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BUG: KCSAN: data-race in __put_mountpoint / link_path_walk

write to 0xffff88821aab6780 of 4 bytes by task 29060 on cpu 0:
 __put_mountpoint+0x93/0x130 fs/namespace.c:776
 put_mountpoint fs/namespace.c:787 [inline]
 attach_recursive_mnt+0x898/0x1210 fs/namespace.c:2181
 graft_tree fs/namespace.c:2237 [inline]
 do_loopback fs/namespace.c:2365 [inline]
 path_mount+0x1258/0x1c70 fs/namespace.c:3186
 do_mount fs/namespace.c:3205 [inline]
 __do_sys_mount fs/namespace.c:3413 [inline]
 __se_sys_mount+0x126/0x180 fs/namespace.c:3390
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3390
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88821aab6780 of 4 bytes by task 5075 on cpu 1:
 link_path_walk+0x334/0x730 fs/namei.c:2145
 path_lookupat+0x7b/0x560 fs/namei.c:2332
 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]
 do_faccessat+0x260/0x500 fs/open.c:423
 __do_sys_access fs/open.c:482 [inline]
 __se_sys_access fs/open.c:480 [inline]
 __x64_sys_access+0x33/0x40 fs/open.c:480
 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: 1 PID: 5075 Comm: systemd-udevd Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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