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BUG: KCSAN: data-race in fasync_remove_entry / kill_fasync

read to 0xffff888122785ef8 of 8 bytes by interrupt on cpu 0:
 kill_fasync+0x3f/0x180 fs/fcntl.c:1019
 perf_event_wakeup+0x11f/0x160 kernel/events/core.c:5957
 perf_pending_event+0x12d/0x170 kernel/events/core.c:6013
 irq_work_run_list+0xea/0x140 kernel/irq_work.c:165
 irq_work_run+0x48/0xa0 kernel/irq_work.c:180
 smp_irq_work_interrupt+0x71/0x1c0 arch/x86/kernel/irq_work.c:21
 irq_work_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:861
 __list_del_entry_valid+0x62/0x93 lib/list_debug.c:51
 __list_del_entry include/linux/list.h:131 [inline]
 list_del include/linux/list.h:139 [inline]
 rt_del_uncached_list+0x87/0x170 net/ipv4/route.c:1520
 ipv4_dst_destroy+0x60/0x80 net/ipv4/route.c:1530
 dst_destroy+0xb3/0x210 net/core/dst.c:120
 dst_destroy_rcu+0x1f/0x30 net/core/dst.c:142
 __rcu_reclaim kernel/rcu/rcu.h:222 [inline]
 rcu_do_batch+0x256/0x5b0 kernel/rcu/tree.c:2157
 rcu_core+0x369/0x4d0 kernel/rcu/tree.c:2377
 rcu_core_si+0x12/0x20 kernel/rcu/tree.c:2386
 __do_softirq+0x115/0x33f kernel/softirq.c:292
 invoke_softirq kernel/softirq.c:373 [inline]
 irq_exit+0xbb/0xe0 kernel/softirq.c:413
 exiting_irq arch/x86/include/asm/apic.h:536 [inline]
 smp_apic_timer_interrupt+0xe6/0x280 arch/x86/kernel/apic/apic.c:1137
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:830
 find_watchpoint kernel/kcsan/core.c:67 [inline]
 check_access kernel/kcsan/core.c:388 [inline]
 __tsan_read1+0x40/0x1e0 kernel/kcsan/core.c:527
 strlen+0x31/0x60 lib/string.c:527
 strlen include/linux/string.h:300 [inline]
 insert_entry fs/proc/proc_sysctl.c:161 [inline]
 insert_header+0x272/0x820 fs/proc/proc_sysctl.c:237
 __register_sysctl_table+0x538/0x880 fs/proc/proc_sysctl.c:1366
 register_net_sysctl+0x32/0x50 net/sysctl_net.c:121
 nf_conntrack_standalone_init_sysctl net/netfilter/nf_conntrack_standalone.c:1073 [inline]
 nf_conntrack_pernet_init+0x60b/0x8a0 net/netfilter/nf_conntrack_standalone.c:1118
 ops_init+0x8c/0x240 net/core/net_namespace.c:137
 setup_net+0x191/0x4b0 net/core/net_namespace.c:335
 copy_net_ns+0x1e3/0x390 net/core/net_namespace.c:476
 create_new_namespaces+0x2e2/0x4b0 kernel/nsproxy.c:103
 unshare_nsproxy_namespaces+0xb9/0x170 kernel/nsproxy.c:202
 ksys_unshare+0x2dc/0x710 kernel/fork.c:2827
 __do_sys_unshare kernel/fork.c:2895 [inline]
 __se_sys_unshare kernel/fork.c:2893 [inline]
 __x64_sys_unshare+0x28/0x40 kernel/fork.c:2893
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff888122785ef8 of 8 bytes by task 31799 on cpu 1:
 fasync_remove_entry+0xba/0x120 fs/fcntl.c:882
 fasync_helper+0xcf/0xdc fs/fcntl.c:981
 perf_fasync+0x6c/0xa0 kernel/events/core.c:5917
 __fput+0x46a/0x520 fs/file_table.c:277
 ____fput+0x1f/0x30 fs/file_table.c:313
 task_work_run+0xf6/0x130 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop+0x2b4/0x2c0 arch/x86/entry/common.c:163
 prepare_exit_to_usermode arch/x86/entry/common.c:194 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:274 [inline]
 do_syscall_64+0x353/0x370 arch/x86/entry/common.c:300
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 31799 Comm: syz-executor.2 Not tainted 5.4.0-rc6+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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