
> [<ffffffff81681249>] system_call_fastpath+0x16/0x1b
> [<ffffffffffffffff>] 0xffffffffffffffff

I ran "strace vgs" and that helped sort out what was going on, it's got nothing 
to do with the kernel.

The system that hung was using "use_lvmetad=0" in lvm.conf with the default 
"global_filter" setting, so when running the "vgs" command it was going out and 
scanning all block devices to see if they were part of LVM, including the iscsi 
device which was no longer accessible since the target had been taken down.  The 
open() on that device hung until it hit the 900 sec timeout, then it continued on.

The working system had "use_lvmetad=1", so it wasn't scanning all block devices. 
  Setting an explicit "global_filter" value also worked to prevent it from 
trying to scan the iscsi device.

Sorry for the noise.

Chris