KCSAN: data-race in __alloc_file / __alloc_file

[syzbot <syzbot+3ef049d50587836c0606@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    05f22368 x86, kcsan: Enable KCSAN for x86
git tree:       https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=10d7fd88e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
dashboard link: https://syzkaller.appspot.com/bug?extid=3ef049d50587836c0606
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

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+3ef049d50587836c0606@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __alloc_file / __alloc_file

write to 0xffff8880bb157398 of 4 bytes by task 10993 on cpu 0:
  get_cred include/linux/cred.h:253 [inline]
  __alloc_file+0x74/0x210 fs/file_table.c:105
  alloc_empty_file+0x8f/0x180 fs/file_table.c:151
  alloc_file+0x4e/0x2b0 fs/file_table.c:193
  alloc_file_pseudo+0x11c/0x1b0 fs/file_table.c:232
  anon_inode_getfile fs/anon_inodes.c:91 [inline]
  anon_inode_getfile+0x103/0x1d0 fs/anon_inodes.c:74
  __do_sys_perf_event_open+0xd32/0x1ac0 kernel/events/core.c:11100
  __se_sys_perf_event_open kernel/events/core.c:10867 [inline]
  __x64_sys_perf_event_open+0x70/0x90 kernel/events/core.c:10867
  do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff8880bb157398 of 4 bytes by task 11004 on cpu 1:
  get_cred include/linux/cred.h:253 [inline]
  __alloc_file+0x74/0x210 fs/file_table.c:105
  alloc_empty_file+0x8f/0x180 fs/file_table.c:151
  path_openat+0x74/0x36e0 fs/namei.c:3514
  do_filp_open+0x11e/0x1b0 fs/namei.c:3555
  do_sys_open+0x3b3/0x4f0 fs/open.c:1097
  __do_sys_open fs/open.c:1115 [inline]
  __se_sys_open fs/open.c:1110 [inline]
  __x64_sys_open+0x55/0x70 fs/open.c:1110
  do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 11004 Comm: syz-executor.5 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
==================================================================


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