In my opinion, there is a fundamental problem wrt interworking between
udev and lvm during the init phase. Looking at scripts and source code I
found nothing which prevents the following to happen:
1) kernel boots with initial ramdisk
2) udevd of initial ramdisk is started by initrd.img/scripts/init-top/udev
3) udevd handles the initial events of the block devices (partitions) produced
by "udevadm trigger --action=add ..." invoked by
"initrd.img/scripts/init-top/udev". Please note that this is executed in the
background using "&".
4) /lib/udev/rules.d/85-lvm2.rules intercepts add events for LVM partitions and
issues "watershed sh -c '/sbin/lvm vgscan; /sbin/lvm vgchange -a y'"
5) "lvm vgchange -a y" sends a LV resume request to the device-mapper in the
kernel and waits for completition notification by udevd via semaphore
6) initrd.img/scripts/init-bottom/udev gets executed which invokes "udevadm
control --exit". After this point, the initrd-udevd is not handling events
anymore.
7) kernel delivers the mapper-task-completion-event to udevd which does not
handle events anymore, and therefore does not decrement the semaphore
8) having already waited 60 seconds (event_timeout) for "watershed sh -c
'/sbin/lvm vgscan; /sbin/lvm vgchange -a y'" to complete, udevd just gives up
and continues termination
9) according to udev-173/udev/udevd.c:1631 the remaining queued events
(including the semaphore decrement) are discarded by means of
"event_queue_cleanup".
10) after termination of udevd "udevadm control --exit" also terminates and the
boot process continues
In the end, the rootfs-udevd does not see the semaphore decrement event
anymore, so "lvm vgscan -a y" still hangs. Some other bugs like #631795
#797226 #581566 might be related to this timing problem.
Also adding udev to the bug. Please set to invalid if e.g. lvm is
required to wait for the logical volume setup to finish in
initrd.img/scripts/init-premount/lvm2.
** Also affects: udev (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/802626
Title:
boot hangs at initrd
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