sched/deadline: only set free_cpus for online runqueues

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

sched/deadline: only set free_cpus for online runqueues

Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus

to reflect rd->online") introduced the cpudl_set/clear_freecpu

functions to allow the cpu_dl::free_cpus mask to be manipulated

by the deadline scheduler class rq_on/offline callbacks so the

mask would also reflect this state.

Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask

from cpudl_find()") removed the check of the cpu_active_mask to

save some processing on the premise that the cpudl::free_cpus

mask already reflected the runqueue online state.

Unfortunately, there are cases where it is possible for the

cpudl_clear function to set the free_cpus bit for a CPU when the

deadline runqueue is offline. When this occurs while a CPU is

connected to the default root domain the flag may retain the bad

state after the CPU has been unplugged. Later, a different CPU

that is transitioning through the default root domain may push a

deadline task to the powered down CPU when cpudl_find sees its

free_cpus bit is set. If this happens the task will not have the

opportunity to run.

One example is outlined here:

https://lore.kernel.org/lkml/[email protected]

Another occurs when the last deadline task is migrated from a

CPU that has an offlined runqueue. The dequeue_task member of

the deadline scheduler class will eventually call cpudl_clear

and set the free_cpus bit for the CPU.

This commit modifies the cpudl_clear function to be aware of the

online state of the deadline runqueue so that the free_cpus mask

can be updated appropriately.

It is no longer necessary to manage the mask outside of the

cpudl_set/clear functions so the cpudl_set/clear_freecpu

functions are removed. In addition, since the free_cpus mask is

now only updated under the cpudl lock the code was changed to

use the non-atomic __cpumask functions.