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BUG: KCSAN: data-race in exit_signals / prepare_signal

write to 0xffff8881324270ec of 4 bytes by task 25508 on cpu 1:
 exit_signals+0xe1/0x4a0 kernel/signal.c:2873
 do_exit+0x194/0x1560 kernel/exit.c:781
 do_group_exit+0xce/0x1a0 kernel/exit.c:922
 get_signal+0xf83/0x15d0 kernel/signal.c:2781
 arch_do_signal_or_restart+0x2a/0x220 arch/x86/kernel/signal.c:789
 handle_signal_work kernel/entry/common.c:147 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
 exit_to_user_mode_prepare+0x104/0x170 kernel/entry/common.c:208
 __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:301
 do_syscall_64+0x40/0x50 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881324270ec of 4 bytes by task 1777 on cpu 0:
 sig_task_ignored kernel/signal.c:94 [inline]
 sig_ignored kernel/signal.c:119 [inline]
 prepare_signal+0x872/0xd80 kernel/signal.c:962
 __send_signal+0x64/0x760 kernel/signal.c:1082
 send_signal+0x38e/0x3d0 kernel/signal.c:1242
 do_send_sig_info kernel/signal.c:1291 [inline]
 group_send_sig_info+0xe8/0x140 kernel/signal.c:1414
 kill_pid_info kernel/signal.c:1448 [inline]
 kill_proc_info kernel/signal.c:1465 [inline]
 kill_something_info kernel/signal.c:1562 [inline]
 __do_sys_kill kernel/signal.c:3691 [inline]
 __se_sys_kill+0xeb/0x400 kernel/signal.c:3685
 __x64_sys_kill+0x2d/0x40 kernel/signal.c:3685
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 1777 Comm: syz-executor.3 Not tainted 5.12.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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