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BUG: KCSAN: data-race in complete_signal / get_signal

read to 0xffff8880b81870a4 of 4 bytes by interrupt on cpu 0:
 wants_signal kernel/signal.c:972 [inline]
 complete_signal+0x1c9/0x5e0 kernel/signal.c:1008
 send_sigqueue+0x2cf/0x4b0 kernel/signal.c:1879
 posix_timer_event kernel/time/posix-timers.c:328 [inline]
 posix_timer_fn+0x10d/0x230 kernel/time/posix-timers.c:354
 __run_hrtimer kernel/time/hrtimer.c:1517 [inline]
 __hrtimer_run_queues+0x274/0x5f0 kernel/time/hrtimer.c:1579
 hrtimer_interrupt+0x22a/0x480 kernel/time/hrtimer.c:1641
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1110 [inline]
 smp_apic_timer_interrupt+0xdc/0x280 arch/x86/kernel/apic/apic.c:1135
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
 arch_local_irq_enable arch/x86/include/asm/paravirt.h:762 [inline]
 get_user_pages_fast+0x184/0x340 mm/gup.c:2435
 get_futex_key+0x18d/0xa10 kernel/futex.c:568
 futex_wake+0xaf/0x350 kernel/futex.c:1666
 do_futex+0x2be/0x18d0 kernel/futex.c:3886
 mm_release+0x1ef/0x210 kernel/fork.c:1309
 exit_mm_release+0x2f/0x40 kernel/fork.c:1326
 exit_mm kernel/exit.c:440 [inline]
 do_exit+0x2a8/0x18c0 kernel/exit.c:784
 do_group_exit+0xb4/0x1c0 kernel/exit.c:895
 __do_sys_exit_group kernel/exit.c:906 [inline]
 __se_sys_exit_group kernel/exit.c:904 [inline]
 __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:904
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff8880b81870a4 of 4 bytes by task 20803 on cpu 1:
 get_signal+0x7c7/0x1320 kernel/signal.c:2714
 do_signal+0x2f/0x6c0 arch/x86/kernel/signal.c:815
 exit_to_usermode_loop+0x250/0x2c0 arch/x86/entry/common.c:160
 prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
 do_syscall_64+0x384/0x3a0 arch/x86/entry/common.c:304
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 20803 Comm: syz-executor.0 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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