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BUG: KCSAN: data-race in mISDN_ioctl / mISDN_read

read to 0xffff88813ad7a8c0 of 4 bytes by task 11832 on cpu 1:
 mISDN_read+0x128/0x4a0 drivers/isdn/mISDN/timerdev.c:108
 vfs_read+0x1a1/0x6a0 fs/read_write.c:480
 ksys_read+0xe8/0x1a0 fs/read_write.c:620
 __do_sys_read fs/read_write.c:630 [inline]
 __se_sys_read fs/read_write.c:628 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:628
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

write to 0xffff88813ad7a8c0 of 4 bytes by task 11833 on cpu 0:
 misdn_add_timer drivers/isdn/mISDN/timerdev.c:175 [inline]
 mISDN_ioctl+0x2d9/0x590 drivers/isdn/mISDN/timerdev.c:233
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:856
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:856
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

value changed: 0x00000001 -> 0x00000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 11833 Comm: syz-executor.2 Not tainted 5.19.0-rc1-syzkaller-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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