[SRU][I][PATCH 0/1] kernel panic: NULL pointer dereference in wb_timer_f()

[Andrea Righi]

BugLink: https://bugs.launchpad.net/bugs/1947557

[Impact]

It is possible to trigger a kernel panic with the latest impish kernel running
systemd autopkgtest using --enable-kvm with the test instances created by
systemd during the autotest. The panic happens in the host, not in the guest VM
executed by systemd.

[Test case]

Add --enable-kvm to the options in test/testdata/test-functions (systemd), run
`sudo autopkgtest . -- null`, wait for the panic to happen.

[Fix]

https://lore.kernel.org/lkml/YW6N2qXpBU3oc50q@arighi-desktop/T/#u

[Regression potential]

The fix is addressing a race in the block layer (in the buffered write
throttling code - block/blk-wbt.c) between a disk being released and the timer
callback that periodically checks if the latency for a specific block device
has been exceeded. If the fix is not correct we may still have a race in this
code, that can still show potential kernel panics in the block layer subsystem.