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BUG: KCSAN: data-race in mISDN_ioctl / mISDN_poll

write to 0xffff88810768d5c0 of 4 bytes by task 4873 on cpu 1:
 misdn_add_timer drivers/isdn/mISDN/timerdev.c:175 [inline]
 mISDN_ioctl+0x2fd/0x540 drivers/isdn/mISDN/timerdev.c:233
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl+0xc9/0x140 fs/ioctl.c:856
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:856
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffff88810768d5c0 of 4 bytes by task 4872 on cpu 0:
 mISDN_poll+0xb9/0x1c0 drivers/isdn/mISDN/timerdev.c:146
 vfs_poll include/linux/poll.h:88 [inline]
 do_pollfd fs/select.c:873 [inline]
 do_poll fs/select.c:921 [inline]
 do_sys_poll+0x630/0xbe0 fs/select.c:1015
 __do_sys_ppoll fs/select.c:1121 [inline]
 __se_sys_ppoll+0x1af/0x1f0 fs/select.c:1101
 __x64_sys_ppoll+0x67/0x80 fs/select.c:1101
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x00000000 -> 0x00000001

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 4872 Comm: syz-executor.1 Not tainted 6.3.0-rc7-syzkaller-00043-gaf67688dca57 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/30/2023
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