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BUG: KCSAN: data-race in futex_wait_queue_me / prepare_signal

write to 0xffff8881008660ac of 4 bytes by task 1731 on cpu 0:
 freezer_do_not_count include/linux/freezer.h:109 [inline]
 freezable_schedule include/linux/freezer.h:171 [inline]
 futex_wait_queue_me+0x198/0x260 kernel/futex.c:2606
 futex_wait+0x143/0x430 kernel/futex.c:2708
 do_futex+0x9e8/0x1e10 kernel/futex.c:3734
 __do_sys_futex kernel/futex.c:3797 [inline]
 __se_sys_futex+0x2a8/0x390 kernel/futex.c:3763
 __x64_sys_futex+0x74/0x80 kernel/futex.c:3763
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881008660ac of 4 bytes by task 1734 on cpu 1:
 sig_task_ignored kernel/signal.c:93 [inline]
 sig_ignored kernel/signal.c:118 [inline]
 prepare_signal+0x872/0xd80 kernel/signal.c:960
 __send_signal+0x64/0x760 kernel/signal.c:1080
 send_signal+0x38e/0x3d0 kernel/signal.c:1240
 do_send_sig_info kernel/signal.c:1289 [inline]
 do_send_specific+0x13d/0x1c0 kernel/signal.c:3825
 do_tkill kernel/signal.c:3851 [inline]
 __do_sys_tgkill kernel/signal.c:3870 [inline]
 __se_sys_tgkill kernel/signal.c:3864 [inline]
 __x64_sys_tgkill+0x108/0x140 kernel/signal.c:3864
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1734 Comm: syz-fuzzer Not tainted 5.12.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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