KCSAN: data-race in __ext4_new_inode / _find_next_bit.constprop.0

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 941e0d91 mm, kcsan: Instrument SLAB/SLUB free with ASSERT_..
git tree: https://github.com/google/ktsan.git kcsan
console output: https://syzkaller.appspot.com/x/log.txt?x=10139381e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=b313b4492ea15bae
dashboard link: https://syzkaller.appspot.com/bug?extid=f7a3e358de2cbccfe4b4
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [adilger...@dilger.ca linux...@vger.kernel.org linux-...@vger.kernel.org ty...@mit.edu 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+f7a3e3...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __ext4_new_inode / _find_next_bit.constprop.0

write to 0xffff888125edf028 of 8 bytes by task 8925 on cpu 0:
instrument_write include/linux/instrumented.h:42 [inline]
__test_and_set_bit include/asm-generic/bitops/instrumented-non-atomic.h:71 [inline]
__test_and_set_bit_le include/asm-generic/bitops/le.h:90 [inline]
__ext4_new_inode+0xf53/0x2f30 fs/ext4/ialloc.c:942
ext4_symlink+0x322/0x9e0 fs/ext4/namei.c:3287
vfs_symlink fs/namei.c:4201 [inline]
vfs_symlink+0x216/0x310 fs/namei.c:4187
do_symlinkat+0x1ac/0x1e0 fs/namei.c:4228
__do_sys_symlink fs/namei.c:4247 [inline]
__se_sys_symlink fs/namei.c:4245 [inline]
__x64_sys_symlink+0x3c/0x50 fs/namei.c:4245
do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888125edf028 of 8 bytes by task 8923 on cpu 1:
_find_next_bit.constprop.0+0x61/0x160 lib/find_bit.c:39
find_next_zero_bit+0x35/0x50 lib/find_bit.c:87
find_next_zero_bit_le include/asm-generic/bitops/le.h:15 [inline]
find_inode_bit.isra.0+0x7d/0x310 fs/ext4/ialloc.c:713
__ext4_new_inode+0xd5f/0x2f30 fs/ext4/ialloc.c:912
ext4_symlink+0x322/0x9e0 fs/ext4/namei.c:3287
vfs_symlink fs/namei.c:4201 [inline]
vfs_symlink+0x216/0x310 fs/namei.c:4187
do_symlinkat+0x1ac/0x1e0 fs/namei.c:4228
__do_sys_symlink fs/namei.c:4247 [inline]
__se_sys_symlink fs/namei.c:4245 [inline]
__x64_sys_symlink+0x3c/0x50 fs/namei.c:4245
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: 1 PID: 8923 Comm: syz-executor.1 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.