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BUG: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult

write to 0xffffffff85c7d080 of 8 bytes by interrupt on cpu 0:
 rcu_report_exp_cpu_mult+0x4f/0xa0 kernel/rcu/tree_exp.h:244
 rcu_report_exp_rdp+0x6c/0x90 kernel/rcu/tree_exp.h:254
 rcu_exp_handler+0xe5/0x190 kernel/rcu/tree_exp.h:616
 flush_smp_call_function_queue+0x18c/0x2b0 kernel/smp.c:248
 generic_smp_call_function_single_interrupt+0x1c/0x49 kernel/smp.c:193
 smp_call_function_single_interrupt+0x3f/0x100 arch/x86/kernel/smp.c:262
 call_function_single_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:851
 should_watch kernel/kcsan/core.c:206 [inline]
 check_access kernel/kcsan/core.c:458 [inline]
 __tsan_read1+0xb0/0x100 kernel/kcsan/core.c:586
 strchr+0x39/0x70 lib/string.c:411
 __register_sysctl_table+0x3a8/0x880 fs/proc/proc_sysctl.c:1350
 register_net_sysctl+0x32/0x50 net/sysctl_net.c:121
 ipv6_sysctl_net_init+0x27f/0x360 net/ipv6/sysctl_net_ipv6.c:243
 ops_init+0x8c/0x240 net/core/net_namespace.c:137
 setup_net+0x191/0x4b0 net/core/net_namespace.c:335
 copy_net_ns+0x1e7/0x400 net/core/net_namespace.c:476
 create_new_namespaces+0x2e2/0x550 kernel/nsproxy.c:103
 unshare_nsproxy_namespaces+0xb9/0x170 kernel/nsproxy.c:202
 ksys_unshare+0x2dc/0x710 kernel/fork.c:2935
 __do_sys_unshare kernel/fork.c:3003 [inline]
 __se_sys_unshare kernel/fork.c:3001 [inline]
 __x64_sys_unshare+0x28/0x40 kernel/fork.c:3001
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff85c7d080 of 8 bytes by task 10158 on cpu 1:
 _find_next_bit lib/find_bit.c:39 [inline]
 find_next_bit+0x57/0xe0 lib/find_bit.c:70
 sync_rcu_exp_select_node_cpus+0x28e/0x510 kernel/rcu/tree_exp.h:375
 sync_rcu_exp_select_cpus+0x30c/0x590 kernel/rcu/tree_exp.h:439
 rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:575 [inline]
 wait_rcu_exp_gp+0x25/0x40 kernel/rcu/tree_exp.h:589
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2264
 worker_thread+0xa0/0x800 kernel/workqueue.c:2410
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 10158 Comm: kworker/1:7 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: rcu_gp wait_rcu_exp_gp
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