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BUG: KCSAN: data-race in pcpu_alloc / pcpu_free_area

read-write to 0xffffffff86e8f9bc of 4 bytes by interrupt on cpu 0:
 pcpu_update_empty_pages mm/percpu.c:606 [inline]
 pcpu_block_update_hint_free mm/percpu.c:1040 [inline]
 pcpu_free_area+0x52a/0x5c0 mm/percpu.c:1298
 free_percpu+0x1d5/0xa50 mm/percpu.c:2281
 free_vfsmnt fs/namespace.c:612 [inline]
 delayed_free_vfsmnt+0xa3/0xe0 fs/namespace.c:619
 rcu_do_batch kernel/rcu/tree.c:2535 [inline]
 rcu_core+0x57e/0xbb0 kernel/rcu/tree.c:2786
 rcu_core_si+0x9/0x10 kernel/rcu/tree.c:2803
 __do_softirq+0x158/0x2e3 kernel/softirq.c:558
 run_ksoftirqd+0x1f/0x30 kernel/softirq.c:921
 smpboot_thread_fn+0x308/0x4a0 kernel/smpboot.c:164
 kthread+0x1a9/0x1e0 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30

read to 0xffffffff86e8f9bc of 4 bytes by task 2063 on cpu 1:
 pcpu_alloc+0x9bb/0xfb0 mm/percpu.c:1877
 __alloc_percpu+0x20/0x30 mm/percpu.c:1949
 alloc_vfsmnt+0xcf/0x2e0 fs/namespace.c:214
 clone_mnt+0x40/0x880 fs/namespace.c:1104
 copy_tree+0x313/0x870 fs/namespace.c:1921
 copy_mnt_ns+0x116/0x600 fs/namespace.c:3480
 create_new_namespaces+0x88/0x450 kernel/nsproxy.c:78
 unshare_nsproxy_namespaces+0xe2/0x120 kernel/nsproxy.c:226
 ksys_unshare+0x38c/0x6e0 kernel/fork.c:3132
 __do_sys_unshare kernel/fork.c:3203 [inline]
 __se_sys_unshare kernel/fork.c:3201 [inline]
 __x64_sys_unshare+0x1b/0x20 kernel/fork.c:3201
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000005 -> 0x00000006

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 2063 Comm: syz-executor.0 Not tainted 5.18.0-rc7-syzkaller-00136-g3b5e1590a267-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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