All the detail is in Bug #1276705.
(1) Currently finit_module() of mptsas kernel module does need more than
30 seconds to initialize LSI SAS1068E disk.
(2) Currently systemd-udevd unconditionally sends SIGKILL upon hardcoded
30 seconds timeout. As a result, finit_module() of mptsas kernel
module receives SIGKILL when waiting for error handler thread to be
started.
(3) Before commit 786235ee was applied, finit_module() receiving SIGKILL
was no problem because kthread_create() ignored SIGKILL when waiting
for error handler thread to be started. But after commit 786235ee was
applied, finit_module() receiving SIGKILL is a problem because
kthread_create() no longer ignores SIGKILL when waiting for error
handler thread to be started. As a result, finit_module() of mptsas
kernel module failed to initialize LSI SAS1068E disk, leading to
a boot failure.
Commit 786235ee was meant for helping OOM killer to terminate the victim
process immediately when the victim process is unable to be terminated
due to waiting for kthreadd process to complete memory allocation.
Kernel developers think that it is a systemd's bug because any thread
who received SIGKILL has a right to terminate immediately. Therefore,
reverting commit 786235ee is not acceptable for kernel developers.
On the other hand, systemd developers think that it is a kernel's bug
because finit_module() should return within 30 seconds. Therefore,
changing to longer timeout is not acceptable for systemd developers.
Since there was no time to wait for systemd to allow longer timeout,
Bug #1276705 used a SAUCE patch that allows kthread_create() to ignore
SIGKILL up to 10 seconds. We used a SAUCE patch for Ubuntu 14.04, but
we don't want to carry this SAUCE patch forever.
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https://bugs.launchpad.net/bugs/1297248
Title:
Wait timeout for systemd-udevd worker process is too short
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