CVE-2024-46839

In the Linux kernel, the following vulnerability has been resolved:

workqueue: Improve scalability of workqueue watchdog touch

On a ~2000 CPU powerpc system, hard lockups have been observed in the
workqueue code when stop_machine runs (in this case due to CPU hotplug).
This is due to lots of CPUs spinning in multi_cpu_stop, calling
touch_nmi_watchdog() which ends up calling wq_watchdog_touch().
wq_watchdog_touch() writes to the global variable wq_watchdog_touched,
and that can find itself in the same cacheline as other important
workqueue data, which slows down operations to the point of lockups.

In the case of the following abridged trace, worker_pool_idr was in
the hot line, causing the lockups to always appear at idr_find.

watchdog: CPU 1125 self-detected hard LOCKUP @ idr_find
Call Trace:
get_work_pool
__queue_work
call_timer_fn
run_timer_softirq
__do_softirq
do_softirq_own_stack
irq_exit
timer_interrupt
decrementer_common_virt
* interrupt: 900 (timer) at multi_cpu_stop
multi_cpu_stop
cpu_stopper_thread
smpboot_thread_fn
kthread

Fix this by having wq_watchdog_touch() only write to the line if the
last time a touch was recorded exceeds 1/4 of the watchdog threshold.