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BUG: KCSAN: data-race in exit_signals / prepare_signal

write to 0xffff88811ebea024 of 4 bytes by task 684 on cpu 0:
 exit_signals+0x124/0x580 kernel/signal.c:2831
 do_exit+0x19f/0x16e0 kernel/exit.c:761
 do_group_exit+0xcb/0x180 kernel/exit.c:903
 get_signal+0x106d/0x1650 kernel/signal.c:2743
 do_signal+0x25/0x270 arch/x86/kernel/signal.c:810
 exit_to_usermode_loop arch/x86/entry/common.c:235 [inline]
 __prepare_exit_to_usermode+0x127/0x230 arch/x86/entry/common.c:269
 __syscall_return_slowpath+0x47/0x60 arch/x86/entry/common.c:352
 do_syscall_64+0x5d/0xb0 arch/x86/entry/common.c:393
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811ebea024 of 4 bytes by task 9237 on cpu 1:
 sig_task_ignored kernel/signal.c:94 [inline]
 sig_ignored kernel/signal.c:119 [inline]
 prepare_signal+0x88d/0xdb0 kernel/signal.c:961
 __send_signal+0x64/0x7d0 kernel/signal.c:1081
 send_signal+0x398/0x3e0 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+0x7b/0xc0 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:3649 [inline]
 __se_sys_kill+0xd3/0x450 kernel/signal.c:3643
 __x64_sys_kill+0x2d/0x40 kernel/signal.c:3643
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9237 Comm: syz-executor.5 Not tainted 5.8.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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