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BUG: KCSAN: data-race in lookup_fast / retain_dentry

write to 0xffff88805279cc00 of 4 bytes by task 15583 on cpu 0:
 retain_dentry+0x1db/0x260 fs/dcache.c:659
 dput+0x10e/0x420 fs/dcache.c:873
 filename_create+0x138/0x3c0 fs/namei.c:3492
 user_path_create fs/namei.c:3524 [inline]
 do_mkdirat+0x5a/0x220 fs/namei.c:3664
 __do_sys_mkdir fs/namei.c:3688 [inline]
 __se_sys_mkdir fs/namei.c:3686 [inline]
 __x64_sys_mkdir+0x32/0x40 fs/namei.c:3686
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88805279cc00 of 4 bytes by task 15597 on cpu 1:
 d_revalidate fs/namei.c:767 [inline]
 lookup_fast+0x15f/0x370 fs/namei.c:1493
 walk_component+0x5c/0x350 fs/namei.c:1856
 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]
 do_mount fs/namespace.c:3202 [inline]
 __do_sys_mount fs/namespace.c:3413 [inline]
 __se_sys_mount+0xf1/0x180 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 15597 Comm: syz-executor.5 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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