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BUG: KCSAN: data-race in syscall_trace_enter / zap_other_threads

write to 0xffff88812fd3c750 of 8 bytes by task 9194 on cpu 0:
 sigaddset include/linux/signal.h:66 [inline]
 zap_other_threads+0x116/0x1e0 kernel/signal.c:1362
 do_group_exit+0x145/0x1a0 kernel/exit.c:917
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:933
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:931
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:931
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88812fd3c750 of 8 bytes by task 9195 on cpu 1:
 sigismember include/linux/signal.h:84 [inline]
 __fatal_signal_pending include/linux/sched/signal.h:376 [inline]
 fatal_signal_pending include/linux/sched/signal.h:381 [inline]
 ptrace_report_syscall include/linux/tracehook.h:79 [inline]
 tracehook_report_syscall_entry include/linux/tracehook.h:105 [inline]
 arch_syscall_enter_tracehook include/linux/entry-common.h:98 [inline]
 syscall_trace_enter+0xf2/0x290 kernel/entry/common.c:60
 __syscall_enter_from_user_work kernel/entry/common.c:89 [inline]
 syscall_enter_from_user_mode+0x1c/0x20 kernel/entry/common.c:107
 do_syscall_64+0x11/0x50 arch/x86/entry/common.c:41
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9195 Comm: syz-executor.4 Not tainted 5.12.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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