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BUG: KCSAN: data-race in do_signal_stop / prepare_signal

write to 0xffff888105db802c of 4 bytes by task 10924 on cpu 0:
 freezer_count include/linux/freezer.h:121 [inline]
 freezable_schedule include/linux/freezer.h:173 [inline]
 do_signal_stop+0x56f/0x6e0 kernel/signal.c:2396
 get_signal+0x788/0x14e0 kernel/signal.c:2631
 arch_do_signal_or_restart+0x2a/0x270 arch/x86/kernel/signal.c:811
 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 0xffff888105db802c of 4 bytes by interrupt on cpu 1:
 sig_task_ignored kernel/signal.c:94 [inline]
 sig_ignored kernel/signal.c:119 [inline]
 prepare_signal+0x872/0xd80 kernel/signal.c:962
 send_sigqueue+0xea/0x430 kernel/signal.c:1861
 posix_timer_event kernel/time/posix-timers.c:354 [inline]
 posix_timer_fn+0xe8/0x210 kernel/time/posix-timers.c:380
 __run_hrtimer+0x163/0x460 kernel/time/hrtimer.c:1537
 __hrtimer_run_queues kernel/time/hrtimer.c:1601 [inline]
 hrtimer_interrupt+0x380/0xaf0 kernel/time/hrtimer.c:1663
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1089 [inline]
 __sysvec_apic_timer_interrupt+0x6f/0x200 arch/x86/kernel/apic/apic.c:1106
 sysvec_apic_timer_interrupt+0x69/0x80 arch/x86/kernel/apic/apic.c:1100
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:632
 watchpoint_slot kernel/kcsan/encoding.h:85 [inline]
 find_watchpoint kernel/kcsan/core.c:110 [inline]
 check_access kernel/kcsan/core.c:612 [inline]
 __tsan_read8+0xf/0x180 kernel/kcsan/core.c:840
 timekeeping_get_delta kernel/time/timekeeping.c:290 [inline]
 timekeeping_get_ns kernel/time/timekeeping.c:386 [inline]
 ktime_get_with_offset+0x12e/0x1f0 kernel/time/timekeeping.c:872
 ktime_get_real include/linux/timekeeping.h:78 [inline]
 __net_timestamp include/linux/skbuff.h:3795 [inline]
 netif_rx_internal+0x200/0x260 net/core/dev.c:4824
 netif_rx_ni+0x21/0x1d0 net/core/dev.c:4890
 dev_loopback_xmit+0x1c4/0x200 net/core/dev.c:3894
 ip_mc_finish_output+0x122/0x260 net/ipv4/ip_output.c:357
 NF_HOOK include/linux/netfilter.h:301 [inline]
 ip_mc_output+0x18c/0x4d0 net/ipv4/ip_output.c:396
 dst_output include/net/dst.h:448 [inline]
 ip_local_out net/ipv4/ip_output.c:126 [inline]
 ip_send_skb+0x6e/0xe0 net/ipv4/ip_output.c:1568
 udp_send_skb+0x62d/0x860 net/ipv4/udp.c:953
 udp_sendmsg+0x12b0/0x1560 net/ipv4/udp.c:1240
 inet_sendmsg+0x5f/0x80 net/ipv4/af_inet.c:821
 sock_sendmsg_nosec net/socket.c:654 [inline]
 sock_sendmsg net/socket.c:674 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2350
 ___sys_sendmsg net/socket.c:2404 [inline]
 __sys_sendmmsg+0x315/0x4b0 net/socket.c:2490
 __do_sys_sendmmsg net/socket.c:2519 [inline]
 __se_sys_sendmmsg net/socket.c:2516 [inline]
 __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2516
 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: 10942 Comm: syz-executor.4 Not tainted 5.12.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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