syzbot has bisected this issue to:

commit 3253cb49cbad4772389d6ef55be75db1f97da910
Author: Sebastian Andrzej Siewior [off-list ref]
Date:   Thu Sep 4 14:25:25 2025 +0000

    softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=17566a7c580000
start commit:   7f7072574127 Merge tag 'kbuild-6.18-1' of git://git.kernel..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=14d66a7c580000
console output: https://syzkaller.appspot.com/x/log.txt?x=10d66a7c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b78ebc06b51acd7e
dashboard link: https://syzkaller.appspot.com/bug?extid=8715dd783e9b0bef43b1
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16ba76e2580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17741ee2580000

Reported-by: syzbot+8715dd783e9b0bef43b1@syzkaller.appspotmail.com
Fixes: 3253cb49cbad ("softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection