Public bug reported:
What I expected to happen: Loops can be successfully created and removed from
multiple different threads without any adverse effects
What happened instead: Udev processes ended up in D state, which soft locked
the system (unable to reboot without resorting to "echo b >
/proc/sysrq-trigger", various other commands [losetup -a, mount -l] would hang)
The use case that this was originally reproduced on required frequent
setup and teardown of loops in order to persist and access various
snapshots of backup files. Through various methods of analysis, we were
able to create a script that recreates the bad behavior relatively
quickly (see attachment testudevLoopOnly.php).
We captured some stack traces from a locked up system, and found that
many of the calls were originating from loop.c. Investigating that file
(specifically lo_setup and lo_release functions) showed that the 4.4
usage of mutexes is different from earlier versions of the kernel
package (4.4.0.142.148), which doesn't exhibit this same behavior. It's
also different from the 4.15 version of the kernel package, which also
does not exhibit the same (incorrect) behavior.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: Incomplete
** Attachment added: "Contains system info, and script to reproduce the issue"
https://bugs.launchpad.net/bugs/1834693/+attachment/5274042/+files/UdevSoftlock.zip
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https://bugs.launchpad.net/bugs/1834693
Title:
Simultaneous creation and teardown of separate loops causes udev
softlock
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