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BUG: KCSAN: data-race in check_kill_permission / ptrace_stop

write to 0xffff88808d0da02c of 4 bytes by task 12728 on cpu 0:
 freezer_do_not_count include/linux/freezer.h:109 [inline]
 freezable_schedule include/linux/freezer.h:171 [inline]
 ptrace_stop+0x370/0x650 kernel/signal.c:2220
 ptrace_signal kernel/signal.c:2495 [inline]
 get_signal+0x80a/0x14e0 kernel/signal.c:2673
 arch_do_signal_or_restart+0x2a/0x270 arch/x86/kernel/signal.c:789
 handle_signal_work kernel/entry/common.c:147 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
 exit_to_user_mode_prepare+0x11a/0x1b0 kernel/entry/common.c:208
 __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:301
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88808d0da02c of 4 bytes by task 12730 on cpu 1:
 check_kill_permission+0x44/0x2d0 kernel/signal.c:838
 do_send_specific kernel/signal.c:3817 [inline]
 do_tkill kernel/signal.c:3849 [inline]
 __do_sys_tkill kernel/signal.c:3884 [inline]
 __se_sys_tkill+0xd5/0x180 kernel/signal.c:3878
 __x64_sys_tkill+0x2d/0x40 kernel/signal.c:3878
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 12730 Comm: syz-executor.3 Not tainted 5.12.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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