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BUG: KCSAN: data-race in futex_wait_queue_me / prepare_signal

read to 0xffff8880905360a4 of 4 bytes by task 20842 on cpu 0:
 sig_task_ignored kernel/signal.c:94 [inline]
 sig_ignored kernel/signal.c:119 [inline]
 prepare_signal+0x1f5/0x7a0 kernel/signal.c:956
 __send_signal+0x96/0x870 kernel/signal.c:1076
 send_signal+0x224/0x2b0 kernel/signal.c:1236
 __group_send_sig_info kernel/signal.c:1275 [inline]
 do_notify_parent+0x55b/0x5e0 kernel/signal.c:1992
 exit_notify kernel/exit.c:670 [inline]
 do_exit+0x1723/0x18f0 kernel/exit.c:838
 do_group_exit+0xb4/0x1c0 kernel/exit.c:921
 __do_sys_exit_group kernel/exit.c:932 [inline]
 __se_sys_exit_group kernel/exit.c:930 [inline]
 __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:930
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff8880905360a4 of 4 bytes by task 20839 on cpu 1:
 freezer_do_not_count include/linux/freezer.h:109 [inline]
 freezable_schedule include/linux/freezer.h:171 [inline]
 futex_wait_queue_me+0x17d/0x290 kernel/futex.c:2627
 futex_wait+0x19b/0x3f0 kernel/futex.c:2733
 do_futex+0xe9/0x18d0 kernel/futex.c:3644
 __do_sys_futex kernel/futex.c:3705 [inline]
 __se_sys_futex kernel/futex.c:3673 [inline]
 __x64_sys_futex+0x2cd/0x3f0 kernel/futex.c:3673
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 20839 Comm: syz-executor.5 Not tainted 5.4.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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