KCSAN: data-race in lookup_fast / vfs_unlink

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 40959e34 kcsan: Avoid blocking producers in prepare_report()
git tree: https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=10752a03e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=1ab2c758651b11f6
dashboard link: https://syzkaller.appspot.com/bug?extid=0b7e1411f196a7280f7d
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [linux-...@vger.kernel.org linux-...@vger.kernel.org vi...@zeniv.linux.org.uk el...@google.com]

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+0b7e14...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in lookup_fast / vfs_unlink

write to 0xffff88812527cf00 of 4 bytes by task 30166 on cpu 1:
dont_mount include/linux/dcache.h:352 [inline]
vfs_unlink+0x267/0x3e0 fs/namei.c:4075
do_unlinkat+0x33f/0x550 fs/namei.c:4137
__do_sys_unlink fs/namei.c:4184 [inline]
__se_sys_unlink fs/namei.c:4182 [inline]
__x64_sys_unlink+0x38/0x50 fs/namei.c:4182
do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812527cf00 of 4 bytes by task 4029 on cpu 0:
d_revalidate fs/namei.c:765 [inline]
lookup_fast+0x1cd/0x6e0 fs/namei.c:1690
walk_component+0x73/0x9a0 fs/namei.c:1911
lookup_last fs/namei.c:2391 [inline]
path_lookupat.isra.0+0x129/0x2e0 fs/namei.c:2436
filename_lookup+0x145/0x2b0 fs/namei.c:2466
user_path_at_empty+0x47/0x60 fs/namei.c:2746
do_readlinkat+0x89/0x230 fs/stat.c:409
__do_sys_readlink fs/stat.c:442 [inline]
__se_sys_readlink fs/stat.c:439 [inline]
__x64_sys_readlink+0x4c/0x60 fs/stat.c:439
do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 4029 Comm: udevd Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.