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BUG: KCSAN: data-race in retain_dentry / step_into

write to 0xffff8881205fa540 of 4 bytes by task 8709 on cpu 1:
 retain_dentry+0x1db/0x260 fs/dcache.c:659
 dput+0x10e/0x420 fs/dcache.c:873
 path_umount+0xf8/0xa80 fs/namespace.c:1738
 ksys_umount fs/namespace.c:1754 [inline]
 __do_sys_umount fs/namespace.c:1759 [inline]
 __se_sys_umount fs/namespace.c:1757 [inline]
 __x64_sys_umount+0x8b/0xb0 fs/namespace.c:1757
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881205fa540 of 4 bytes by task 8696 on cpu 0:
 __d_entry_type include/linux/dcache.h:391 [inline]
 d_is_symlink include/linux/dcache.h:421 [inline]
 step_into+0x158/0xe70 fs/namei.c:1694
 walk_component+0x190/0x350 fs/namei.c:1866
 lookup_last fs/namei.c:2309 [inline]
 path_lookupat+0x12d/0x560 fs/namei.c:2333
 filename_lookup+0xf2/0x380 fs/namei.c:2366
 user_path_at_empty+0x3b/0x50 fs/namei.c:2646
 user_path_at include/linux/namei.h:59 [inline]
 ksys_umount fs/namespace.c:1751 [inline]
 __do_sys_umount fs/namespace.c:1759 [inline]
 __se_sys_umount fs/namespace.c:1757 [inline]
 __x64_sys_umount+0x65/0xb0 fs/namespace.c:1757
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 8696 Comm: syz-fuzzer Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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