CVE-2025-40163

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

sched/deadline: Stop dl_server before CPU goes offline

IBM CI tool reported kernel warning1 ↗ when running a CPU removal operation through drmgr2 ↗. i.e "drmgr -c cpu -r -q 1"

WARNING: CPU: 0 PID: 0 at kernel/sched/cpudeadline.c:219 cpudl_set+0x58/0x170 NIP [c0000000002b6ed8] cpudl_set+0x58/0x170 LR [c0000000002b7cb8] dl_server_timer+0x168/0x2a0 Call Trace: [c000000002c2f8c0] init_stack+0x78c0/0x8000 (unreliable) [c0000000002b7cb8] dl_server_timer+0x168/0x2a0 [c00000000034df84] __hrtimer_run_queues+0x1a4/0x390 [c00000000034f624] hrtimer_interrupt+0x124/0x300 [c00000000002a230] timer_interrupt+0x140/0x320

Git bisects to: commit 4ae8d9aa9f9d ("sched/deadline: Fix dl_server getting stuck")

This happens since:

• dl_server hrtimer gets enqueued close to cpu offline, when kthread_park enqueues a fair task.
• CPU goes offline and drmgr removes it from cpu_present_mask.
• hrtimer fires and warning is hit.

Fix it by stopping the dl_server before CPU is marked dead.

[sshegde: wrote the changelog and tested it]