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BUG: KCSAN: data-race in exit_signals / prepare_signal

read-write to 0xffff88812a9a5024 of 4 bytes by task 11970 on cpu 1:
 exit_signals+0x10d/0x540 kernel/signal.c:2845
 do_exit+0x19a/0x1670 kernel/exit.c:762
 do_group_exit+0xc8/0x170 kernel/exit.c:903
 get_signal+0xf9b/0x1510 kernel/signal.c:2757
 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+0x116/0x200 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 0xffff88812a9a5024 of 4 bytes by task 8741 on cpu 0:
 sig_task_ignored kernel/signal.c:94 [inline]
 sig_ignored kernel/signal.c:119 [inline]
 prepare_signal+0x891/0xda0 kernel/signal.c:961
 __send_signal+0x64/0x780 kernel/signal.c:1081
 send_signal+0x39f/0x3d0 kernel/signal.c:1241
 do_send_sig_info+0x9b/0xe0 kernel/signal.c:1290
 group_send_sig_info kernel/signal.c:1413 [inline]
 kill_pid_info+0x7f/0x100 kernel/signal.c:1447
 kill_proc_info kernel/signal.c:1464 [inline]
 kill_something_info kernel/signal.c:1561 [inline]
 __do_sys_kill kernel/signal.c:3663 [inline]
 __se_sys_kill+0xd3/0x450 kernel/signal.c:3657
 __x64_sys_kill+0x2d/0x40 kernel/signal.c:3657
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 8741 Comm: syz-executor.5 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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