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BUG: KCSAN: data-race in exit_signals / prepare_signal

read-write to 0xffff88810a71c064 of 4 bytes by task 13263 on cpu 0:
 exit_signals+0x102/0x530 kernel/signal.c:2846
 do_exit+0x15d/0x1630 kernel/exit.c:765
 do_group_exit+0xc8/0x170 kernel/exit.c:906
 get_signal+0xf9b/0x1510 kernel/signal.c:2758
 arch_do_signal+0x25/0x260 arch/x86/kernel/signal.c:811
 exit_to_user_mode_loop kernel/entry/common.c:161 [inline]
 exit_to_user_mode_prepare+0xde/0x170 kernel/entry/common.c:191
 irqentry_exit_to_user_mode+0x5/0x20 kernel/entry/common.c:279
 irqentry_exit+0xe/0x40 kernel/entry/common.c:367
 asm_sysvec_reschedule_ipi+0x12/0x20 arch/x86/include/asm/idtentry.h:636

read to 0xffff88810a71c064 of 4 bytes by task 8471 on cpu 1:
 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+0x64/0x780 kernel/signal.c:1082
 send_signal+0x39f/0x3d0 kernel/signal.c:1242
 do_send_sig_info+0x9b/0xe0 kernel/signal.c:1291
 group_send_sig_info kernel/signal.c:1414 [inline]
 kill_pid_info+0x7f/0x100 kernel/signal.c:1448
 kill_proc_info kernel/signal.c:1465 [inline]
 kill_something_info kernel/signal.c:1562 [inline]
 __do_sys_kill kernel/signal.c:3664 [inline]
 __se_sys_kill+0xd3/0x450 kernel/signal.c:3658
 __x64_sys_kill+0x2d/0x40 kernel/signal.c:3658
 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: 1 PID: 8471 Comm: syz-executor.1 Not tainted 5.10.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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