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BUG: KCSAN: data-race in __put_mountpoint / dput

write to 0xffff8880824a5a80 of 4 bytes by task 1966 on cpu 0:
 __put_mountpoint+0x93/0x130 fs/namespace.c:776
 put_mountpoint fs/namespace.c:787 [inline]
 umount_mnt fs/namespace.c:838 [inline]
 umount_tree+0x812/0x8e0 fs/namespace.c:1522
 do_umount fs/namespace.c:1642 [inline]
 ksys_umount+0x9c6/0xa80 fs/namespace.c:1748
 __do_sys_umount fs/namespace.c:1759 [inline]
 __se_sys_umount fs/namespace.c:1757 [inline]
 __x64_sys_umount+0x2c/0x40 fs/namespace.c:1757
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880824a5a80 of 4 bytes by task 1957 on cpu 1:
 fast_dput fs/dcache.c:746 [inline]
 dput+0x2f/0x420 fs/dcache.c:865
 cleanup_mnt+0x265/0x2e0 fs/namespace.c:1117
 __cleanup_mnt+0x15/0x20 fs/namespace.c:1125
 task_work_run+0x8e/0x110 kernel/task_work.c:123
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop arch/x86/entry/common.c:216 [inline]
 __prepare_exit_to_usermode+0x1c8/0x230 arch/x86/entry/common.c:246
 __syscall_return_slowpath arch/x86/entry/common.c:329 [inline]
 do_syscall_64+0xb9/0x120 arch/x86/entry/common.c:368
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1957 Comm: syz-executor.3 Not tainted 5.8.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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