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BUG: KCSAN: data-race in do_notify_parent_cldstop / do_signal_stop

write to 0xffff88812b1afaa0 of 4 bytes by task 18058 on cpu 1:
 do_signal_stop+0xfb/0x6d0 kernel/signal.c:2339
 get_signal+0x6e4/0x15f0 kernel/signal.c:2717
 arch_do_signal+0x25/0x270 arch/x86/kernel/signal.c:811
 exit_to_user_mode_loop kernel/entry/common.c:161 [inline]
 exit_to_user_mode_prepare+0x124/0x210 kernel/entry/common.c:192
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:267
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812b1afaa0 of 4 bytes by task 18042 on cpu 0:
 do_notify_parent_cldstop+0x1b2/0x280 kernel/signal.c:2057
 get_signal+0x11f6/0x15f0 kernel/signal.c:2591
 arch_do_signal+0x25/0x270 arch/x86/kernel/signal.c:811
 exit_to_user_mode_loop kernel/entry/common.c:161 [inline]
 exit_to_user_mode_prepare+0x124/0x210 kernel/entry/common.c:192
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:267
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 18042 Comm: syz-executor.4 Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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