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BUG: KCSAN: data-race in prepare_signal / ptrace_stop

write to 0xffff888022d5902c of 4 bytes by task 27166 on cpu 1:
 freezer_count include/linux/freezer.h:121 [inline]
 freezable_schedule include/linux/freezer.h:173 [inline]
 ptrace_stop+0x392/0x650 kernel/signal.c:2216
 ptrace_do_notify kernel/signal.c:2269 [inline]
 ptrace_notify+0x109/0x160 kernel/signal.c:2279
 ptrace_report_syscall include/linux/tracehook.h:66 [inline]
 tracehook_report_syscall_entry include/linux/tracehook.h:105 [inline]
 arch_syscall_enter_tracehook include/linux/entry-common.h:96 [inline]
 syscall_trace_enter+0x87/0x2b0 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+0x11/0x80 arch/x86/entry/common.c:41
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888022d5902c of 4 bytes by task 27161 on cpu 0:
 sig_task_ignored kernel/signal.c:94 [inline]
 sig_ignored kernel/signal.c:119 [inline]
 prepare_signal+0x872/0xd80 kernel/signal.c:962
 __send_signal+0x64/0x790 kernel/signal.c:1082
 send_signal+0x398/0x3e0 kernel/signal.c:1242
 __group_send_sig_info kernel/signal.c:1281 [inline]
 do_notify_parent_cldstop+0x23a/0x290 kernel/signal.c:2068
 ptrace_stop+0x2be/0x650 kernel/signal.c:2202
 ptrace_do_notify kernel/signal.c:2269 [inline]
 ptrace_notify+0x109/0x160 kernel/signal.c:2279
 ptrace_report_syscall include/linux/tracehook.h:66 [inline]
 tracehook_report_syscall_exit include/linux/tracehook.h:130 [inline]
 arch_syscall_exit_tracehook include/linux/entry-common.h:295 [inline]
 syscall_exit_work+0xea/0x200 kernel/entry/common.c:257
 syscall_exit_to_user_mode_prepare kernel/entry/common.c:284 [inline]
 __syscall_exit_to_user_mode_work kernel/entry/common.c:289 [inline]
 syscall_exit_to_user_mode+0x36/0x40 kernel/entry/common.c:302
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 27161 Comm: syz-executor.2 Not tainted 5.11.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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