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BUG: KCSAN: data-race in d_set_mounted / link_path_walk

write to 0xffff88801663cc00 of 4 bytes by task 16143 on cpu 0:
 d_set_mounted+0x112/0x190 fs/dcache.c:1466
 get_mountpoint+0x1ac/0x2f0 fs/namespace.c:740
 lock_mount+0x86/0x1d0 fs/namespace.c:2203
 do_loopback fs/namespace.c:2352 [inline]
 path_mount+0xbb1/0x1c10 fs/namespace.c:3202
 do_mount fs/namespace.c:3221 [inline]
 __do_sys_mount fs/namespace.c:3429 [inline]
 __se_sys_mount+0x23d/0x2e0 fs/namespace.c:3406
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3406
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88801663cc00 of 4 bytes by task 16144 on cpu 1:
 __d_entry_type include/linux/dcache.h:392 [inline]
 d_can_lookup include/linux/dcache.h:407 [inline]
 link_path_walk+0x48e/0x730 fs/namei.c:2196
 path_lookupat+0x7b/0x550 fs/namei.c:2335
 filename_lookup+0xf2/0x380 fs/namei.c:2369
 user_path_at_empty+0x3b/0x50 fs/namei.c:2649
 user_path_at include/linux/namei.h:59 [inline]
 __do_sys_pivot_root fs/namespace.c:3704 [inline]
 __se_sys_pivot_root+0xd8/0xf80 fs/namespace.c:3688
 __x64_sys_pivot_root+0x2d/0x40 fs/namespace.c:3688
 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: 16144 Comm: syz-executor.0 Not tainted 5.11.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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