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BUG: KCSAN: data-race in d_set_mounted / dput

write to 0xffff88811e75ab40 of 4 bytes by task 16326 on cpu 0:
 d_set_mounted+0xfc/0x190 fs/dcache.c:1459
 get_mountpoint+0x1be/0x300 fs/namespace.c:740
 lock_mount+0x86/0x1d0 fs/namespace.c:2200
 do_new_mount_fc fs/namespace.c:2818 [inline]
 do_new_mount fs/namespace.c:2877 [inline]
 path_mount+0x18ef/0x1c70 fs/namespace.c:3192
 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 0xffff88811e75ab40 of 4 bytes by task 16335 on cpu 1:
 fast_dput fs/dcache.c:746 [inline]
 dput+0x2f/0x420 fs/dcache.c:865
 path_put+0x1b/0x30 fs/namei.c:496
 __do_sys_chdir fs/open.c:502 [inline]
 __se_sys_chdir+0xd3/0x150 fs/open.c:485
 __x64_sys_chdir+0x1b/0x20 fs/open.c:485
 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: 16335 Comm: syz-executor.5 Not tainted 5.9.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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