[linux-lvm] deadlock during lvm snapshot

[Tomaz Beltram]

Hi,

I'm doing backup of a running mongodb using LVM snapshot. Sometimes I
run into a deadlock situation and kernel reports blocked tasks for jbd2,
mongod, dmeventd and my tar doing backup.

This happens very rarely (one in a thousand) but the effect is rather
severe as mongodb stops working. I also can't remove and unmount the
snapshot. I have attached syslog of two occurrences. The stack traces of
the blocked tasks are very similar so I suspect a potential race
condition between the affected tasks.

Sep 15 17:06:53 dss2 kernel: [719277.567904] INFO: task jbd2/dm-2-8:9048
blocked for more than 120 seconds.
Sep 15 17:06:53 dss2 kernel: [719277.568130] INFO: task mongod:23239
blocked for more than 120 seconds.
Sep 15 17:06:53 dss2 kernel: [719277.568267] INFO: task mongod:23242
blocked for more than 120 seconds.
Sep 15 17:06:53 dss2 kernel: [719277.568350] INFO: task mongod:23243
blocked for more than 120 seconds.
Sep 15 17:06:53 dss2 kernel: [719277.568397] INFO: task dmeventd:12427
blocked for more than 120 seconds.
Sep 15 17:06:53 dss2 kernel: [719277.568523] INFO: task
kworker/u16:2:31890 blocked for more than 120 seconds.
Sep 15 17:06:53 dss2 kernel: [719277.568713] INFO: task tar:12446
blocked for more than 120 seconds.
Sep 15 17:08:53 dss2 kernel: [719397.567614] INFO: task jbd2/dm-2-8:9048
blocked for more than 120 seconds.
Sep 15 17:08:53 dss2 kernel: [719397.567731] INFO: task mongod:23239
blocked for more than 120 seconds.
Sep 15 17:08:53 dss2 kernel: [719397.567870] INFO: task mongod:23240
blocked for more than 120 seconds.

Nov 25 17:10:56 dss2 kernel: [282360.865020] INFO: task jbd2/dm-4-8:878
blocked for more than 120 seconds.
Nov 25 17:10:56 dss2 kernel: [282360.865624] INFO: task mongod:1652
blocked for more than 120 seconds.
Nov 25 17:10:56 dss2 kernel: [282360.866279] INFO: task mongod:1655
blocked for more than 120 seconds.
Nov 25 17:10:56 dss2 kernel: [282360.866771] INFO: task mongod:1656
blocked for more than 120 seconds.
Nov 25 17:10:56 dss2 kernel: [282360.867294] INFO: task dmeventd:3504
blocked for more than 120 seconds.
Nov 25 17:10:56 dss2 kernel: [282360.867783] INFO: task
kworker/u16:2:8016 blocked for more than 120 seconds.
Nov 25 17:10:56 dss2 kernel: [282360.868351] INFO: task tar:3560 blocked
for more than 120 seconds.
Nov 25 17:10:56 dss2 kernel: [282360.868865] INFO: task
kworker/u16:1:5561 blocked for more than 120 seconds.
Nov 25 17:12:56 dss2 kernel: [282480.868656] INFO: task jbd2/dm-4-8:878
blocked for more than 120 seconds.
Nov 25 17:12:56 dss2 kernel: [282480.869015] INFO: task mongod:1652
blocked for more than 120 seconds.

Full syslogs of two occurrences are attached.

I have Ubuntu 16.04.1 (lvm2 2.02.133-1ubuntu10) with mongod 3.2.9 on a
64bit system.

wbr Tomaz

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